Carol Atkinson – MSc Architecture: AEES
January 2008
University of East London
School of Computing and Technology
Docklands Campus
4-6 University Way
London
E16 2RD
Telephone: 02 082 233000
Graduate School of the Environment
Centre for Alternative Technology
Machynlleth
Powys
SY20 9AZ
Telephone: 01654 704968
1
Carol Atkinson – MSc Architecture: AEES
January 2008
Energy Assessment of a
Straw Bale Building
MSc Architecture:
Advanced Environmental and Energy Studies
Carol Atkinson
January 2008
2
Carol Atkinson – MSc Architecture: AEES
January 2008
Abstract
Around the world, straw bales have been used to build homes for over a hundred
years. In the UK, however, despite the vast acreage of cereal crops grown, it is still a
relatively new construction material.
Proponents of straw bale building claim that it can be used to build an attractive and
healthy home whilst significantly reducing the energy required for both construction
and heating. At a time when climate change and energy supply issues are demanding
huge reductions in energy consumption and carbon emissions, could building with
straw be part of the solution?
This research centres around a small holiday cottage built in 2006/07 with straw bale
walls. Construction notes facilitate a brief review of the energy embodied in the
cottage, focusing mainly on the straw walls. A theoretical analysis of the thermal
performance of buildings follows in an attempt to understand how and why a straw
bale home may reduce heating demand. This analysis is supported by data and
observations from the cottage during 2007/08.
The straw bale home appears to perform well on all levels; construction did have low
embodied energy (although there was still room for improvement) and heating
demand was low (although other elements of the building affect the result).
However, it becomes clear that a great deal of further research is required before the
thermal performance of any type of complete wall is fully understood. This will be
essential if tighter regulations are to effectively reduce or eliminate the need to use
valuable energy for space heating whilst maintaining high standards of thermal
comfort.
3
Carol Atkinson – MSc Architecture: AEES
January 2008
Acknowledgements
•
•
•
•
•
•
•
Mike Thompson – for running such an inspirational course (and for
welcoming mature students!)
Judith Thornton – for support and guidance as course tutor
Paul Teather – for patience, help and support as thesis tutor
Barbara Jones & Bee Rowan – for bringing straw bale building to the UK,
developing it so professionally and promoting it so passionately
Anne & Helen – for allowing me to hang data loggers in their lovely homes
Julie & Tim, Lisa & Adi, Jill & Ian and numerous others – for help with
building the Straw Bale Cabin
Ali & Jill - for proof reading
And lastly, but most importantly
• Richard, Sam & Joe – for all your help and support - thank you!
4
Carol Atkinson – MSc Architecture: AEES
January 2008
Contents
1.
1.1
1.2
1.3
2.
2.1
2.2
2.3
2.4
2.5
3.
3.1
3.2
3.3
3.4
4.
4.1
4.2
5.
5.1
5.2
6.
6.1
6.2
6.3
7.
A
B
C
D
E
F
G
H
I
8.
9.
10.
Abstract
Acknowledgements
Contents
Abbreviations
List of figures
Introduction
The big issues
Building issues
This research
Construction
What is embodied energy?
How much energy?
Building the Straw Bale Cabin
Embodied energy in the Straw Bale Cabin
Carbon
Thermal Insulation
Thermal conductivity
U-value
U-value limitations
Heat loss from a building
Thermal Capacity
Thermal mass
Thickness
Air Flow
Air tightness
Air quality
Further data
Autumn/winter
Spring
Summer
Conclusion
Appendices
Basic theory of heat transfer and thermal comfort
Elevations of the Straw Bale Cabin
Embodied energy calculations for brick and block walls
Data loggers, calibration and positioning
Details of the static caravan
Spring heating study (including details of Eco Lodge)
Winter heating study
Summer heating study
Imperial to metric conversion
References
Bibliography
Glossary of terms
3
4
5
6
7
10
10
11
13
15
15
16
16
23
28
30
30
34
38
41
46
46
51
54
54
56
62
62
66
66
72
75
78
80
81
84
85
88
91
93
94
97
98
5
Carol Atkinson – MSc Architecture: AEES
January 2008
Abbreviations
m
mm
CO2
kWh
kg
OSB
AECB
BRE
IPCC
Metre
Millimetre
Carbon dioxide
Kilowatt hour
Kilograms
Oriented strand board
Association of Environment Conscious Builders
Building Research Establishment
Intergovernmental Panel on Climate Change
6
Carol Atkinson – MSc Architecture: AEES
January 2008
List of figures
1.1
1.2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10
2.11
2.12
2.13
2.14
2.15
2.16
2.17
2.18
2.19
2.20
2.21
2.22
2.23
2.24
2.25
2.26
2.27
2.28
2.29
2.30
2.31
2.32
2.33
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
3.13
3.14
3.15
3.16
3.17
The big issues and their impact on the built environment.
The Straw Bale Cabin, East Yorkshire.
Approximate primary embodied energy with author’s assessment of energy rating.
The completed Straw Bale Cabin at Village Farm.
Floor plan of the Straw Bale Cabin.
Steel chassis arriving on site.
Floor joists, base and roof plates.
Half bale being lowered onto base spike between full bale and door fixing point.
Bale plan from north elevation of the Straw Bale Cabin.
Hazel staple used across corners.
Hazel rods to insert into the bales.
Elevation and plan view of door fixing post notched into straw bale.
Completed walls being trimmed.
Strapping used to secure roof plate.
July 2007 – cedar shingle roof completed.
First coat of lime render being worked into the straw.
Second coat of lime completed and scratched.
Final coat of lime render.
Ochre lime wash applied.
First coat of clay worked into the straw at the side of the window, with the hessian visible
The same window reveal after the second coat of clay plaster.
A cross section through the completed straw bale wall (not to scale).
Breathable membrane, rafters and counter battens in the roof.
The first coat of clay being rubbed into the wood fibre boards.
Sheep’s wool insulation fitted between the floor joists.
Cork over the insulated joists onto which the floor boards were laid.
The lounge area of the main room.
The kitchen area of the main room.
Materials used to build the outer shell of the Straw Bale Cabin.
Pie chart showing the energy rating of materials used to build the Straw Bale Cabin.
Figures used in pie chart (Fig 2.28)
Energy embodied in the straw walls of the Straw Bale Cabin.
Graph showing energy embodied in the walls of the Straw Bale Cabin and other walls.
Table comparing roofing materials for the Straw Bale Cabin.
Table comparing roof insulation for the Straw Bale Cabin.
Graphs reproduced from Boyle, 2004 pages 43 and 44.
A summary of the main thermal conductivity test results carried out on straw bales.
Bales laid flat.
Bales laid on edge.
Simplified diagram to illustrate how heat may be conducted more easily along stems
A straw bale laid flat and viewed from the cut side.
The same straw bale laid flat and viewed from the opposite, folded side.
The same straw bale laid flat and split open to be viewed from the centre of the bale.
Thermal conductivity of straw and other insulation materials
U-value calculation for the walls of the Straw Bale Cabin.
U-value calculation for a straw bale wall with internal clay plaster and external lime
render amended for the plaster/straw bond.
Calculation of the average u-value for the walls of the Straw Bale Cabin.
U-values achieved for elements of the Straw Bale Cabin compared to a range of building
standards.
Examples of insulation analysed by installation method.
Internal surface area of the Straw Bale Cabin.
Heat loss calculation for the Straw Bale Cabin.
Diagram to show how the heat loss calculation disregards the corners of the building.
7
Carol Atkinson – MSc Architecture: AEES
3.18
3.19
3.20
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
A.1
B.1
B.2
C.1
C.2
D.1
D.2
D.3
D.4
E.1
F.1
F.2
F.3
F.4
F.5
January 2008
Temperature at the Cabin from 5th – 7th January 2008
Temperature at the Cabin from 7th – 9th January 2008
Area analysis of the fabric of the Straw Bale Cabin and heat loss through it.
Specific heat capacity of building materials.
Volume specific heat capacity of building materials.
A simplified diagram to highlight the thermal capacity of heavyweight materials.
Reproduced from Thermal Mass for housing: Concrete Solutions for the Changing
Climate (Concrete Centre, 2006).
Temperature inside the Straw Bale Cabin and outside the Cabin on 8th/9th August 2007.
Thermal response in lightweight and heavyweight buildings (from McMullan, 2002).
Inside/outside temperatures at the Cabin during five unoccupied & unheated days in 2007.
Indoor and outdoor temperatures recorded at the Straw Bale Cabin in 2007.
Temperature outside the Cabin and inside a nearby static caravan on 8th/9th August 2007
Temperature inside and outside the Cabin on 14th/15th January 2008.
Calibrated fan mounted in the doorway of the Straw Bale Cabin.
Relative humidity recorded at the Cabin 21st September to 21st December 2007.
Mean average temperature and relative humidity from the 3 month data recorded at the
Cabin 21st September to 21st December 2007.
Analysis of data from logger inside the Cabin from 21st September 2007 to 21st December.
Analysis of data from logger outside the Cabin - 21st September 2007 to 21st December.
Relative humidity and temperature recorded in the Cabin - 18th November 2007.
Relative humidity and temperature recorded in the Cabin - 18 November 2007.
Temperature and relative humidity readings from inside the unoccupied Cabin, outside the
Cabin and inside an unoccupied static caravan on 15th, 16th and 17th October 2007.
Comparison of relative humidity readings from inside the unoccupied Cabin, outside the
Cabin and inside an unoccupied static caravan on 15th, 16th and 17th October 2007.
Position of data loggers on 13th to 18th October 2007.
Notes from the weather diary on 13th to 18th October 2007.
Temperatures recorded at five minute intervals on 13th – 18th October 2007.
Diagrammatic representation of the heat loss from the Straw Bale Cabin on 16th – 17th
October 2007.
Temperatures recorded at five minute intervals from 3:15pm on 18th October to 6:00pm on
the 19th October 2007.
Explanation of the temperature changes graphed in Figure 6.5.
Temperature readings at the Straw Bale Cabin and static caravan on 7th – 10th August
2007.
Highest lowest and average temperatures recorded on 7th – 10th August 2007.
Cooling strategies in a variety of buildings.
Heat transfer in the UK.
East and west elevations of the Straw Bale Cabin.
North and south elevations of the Straw Bale Cabin
Embodied energy in the wall of the Straw Bale Cabin if it had been made of bricks and
blocks with foamed glass insulation
Embodied energy in the wall of the Straw Bale Cabin if it had been made of bricks and
blocks with mineral fibre insulation
Lascar data logger in its usual position suspended from the beam in the lounge/kitchen
area of the Straw Bale Cabin.
Temperature recordings outside the Straw Bale Cabin on 21st – 23rd April 2007
Temperature recordings outside the Straw Bale Cabin on 7th – 9th August 2007.
Lascar data logger in its usual position suspended under the porch along the south facade
of the Straw Bale Cabin.
North elevation of the Willerby Lyndhurst static caravan
Eco Lodge (Oak Cabin) at Flaxton, near York.
The approximate dimensions of the holiday homes.
Dates, times and temperatures at the short heating studies in March and April 2007.
The first 515 minutes of each study
The first 515 minutes from the study of the Straw Bale Cabin and Eco Lodge adjusted to
the same start temperature.
8
Carol Atkinson – MSc Architecture: AEES
G.1
G.2
G.3
G.4
G.5
H.1
H.2
January 2008
Loggers in position
Window insulated with a duvet to minimise heat loss
Weather and external temperatures on 19th – 22nd November 2007.
Temperatures recorded at five minute intervals from 2pm on 19th November to 7am on
22nd November 2007.
Diagrammatic representation of the heat loss from the Straw Bale Cabin on 21st – 23rd
November 2007.
Inside temperatures (oC) at the static caravan and outside temperatures under the south
facing porch at the Straw Bale Cabin on 20th August to 1st September 2007.
Inside temperatures (oC) at the Straw Bale Cabin and outside temperatures under the south
facing porch at the Cabin on 20th to 26th August 2007.
* denotes term further explained in glossary on page 98.
9
Carol Atkinson – MSc Architecture: AEES
January 2008
1. Introduction
1.1 The big issues!
The majority of scientists now agree that the earth’s climate is changing and that
carbon emissions must be reduced to limit the most harmful effects of that change.
At the same time, supplies of oil are about to peak. Oil and other fossil fuels have
powered the developed nations over the last 150 years, releasing the greenhouse
gases* that are now thought to be the cause of climate change.
Together, these issues will drive the quest for alternative energy sources and greater
energy efficiency.
Climate Change
The UK climate is mild; warm summers, cool winters and plentiful rainfall
throughout the year. The weather is significantly warmer than other countries of
similar latitude due to the Gulf Stream*.
The Intergovernmental Panel on Climate Change* (IPCC) issued its Fourth
Assessment Report on 17 November 2007 1 . It states that the warming of the climate
system is unequivocal and that human actions are “very likely” the cause. It predicts
global temperature rises of between 1.1oC and 6.4oC over the 21st century with more
frequent extreme weather conditions such as heat waves, heavy rain and droughts.
Other scientists, however, predict that, due to global warming induced changes to the
Gulf Stream, the UK will experience an initial warming followed by much colder
temperatures (Joyce, 2007).
One way or another, weather patterns in the UK are likely to change during the
lifetime of our children.
The IPCC also states that the likely amount of temperature rise could vary greatly
depending on the fossil intensity of human activity over the next century. Therefore,
greenhouse gas emissions must be drastically reduced if we are to limit the extent of
climate change.
With this in mind, the Government’s Climate Change Bill introduced to Parliament
in November 2007 2 calls for a 60% cut in the UK’s carbon emissions by 2050.
Peak oil*
It is extremely difficult to accurately assess world oil reserves, to forecast their
extraction rate and to predict future demand. Many experts claim that oil production
is peaking now, whilst more optimistic models point to a 2030 peak.
In the UK, oil production has been in decline since 2002 (DTI, 2006 page 62) and in
2005, gas consumption exceeded production (DTI, 2006 page 12).
1
2
www.ipcc.ch
(http://www.defra.gov.uk/environment/climatechange/uk/legislation/index.htm accessed 29/30/07)
10
Carol Atkinson – MSc Architecture: AEES
January 2008
Market forces will inevitably result in price rises as world supplies dwindle and the
distribution of fossil fuels over huge distances from point of extraction to point of
use will have many practical, financial, political and environmental implications.
It seems reasonable to conclude that oil will be in short supply within 20 years and
all energy prices will rise significantly.
1.2 Building issues
The majority of UK citizens spend their time in one building or another, whether at
home or at work. Therefore, it is not surprising that a high percentage of energy is
consumed in buildings.
How can we provide the shelter we need in a changing climate whilst reducing both
energy demand and carbon emissions?
Energy in use
In 2005, over 27% of final energy consumption in the UK was accounted for by the
domestic sector (DTI, 2006 page 13). It is difficult to establish exactly how this
energy is used in homes but the DTI 1995 estimate was that 57% of domestic
delivered energy was used for space heating (Boyle et al, 2003 page 113).
It could be argued that if the UK climate warms appreciably, the demand for space
heating would reduce without the need for additional measures. However,
uncertainty over the effect of climate change, a need to quickly reduce emissions by
at least 60%, a rising population and an increasing number of households mean that
steps must be taken to save energy.
The simplest way to save energy is to use less; adapt to lower indoor temperatures,
wear more clothes or restrict heating to occupied spaces for example. Savings could
also be made by installing more efficient heating systems, improving existing
buildings or replacing them with new buildings that require less energy for heating.
Energy in construction
Today, there are 25.8 million homes in the UK. However, the Government expects 7
million new homes to be built by 2050 (Boardman, 2007 pages 57 and 38). As 10%
of total energy use in the UK is embodied in construction materials (Harris, 2005)
significant amounts of energy could be used and carbon emissions generated by the
building of these homes unless materials and methods requiring less energy can be
employed.
Timber is currently the most widely used renewable building material but it takes
over 50 years to grow. A large enough supply, therefore, can not be grown in the
time available. Straw, however, grows in a much shorter time frame…………
Building with straw
Straw is the dead, dried stems of cereals. It is left over after the grain is removed in
the harvesting process. Wheat, barley and oats are the most commonly grown cereal
crops in the UK. Their straw is currently used for animal feed and bedding or
incorporated back into the soil as a fertiliser. A certain amount would still be
required for these purposes.
11
Carol Atkinson – MSc Architecture: AEES
January 2008
However, in 2006, nearly 3 million hectares of cereal crops were grown in the UK. 3
Each hectare can yield approximately four to six tonnes of straw. As an average
home could be built with less than 10 tonnes of straw, that is a vast quantity of
potential building material!
Straw bale buildings were first constructed in the USA in the late 1800’s, when
baling machines were invented (Jones, 2002 page 13). As there were no trees, straw
was the only material available to the European settlers on the Nebraska plains. The
oldest bale house still standing there today was built in 1903 (King, 2006 page xxi)
and the oldest European straw bale house was built in France in 1921 (Steen, 2000
page iv).
In the 1940’s a combination of war and the popularity of cement led to the virtual
extinction of straw bale building – until a revival by US green building pioneers in
the 1970’s. The first straw bale building in the UK was built in 1994 and there are
now over fifty of them. 4
It is claimed that building with straw can
•
•
•
save energy in construction because of its low level of embodied energy,
save energy in heating and cooling buildings because of its insulating
properties, and it can
remove carbon dioxide from the atmosphere as it grows, sequestering it in the
fabric of the building itself.
Figure 1.1 The big issues and their impact on the built environment
Can straw bales be part of the building solution?
3
http://statistics.defra.gov.uk/esg/quick/agri.asp
Amazon Nails alone have been involved with 50 straw bale construction projects
www.srawbalefutures.org.uk accessed 14/1/08
4
12
Carol Atkinson – MSc Architecture: AEES
January 2008
1.3 This research
In the long term straw could be used for building new dwellings and for retroinsulating existing homes – as bales or straw products. However, as these are early
days for straw bale building in the UK, a greater understanding will be gained by
first observing and understanding its performance in new buildings.
Although this work focuses mainly on dwellings, many aspects could apply equally
to commercial or industrial buildings.
Methodology
Since straw bale building was first pioneered the basic building technique has
remained as straightforward as stacking the bales and plastering both sides (King,
2006 page xxvii).
There have been straw bale studies in the USA, Canada, Germany, Austria, Denmark
and the UK. However, techniques are continually developing as the knowledge base
grows and some of this research is not directly applicable to current best practice.
This thesis aims to bring together various strands of research into straw and other
building materials from an energy perspective.
The review of these studies will be made in light of observations, notes and records
from the construction of The Straw Bale Cabin between June 2006 and March 2007
and from temperature and relative humidity data captured there between February
2007 and January 2008.
The theoretical analysis is based on steady state values and comparison of single
factors. However, actual measurements in the Cabin include the dynamic
performance of all factors.
The Straw Bale Cabin
The Straw Bale Cabin (Fig 1.2) is a small holiday cottage situated at Village Farm,
Brind, near Howden, East Yorkshire. Its construction is outlined in Chapter 2.
Figure 1.2 The Straw Bale Cabin, East Yorkshire
Chapter 2 also considers the energy embodied in building materials generally and
reviews the energy required to build the Cabin and more particularly the straw bale
walls.
13
Carol Atkinson – MSc Architecture: AEES
January 2008
With energy saving in mind, the thermal response of the Cabin during 2007 is the
main focus of this thesis. (Appendix A provides the basic theory of heat transfer and
thermal comfort).
The heating (and cooling) of buildings depends on three key factors
•
•
•
Thermal insulation (to keep heat in (or out in the summer))
Thermal capacity (to store heat and moderate temperature swings), and
controlled air flow (to reduce heat loss on air change)
Chapters 3, 4 and 5 consider these factors in turn. Where possible, reference is made
to the Straw Bale Cabin and actual data is used to illustrate relevant points. Further
data from the Cabin is discussed in Chapter 6 and on occasion, comparative data
from other buildings is used to widen the discussion.
The Straw Bale Cabin is technically a caravan – narrow enough to be loaded onto a
lorry and transported away should the authorities demand it. Several comparisons,
therefore, are made to a nearby, similar sized, conventional static caravan, also used
as holiday accommodation. Appendix E gives provides further details on this
caravan.
Chapter 7 concludes the energy assessment of straw bale building and summarises
the limitations of this study and the further research necessary.
14
Carol Atkinson – MSc Architecture: AEES
January 2008
2. Construction
2.1 What is embodied energy?
The embodied energy of a building material is the total energy required to produce it.
Depending on the material, that could include the energy needed for growing,
recycling, extracting, processing and transport. Materials processed at high
temperatures such as metals and plastics have the highest embodied energy.
Material
Copper
Steel (ore)
Aluminium
Plastic
Steel (recycled)
Glass
Fibre cement slates
Clay tiles
Bricks
Plastic insulation
Plaster board
Imported softwood
Concrete
Sand cement render
Mineral fibre insulation
UK green oak
UK softwood (air dried)
Sheep's wool
kWh/m3
133,000
80,000
55,868
47,000
29,669
23,000
12,783
1,520
1,462
1,125
900
754
600
400
230
220
110
30
Energy rating
extremely high
extremely high
extremely high
extremely high
very high
very high
very high
High
High
High
Medium
Medium
Medium
Medium
Low
Low
Low
very low
Figure 2.1 Approximate primary embodied energy (GBB, 2006 page 277) with author’s assessment
of energy rating.
Ideally, the energy required for delivery to site and installation should also be
included in the total embodied energy calculation. Some calculations even go as far
as to include energy for repair and maintenance over the life of the material and
energy for final demolition and disposal.
In practice the embodied energy calculation is effected by many generalisations and
estimates; different companies using different raw materials, process methods and
fuels, different transport modes, distances and fuels, commercial vested interests and
secrecy. The figures also change with innovation and increasing efficiency. The UK
cement industry, for example, recently claimed to have reduced its carbon footprint
by 29% 5 and in a press release dated November 2007 a local tile company claimed
to be using 100% renewable energy. 6
5
6
www.newbuilder.co.uk/news/NewsFullStory.asp?ID=2205
www.sandtoft.com
15
Carol Atkinson – MSc Architecture: AEES
January 2008
As yet there is no standardised or widely accepted method of calculation (Borer,
2005 page 95). However, the figures do provide a useful guide especially where there
are large differences between materials. They highlight the potential high energy cost
locked up in buildings.
2.2 How much energy?
The embodied energy in the average house is often quoted at 100,000 kWh. If the
annual energy demand of that house is 20,000 kWh, the energy embodied in
construction soon becomes insignificant. However, if energy in use could be reduced
to 5,000 kWh per annum in a more efficient house, the embodied energy then
becomes a major part of the building’s lifetime energy consumption (Borer, 2005
page 97). If 50,000 kWh could be saved on the construction of this more efficient
home it would provide the energy to run the building for an additional 10 years,
significantly reducing carbon dioxide (CO2) emissions in the process.
Construction has an immediate effect on emissions. The 7 million new homes
expected by 2050 will cause a large spike in CO2 emissions if levels of embodied
energy remain as currently.
It is beyond the scope of this thesis to calculate the total energy embodied in the
Straw Bale Cabin. However, using the energy ratings established in section 2.1 as a
guide, the basic fabric of the building is assessed.
First, it is essential to know how the Cabin was constructed.
2.3 Building the Straw Bale Cabin
The Straw Bale Cabin (Fig 2.2) is a small holiday cottage with one bedroom. Its
external dimensions are approximately 4m by 10m and its internal dimensions are
approximately 9m by 3m. The elevations are reproduced in Appendix B.
Figure 2.2 The completed Straw Bale Cabin at Village Farm.
Figure 2.3 Floor plan of the Straw Bale Cabin (4m x 10m).
16
Carol Atkinson – MSc Architecture: AEES
January 2008
The Cabin was built using the load bearing method* as taught by Yorkshire based
straw bale consultancy Amazon Nails. 7 The basic steps are described below.
Walls
During dry weather in August 2005 wheat straw was baled on a farm 2 miles away
from the construction site. The bales were approximately 1m long, 0.475m wide and
0.4m high. They were baled as tightly as possible, secured with two polypropylene
strings and stored in a shed at Village Farm until June 2006 when building
commenced.
Due to planning difficulties, the Cabin had to be a mobile, temporary structure. It is
constructed on a steel caravan chassis (Fig 2.4) rather than on permanent
foundations. Floor joists were bolted to the chassis and the base plate for the walls
was screwed on to the joists (Fig 2.5). The base plate was insulated with sheep’s
wool from a local flock.
Figure 2.4 Steel chassis arriving on site.
Figure 2.5 Floor joists, base and roof plates.
The bales were stacked flat*. The first course was secured to the base plate by means
of two hazel spikes (0.35m) per bale (Fig 2.6). The cut side* of the bale was placed
to the outside of the building on the first, third and fifth courses. On the second and
fourth courses the folded side of the bale was placed to the outside. As the bales are
denser on the cut side, alternating them in this way balances the density of the wall.
It is also much easier to work the plaster into the cut side - alternating courses
therefore, also balances plaster adhesion.
The bale positions were staggered so that full bales were centred over the joint of two
bales on the course below (Fig 2.7). Each bale was trimmed to make the ends as
square as possible, minimising gaps between them. Half bales were made with a
baling needle* and any remaining gaps were stuffed with loose straw. The completed
building used 96 full bales and 45 half bales of straw (approximately 2.7 tonnes).
7
www.amazonails.co.uk
17
Carol Atkinson – MSc Architecture: AEES
Figure 2.6 Half bale being lowered onto base
spike between full bale and door fixing point.
January 2008
Figure 2.7 Bale plan from north elevation
of the Straw Bale Cabin.
Hazel was used to stabilise the walls. Each corner was pinned with a hazel staple
(Fig 2.8) and two hazel rods (1m) (Fig 2.9) were inserted into the centre of each bale
on the third and fifth courses. Timber posts (100mm square) were used to provide
fixing points for windows and doors. They were built into the centre of the walls on
either side of each opening and the ends of the bales were carefully notched between
the strings so that they wrapped three sides of the posts (Fig 2.10).
Figure 2.8 Hazel staple used across corners.
Figure 2.9 Hazel rods to insert into the bales.
Figure 2.10 Elevation and plan view of door fixing post notched into straw bale.
On the fifth day, the roof plate was lowered onto the straw bales and insulated with
loose straw. The walls were trimmed (Fig 2.11) to provide a firm base for plastering
and strapped (Fig 2.12) to secure the roof plate to the base plate.
18
Carol Atkinson – MSc Architecture: AEES
Figure 2.11 Completed walls being trimmed.
January 2008
Figure 2.12 Strapping used to secure roof plate.
The roof of Cornish western red cedar shingles was added as soon as possible to keep
the building dry (Fig 2.13). The straps were tightened as the bales compressed under
the weight of the roof. As the bales were tightly baled (density approximately
120kgm3), the total compression of the 2m high wall (5 courses of 0.4m bales) after 5
weeks was only 20mm. The gable ends were filled with customised triangular straw
bales after the roof was in position.
Figure 2.13 July 2007 – cedar shingle roof completed.
To ensure the lime was properly cured before the first frosts of winter, rendering the
external walls began in early August. The first coat was worked directly into the
straw by hand (Fig 2.14). A thicker, levelling, second coat was applied by hand a
week later (Fig 2.15). Both coats were a 3:1 mix of sand and lime putty with added
chopped hemp supplied ready mixed by Womersleys. 8 The lime plaster was
moistened with the misting nozzle of the hosepipe several times a day. After 10 days
a 3mm top coat (finer sand and no added hemp) was applied (Fig 2.16) and finished
with 3 coats of good quality lime wash (Fig 2.17). To facilitate slow curing, the
Cabin was shrouded in tarpaulins and large bales to shelter it from wind, rain and sun
until October.
The lime coat extended over the timber base and roof plates to completely cover the
external facade. To prevent cracks developing, Hessian* strips were used in the first
coat to bridge the timber and straw. It was also used over the plastic strapping and
the damp proof membranes installed under the window sills.
8
www.womersleys.co.uk
19
Carol Atkinson – MSc Architecture: AEES
January 2008
Figure 2.14 First coat of lime render being . Figure 2.15 Second coat of lime completed
worked into the straw.
and scratched.
Figure 2.16 Final coat of lime render.
Figure 2.17 Ochre lime wash applied.
It was late September before clay plastering began on the interior walls. The first
coat was simply milled clay from the local tile works. After soaking in water for 30
minutes the excess water was poured off and the clay mixed up to a thick creamy
consistency. It was worked by hand directly into the straw taking care to plug any
gaps with a straw/clay mix (Fig 2.18).
The second coat of clay plaster included chopped hemp, supplied ready mixed by
Womersleys (Fig 2.19). It was applied by hand in a thicker layer with Hessian once
again bridging all timber/straw joins. Hessian was also used to smooth and
strengthen each window reveal. A 3mm fine top coat was applied when the second
coat was dry. The final floor had not been laid at this point so the temporary floor
boards could be pulled back after each coat to increase airflow in the building and
speed drying. The walls were primed 9 and finished with 3 coats of clay paint 10 .
Figure 2.18 First coat of clay worked into the Figure 2.19 The same window reveal after the
straw at the side of the window, with the
second coat of clay plaster. The small pieces of
hessian visible on the left hand side.
chopped hemp in the plaster are visible.
9
http://www.womersleys.co.uk/pdfs/primer.pdf accessed on 10/1/08
http://www.womersleys.co.uk/pdfs/economicwhite.pdf accessed on 10.1/08
10
20
Carol Atkinson – MSc Architecture: AEES
January 2008
Figure 2.20 A cross section through the completed straw bale wall (not to scale).
Windows and doors
The windows are standard timber windows from the local builder’s merchant fitted
with double glazed units with a 16mm argon filled cavity and low-e* coating.
The doors are also fully glazed; the main central door provides borrowed light to the
bathroom and the French doors to the left hand side of the south elevation open out
onto the decking to provide outdoor living space for the occupants in summer. All
doors are double glazed with a 16mm argon filled cavity and low-e coating.
Roof
The roof consists of 50mm x 150mm softwood rafters attached to the roof plate, with
the weight spread evenly over the full width of the bales. A breathable membrane*
was placed on the rafters (Fig 2.21) followed by timber laths and cedar shingles.
The underside of the roof was insulated with 200mm of sheep’s wool insulation 11 ,
the last 75mm of which was fitted between counter battens perpendicular to the first
125mm. The insulation was held in place by wood fibre boards plastered with three
coats of clay plaster (Fig 2.22). The second coat incorporated a plastic mesh to
prevent cracks appearing at the joints of the boards.
Figure 2.21 Breathable membrane, rafters
and counter battens in the roof.
11
Figure 2.22 The first coat of clay being rubbed
into the wood fibre boards.
.
www.secondnature.co.uk
21
Carol Atkinson – MSc Architecture: AEES
January 2008
Floor
OSB* boards were fastened underneath the floor joists and the space filled with
150mm sheep’s wool insulation (Fig 2.23). A 3mm layer of cork was laid over the
joists (Fig 2.24) before the final floor boards of oak (in the lounge/kitchen area) and
pine (in the bedroom). All floorboards were finished with primer and 2 coats of hard
resin oil. In the entrance and bathroom exterior grade plywood was used and finished
with marmoleum*.
Figure 2.23 Sheep’s wool insulation fitted
between the floor joists.
Figure 2.24 Cork over the insulated joists
onto which the floor boards were laid.
The Cabin was complete for the first visitors to arrive in March 2007 (Fig 2.25 and
Fig 2.26).
Figure 2.25 The lounge area of the main room.
Figure 2.26 The kitchen area of the main room.
22
Carol Atkinson – MSc Architecture: AEES
January 2008
2.4 Embodied energy in the Straw Bale Cabin
The table below (Fig 2.27) lists the materials used to build the outer shell of the
Straw Bale Cabin. It includes the source of the material where this could be
ascertained and the approximate volume. An energy rating has been allocated to each
material in accordance with the banding outlined in section 2.1 earlier.
Foundations
Steel chassis
Supporting timber
Rails around base
Walls
Wheat straw
Hazel
Hessian
Lime plaster (with chopped hemp)
Lime top coat
Lime wash
Milled clay
Clay plaster undercoat (with hemp)
Clay plaster top coat
Timber
OSB
Baler twine
Plastic straps
Nails & screws
Windows & doors
Softwood windows
Double glazed units
Kiln dried English oak
Redwood doors with hardwood frames
Hinges, locks & handles
Roof
Cedar shingles
Stainless steel nails
Lead flashing
Velux roof light
Tyvec breathable membrane
Roof timber
Celenit wood fibre board
Alkali resistant plastic mesh
Thermafleece
Clay plaster undercoat (with hemp)
Clay plaster top coat
Wooden gutters
Galvanised down pipe
Floor
External plywood
Timber joists
OSB
Thermafleece
Cork underlay
Oak floorboards
Pine floorboards
Nails & screws
Source
Volume
(m3)
Embodied energy
(kWh)
UK
Scotland
Scandinavia
0.07
0.45
0.55
Extremely high
Low
Medium
Local
Cumbria
Far East
Derbyshire
Derbyshire
Derbyshire
Local
West Yorkshire
West Yorkshire
Scotland
Far East
Europe
?
?
28.20
0.03
1.35
0.18
0.06
0.23
0.93
0.14
0.86
0.51
-
Very low
Very low
Medium
Medium
Medium
Medium
Very low
Very low
Very low
Low
High
Medium
Extremely high
Extremely high
Europe
UK
UK
Canada
?
0.17
0.04
0.04
0.19
-
Medium
Very high
Medium
Medium
Extremely high
Cornwall
?
?
?
?
Scotland
Italy
Italy
UK
West Yorkshire
West Yorkshire
Canada
UK
1.30
0.01
0.04
0.80
0.49
6.40
0.32
0.10
0.21
-
Very low
Extremely high
Extremely high
Very high
High
Low
Medium
High
Very low
Very low
Very low
Medium
Extremely high
Far East
Scotland
Far East
UK
Portugal
Europe
Scandinavia
?
0.12
0.57
0.44
5.43
0.06
0.24
0.18
-
High
Low
High
Very low
Medium
Medium
Medium
Extremely high
Figure 2.27 Materials used to build the outer shell of the Straw Bale Cabin.
23
Carol Atkinson – MSc Architecture: AEES
January 2008
%
m3
0.07
0%
0.05
0%
1.11
2%
3.72
7%
2.68
6%
43.08
85%
50.71
Figure 2.28 Pie chart showing the energy rating Figure 2.29 Figures used in pie chart (Fig 2.28)
of materials used to build the Straw Bale Cabin.
Energy Rating
extremely high
very high
high
medium
low
very low
Figure 2.28 and Figure 2.29 summarise the energy rating of the materials used to
build the Cabin; 85% of the Cabin shell consists of materials with very low
embodied energy (mainly straw, sheep’s wool and clay plaster). 6% of materials
have low embodied energy (mainly UK softwood) and 7% have medium levels of
embodied energy (mainly lime render and imported timber).
The materials with extremely high embodied energy such as metal nails and screws
and plastic straps are very small by volume – less than 1%. They are functional and
durable and facilitate speedy construction. Glass has very high embodied energy but
its use saves energy by allowing daylight and solar heat gain into the building. It also
provides an outside view which is considered beneficial to the emotional well being
of occupants. Materials with high embodied energy (2%) include engineered sheet
materials such as plywood and OSB and the breathable membrane on the roof.
The Cabin is a small building with a simple form. The exercise has been further
simplified by the omission of
• internal walls (which are also made from materials with low embodied
energy)
• electrical and plumbing fittings (which are similar to those used in other
buildings)
• delivery distances and mileage by contractors driving to site (local was
specified wherever possible)
• equipment, tools and consumables
• waste (which was minimal on this project)
Straw walls
Considering the straw walls in more detail, the total embodied energy has been
calculated as approximately 2,360 kWh, an average of 73 kWh per m3 of wall (Fig
2.30).
The energy embodied in straw is very low. If not used to build the Cabin, the same
amount of energy would have been used to bale this straw and lead it away for
animal bedding. If it had been left on the field as a soil conditioner it would have had
to be chopped before it could be incorporated and the energy required for chopping
would have been similar to that required for baling and leading away.
As the straw was sourced locally (2 miles away), the transport energy is also
minimal.
24
Carol Atkinson – MSc Architecture: AEES
January 2008
In the calculation below, 14 MJ/m3 (less than 4 kWh/m3) is assumed to be the energy
embodied in the straw bales (Minke, 2004, page 10).
If the crop had been grown specifically for the straw rather than for the grain (when
straw is the by-product) the embodied energy of the straw would be greater as it
would have to include planting, crop husbandry and harvesting.
Wheat straw
Hazel
Lime renders
Milled clay basecoat
Clay plasters
Timber (UK softwood - air
dried)
OSB
Total
Average kWh/m3
kWh/m3
4
5
350
30
55
m
28.20
0.03
1.59
0.23
1.07
Embodied energy
kWh
112.80
0.15
556.50
6.90
58.85
110
3000
0.86
0.51
32.49
94.60
1,530.00
2,359.80
3
73
Figure 2.30 Energy embodied in the straw walls of the Straw Bale Cabin.
The energy embodied in the other materials has been estimated as follows;
Hazel – very low as simply sawn by hand and air dried (but transported from
Cumbria in this case so assumed to be slightly higher than the local straw)
Lime render – lime is processed at high temperatures but lower than those used to
produce cement. It is mixed with sand in the ratio of 3:1. Sand cement render has an
embodied energy of 400 kWh/m3 (GBB, 2006 V1 page 277).
Milled clay - from the local tile works where it was dug from the ground and
chopped into small pieces
Clay plaster – from a Yorkshire source with chopped hemp added
Timber - (GBB, 2006 V1 page 277)
OSB – no figure could be found. However, a website which compares the
environmental credentials of building materials stated that the embodied energy was
“high”. 12 Plywood, another engineered sheet material, has an embodied energy of
approximately 5,000 kWh/m3 (Borer, 2005 page 96). Energy is used to collect and
prepare the timber, manufacture the glue and bond the sheet together using heat and
pressure. As OSB is made from waste timber and forestry thinnings, it has been
assumed to have lower embodied energy than plywood.
The high embodied energy associated with the relatively small volume of OSB used
to construct the base and roof plates in the walls of the Cabin completely dominates
the calculation in Figure 2.30 because the other materials have much lower embodied
energy. OSB accounts for less than 2% of the walls volume but 65% of its embodied
energy. It may be possible to replace OSB with a less energy intensive material in
future buildings.
12
(www.greenspec.co.uk)
25
Carol Atkinson – MSc Architecture: AEES
January 2008
For simplification, the calculation does not include the energy expended on tools
(drills, saws, mixers etc), consumables (nails, screws, baler twine, hessian) labourers
and their travel to site, delivery of materials and water usage.
Other walls
If the Cabin walls had been built with the same dimensions but of bricks and blocks
and fully filled foamed glass insulation instead of straw, the embodied energy of the
walls would have been over 11 times higher. Using mineral fibre insulation the walls
would still embody over 5 times the energy of the straw wall (Fig 2.31).
30000
25000
kWh
20000
15000
10000
5000
0
plastered
straw
brick/block &
foamed glass
brick/block &
mineral fibre
Figure 2.31 Graph showing energy embodied in the walls of the Straw Bale Cabin and the energy that
would have been embodied if the walls had been made of bricks, blocks and conventional insulation
products.
The embodied energy calculations for the brick/block walls are given in Appendix C.
They are simplified in that the mortar joints and wall ties have been ignored and no
account has been taken of the substantial foundations that would be required for the
considerably heavier brick/block walls. Therefore, the figures in the graph above are
believed to under estimate the energy embodied in the brick/block walls.
As, in theory, the walls compared above have a similar level of thermal resistance
(see next Chapter), the energy needed for space heating over the life of the building
should be similar. If the straw walls had an expected life span of 50 years (very
conservative given the lifespan of the French house built in 1921), the two
brick/block walls would need to last 550 or 250 years to balance out the initial
energy consumption during construction (11 or 5 times that of the straw building).
The lime façade on the straw walls would require a coat of lime wash every 5 years
or so but the energy cost of this would not be significant. The brick façade should
require very little maintenance.
Other comparisons
If clay roof tiles had been used on the Straw Bale Cabin instead of cedar shingles (all
other roof details remaining the same) the embodied energy of the roof would
increase by over 1700 kWh (Fig 2.32).
26
Carol Atkinson – MSc Architecture: AEES
Roof cover
Cedar shingles
Clay tiles
Difference (kWh)
3
kWh/m
200
1520
January 2008
3
m
1.30
1.30
Embodied energy
kWh
260
1976
1716
Figure 2.32 Table comparing roofing materials for the Straw Bale Cabin. (Embodied energy figures
from GBB, 2006 V1 page 277).
However, the shingles may last only 15 years. To balance out the initial energy cost
the tiles would need to last 114 years (less if considering the energy needed for
several shingle replacements in this time frame). The clay tiles may be preferred for
their superior fire resistance, particularly in an urban setting.
The use of foamed glass insulation in the ceiling instead of sheep’s wool would
increase embodied energy by over 4600 kWh (Fig 2.33). It is 25 times more costly in
energy terms than sheep’s wool. Both materials provide similar levels of insulation
and longevity.
Roof insulation
Sheep’s wool
Foamed glass
Difference (kWh)
kWh/m3
30
751
m3
6.40
6.40
Embodied energy
kWh
192
4806
4614
Figure 2.33 Table comparing roof insulation for the Straw Bale Cabin. (Embodied energy figures
from GBB, 2006 V1 page 277).
A static caravan is very similar in size to the Straw Bale Cabin (see Appendix E). A
caravan’s walls are usually made of aluminium insulated with polystyrene or
polyurethane rigid foam all of which have high embodied energy. However, as the
walls of a caravan are very thin compared to the Cabin (less than one tenth the
width), the embodied energy of the caravan’s walls is probably only 5 times higher.
Added to this, though, the thermal performance of the caravan will be very poor (see
Chapter 3) and therefore energy needed for heating or cooling will also be higher.
Summary
Even when every detail of construction is known, as in the case of the Cabin, a
precise embodied energy calculation remains difficult. However, the work done
indicates that a home with straw bale walls could significantly reduce the energy
required for construction, particularly if renewable materials are used in all elements
of the fabric.
As the Straw Bale Cabin is only small (internal floor area 27m2) home, many
assumptions would be required to scale it up for direct comparison with the average
house described in section 2.2. However, if the walls could be 5 – 11 times less
energy intensive than the conventional alternatives and roof insulation could be 25
times less this suggests that a home built of largely unprocessed materials could
easily be built using under 10,000 kWh - a tenth of the energy currently embodied in
buildings.
27
Carol Atkinson – MSc Architecture: AEES
January 2008
2.5 Carbon
Emissions
Unless the embodied energy discussed above can be generated from renewable
sources (unlikely at present) then construction will inevitably result in carbon
emissions. The calculation of carbon emissions from the construction of homes is
complex due to wide variations in materials, size of dwelling and fuel assumptions.
Estimates vary from 30,000 kg of carbon dioxide (CO2) to 65,000 kg CO2 for every
new dwelling constructed.
The energy embodied in the walls of the Straw Bale Cabin was calculated above as
2,360 kWh, with the factory made OSB having the highest energy requirement. If we
assume that the factory was powered by a natural gas fuelled power station, the CO2
emissions from the wall would be 1,053 kg (2360 x 446g (Boyle, 2004 page 138)).
Storage
Straw and other renewable materials such as hemp and timber can be used to
sequester carbon. CO2 is absorbed from the atmosphere during photosynthesis* and
stored in the plant material for the life of the building.
However, the UK has only 10% forestry cover and produces only 15% of its current
annual timber requirement (FOE, 2002). New timber will take over 50 years to grow.
Straw and hemp can be grown every year in the UK and are dual purpose crops
(producing seed or fibre at the same time as building material).
As 1.36 kg of CO2 is stored in each kilogram of straw (Musset, 2004 page 20), the
Straw Bale Cabin has stored over 3,700 kg of CO2 in its walls (96 full bales plus 45
half bales weighing an average 23 kg per bale). After deducting the emissions
generated to build the complete wall, the net balance of CO2 locked up in the walls is
approximately 2,650 kg (over 80kg/m3).
Low embodied energy combined with carbon sequestration make straw bale walls
carbon negative. In addition, the lime render absorbs CO2 from the atmosphere as it
cures. CO2 was released during the lime production and more was emitted by the fuel
needed to drive the process but the net carbon cost of lime is significantly less than
that of cement.
A hemp/lime company claims that a small detached house (52m2 ground floor area)
with walls made of 300mm hempcrete* (49m3) will lock up 5,400 kg CO2 in
(110kg/m3). 13
As no details of the hemp calculation are provided, a direct comparison cannot be
made with the straw bale calculation. However, the straw walls should store more
CO2 than the hemp walls (rather than less as indicated above) because
•
•
the straw walls are 60% wider (so there is more plant material) and
the straw walls are mostly straw (where as the hemp is completely coated
with lime)
13
http://www.lhoist.co.uk/tradical/pdf/Tradical_Information_Pack.pdf
page 22 accessed on 14/1/08
28
Carol Atkinson – MSc Architecture: AEES
January 2008
Summary of the main points from Chapter 2
•
Embodied energy calculations are crude but they highlight the pronounced
difference between conventional and renewable construction materials in
energy terms.
•
Straw bale walls have very low embodied energy and carbon emissions.
Significant savings of energy and emissions could be made if they were more
widely used in construction.
•
Straw bale walls are carbon negative – they lock away carbon for the life of
the building.
29
Carol Atkinson – MSc Architecture: AEES
January 2008
3. Thermal Insulation
Even in a temperate climate such as the UK, energy for space heating in the colder
months accounts for over half of total domestic delivered energy. This could be
reduced if homes were better designed and constructed to retain heat.
Heat is lost from buildings through the fabric (walls, windows, doors, floor and roof)
and through air flow (controlled or otherwise). To oppose the transfer of heat by
conduction 14 , thermal insulation in the fabric of the building is required.
Figure 3.1 Graphs reproduced from (Boyle, 2004 pages 43 and 44). On the left, a typical 1970’s
poorly insulated house requires net space heating of 13,000 kWh per year. The better insulated house
on the right has a much lower net space heating demand of 4,000 kWh per year. (Note the different
scale for daily energy on the left hand axis of each graph.)
This chapter reviews existing research into straw, with reference where possible to
the Straw Bale Cabin and other insulation and construction materials. Air flow is
discussed later, in Chapter 5.
3.1 Thermal conductivity
Bales consist of straw and air. Straw includes hollow stems, leaves and chaff*
(Wihan, 2007 page 29). There could also be weeds and grains not removed by the
harvesting process. Air is encapsulated in the hollow stems and between the various
fibres in the bale.
Over the last 15 years a number of overseas research projects have considered the
thermal conductivity (W/mK) of straw bales (Fig 3.2).
Study
Date
McCabe (1)
Andersen (2)
Andersen (2)
Germany & Austria (3)
1993
2004
2004
200?
Density
Kg/m3
133
90
75
?
W/mK
Bales flat
0.061
0.060
0.057
0.060
W/mK
Bales on edge
0.054
0.056
0.052
0.045
Figure 3.2 A summary of the main thermal conductivity test results carried out on straw bales
(1)
(2)
(3)
14
(Jones, 2002 page 85) tests performed by guarded hot plate method on single bales (USA)
(Andersen, 2004) Measurement of specific thermal conductivity, λ10, for 100mm of straw according to (ISO 8302:
1991) (Denmark)
(Minke, 2004 page 29) “various tests in Germany and Austria confirmed a value of λ 10,tr = 0.045 W/mK (for vertical
dry straw bales at an average temperature of 10oC)”
See Appendix A for a brief explanation of the principals of heat transfer
30
Carol Atkinson – MSc Architecture: AEES
January 2008
All tests have shown lower thermal conductivity for bales laid on edge (Fig 3.2).
Figure 3.3 Bales laid flat (strings to centre of wall). Figure 3.4 Bales laid on edge (strings visible).
The straw fibres are assumed to be predominantly aligned perpendicular to the wall
(and the heat flow) when bales are laid on edge (Fig 3.4). In bales laid flat the fibres
are assumed to be aligned parallel to the wall (and the heat flow) (Fig 3.3). As
conduction will occur more easily along the parallel stems, this may explain the
higher rate of heat transfer (Fig 3.5).
Figure 3.5 Simplified diagram to illustrate how heat may be conducted more easily along stems laid
in the direction of the heat flow (red arrow).
However, the differences in thermal conductivity could be due to limitations in the
testing procedures. For example, heat in the bales laid on edge could have been
conducted up or down the wall (along the stems) rather than through it and not been
recorded by the testing equipment.
When the bales used to build the Cabin are viewed from the cut side the fibres appear
to be aligned parallel to the wall (Fig 3.6). However, when viewed from the folded
side (Fig 3.7) and the centre of the bale (Fig 3.8) the orientation of the fibres is much
less clear.
31
Carol Atkinson – MSc Architecture: AEES
January 2008
Figure 3.6 A straw bale laid flat and viewed from the cut side. The cuts give the
impression the straw stems are generally aligned parallel to the wall (and the heat flow).
Figure 3.7 The same straw bale laid flat and viewed from the opposite, folded side.
The stems appear to be predominantly aligned perpendicular to the wall (and the heat flow).
Figure 3.8 The same straw bale laid flat and split open to be viewed from the centre of the bale.
The stems appear to be randomly aligned.
The length and general orientation of fibres in the test bales in Figure 3.2 could not
be ascertained.
32
Carol Atkinson – MSc Architecture: AEES
January 2008
The Danish researchers measured a slightly lower conductivity in the less dense
bales. Lower density means less straw and more air (with lower conductivity) and so
this result seems reasonable in a steady state conductivity test. However, more air in
less dense bales may increase the likelihood of air movement, giving rise to greater
convection losses in a real wall (see section 3.3).
It was not possible to source the actual research papers referred to in this analysis,
merely the summary and commentary by the quoted authors. Interpretation of the
research papers does vary a little and the precise conditions and methods of the tests
were not explained. However, as the results are fairly similar, it seems reasonable to
assume that the thermal conductivity of wheat straw bales laid flat is in the region
of 0.060 W/mK.
Further work on the thermal conductivity of straw should include analysis of;
• Type of straw (wheat, barley, oats and other crops)
• How it is grown (soil type, fertiliser, pesticides or organic, weather impact)
• Content (proportion of stem, chaff and leaves)
• Length of fibre
• Density of bales
• How it is harvested, baled and stored
However, as the tests are expensive care must be taken to prioritise. A test at the
National Physical Laboratory costs £2,500 (their guarded hot box is worth over
£300,000). 15 Content, fibre length and density should be the highest priority.
As straw is a multi functional building material, thermal conductivity cannot be
examined in isolation. The inter-related aspects of moisture content, durability and
structural strength must be considered simultaneously.
In thermal conductivity tests moisture content is not relevant. The tests are performed
until the steady state is reached at which point the material is completely dry. This
facilitates comparison between materials but real walls, of any kind, will rarely exist
in a completely dry state in the UK climate. Conductivity, and therefore heat loss,
may increase as levels of moisture rise (CIBSE, 1999 page 160).
The thermal conductivity of straw is not as low as that claimed for materials
commonly used purely as insulation (Fig 3.9).
Material
Straw bale (laid flat)
Fibre glass insulation
Sheep’s wool
Warmcell (recycled paper)
Mineral wool insulation
Rigid polystyrene
Rigid polyurethane foam
Thermal conductivity (λ)
W/mK
0.060
0.040
0.039
0.036
0.032
0.029
0.022
% improvement
on straw
33%
35%
40%
47%
52%
63%
Figure 3.9 Thermal conductivity of straw (as discussed earlier) and other insulation materials (GBB,
2006 V1 page 277).
15
http://markbrinkley.blogspot.com/2006/11/interview-ray-mr-u-value-williams.html
33
Carol Atkinson – MSc Architecture: AEES
January 2008
The values are approximate because of manufacturing variations. Constituents and
density can differ between brands and even between batches of the same brand.
Manufacturers sell their insulation products mainly on their λ values, so they have a
vested interest in the best possible result the testing procedure can offer. However,
there is more to insulation than the laboratory tested λ of an individual material. How
does it perform as part of a real wall?
3.2 U-value
A wall is typically made up of different layers and surfaces that transfer heat at
different rates. A u-value is a measure of the rate of heat transfer that is calculated
from the thermal resistances of each part of the wall. The resistance of a material is
the reciprocal of its conductivity;
R = 1/λ
The total resistance of the wall is the sum of the thermal resistances of all the
components and the u-value is the reciprocal of the total.
U = 1/Rt
The Straw Bale Cabin
The different layers of the Cabin walls are used to calculate the u-value (Fig 3.10).
Wall assembly
Internal surface resistance (1)
Earth plaster (2)
Straw bale
Lime render (2)
External surface resistance (1)
Thickness
m
0.025
0.475
0.025
-
Total thickness of wall
Total resistance of wall
0.525
U-value (u=1/Rt)
0.123
Conductivity (λ)
W/mK
0.800
0.060
0.870
-
Resistance
m2K/W
0.130
0.031
7.917
0.029
0.040
8.147
W/m2K
Figure 3.10 U-value calculation for the walls of the Straw Bale Cabin.
(1)
The resistances of surfaces must also be taken into account. All surfaces hold a boundary layer of stationary air
(2)
which opposes heat flow, thereby providing a degree of thermal insulation against conduction. (BR443, 2006 page
11)
(Minke, 2004 page 29)
The thermal conductivity of the wheat straw bales used to construct the Cabin is
assumed to be 0.060 W/mK as concluded in section 3.1.
As clay and lime are not commonly used construction materials and as their
constituency can vary considerably, it is difficult to find definitive values for thermal
conductivity;
(Wihan, 2007 page 119)
The conductivity of earth plaster could vary from 0.2 W/mK to 0.7 W/mK,
depending on the clay/sand content of the plaster and the amount of straw or hemp
worked in.
34
Carol Atkinson – MSc Architecture: AEES
January 2008
(GBB, 2006 V1 page 277)
Unfired clay bricks have a conductivity of 0.95 W/mK, earth blocks 0.34 W/mK and
lime plaster made with recycled glass 0.378 W/mK.
(Minke, 2004 page 29)
Thermal conductivity is 0.8 W/mK for earth plaster and 0.87 W/mK for lime render.
Minke’s more conservative values are used in the calculation above (Fig 3.10).
As explained in Chapter 2, the actual layers of the Cabin walls are more complicated
than this calculation suggests;
• the internal surface is coated with primer and 3 coats of light coloured clay
paint
• the earth plaster consists of 3 different coats (clay slip, clay & sand reinforced
with chopped hemp, clay and fine sand)
• where straw joins timber (roof plate, windows, doors, electric sockets), the
plaster (internal & external) includes a strip of hessian to prevent cracking
• the plaster (internal and external) includes 15 full height plastic straps
(covered by strips of hessian)
• the straw bales are reinforced centrally with 30mm wide hazel rods (two per
bale)
• the lime render consists of 3 coats (the first two of which are reinforced with
chopped hemp)
• the external surface is coated with 3 coats of light coloured lime wash
However, the wide straw bales completely dominate the u-value calculation. The
paint, plastic strap and hessian are thin materials and would not have a significant
impact on the calculation though they are important to the overall performance of the
wall. The paint could have an effect on the emissivity of the surface but is not
considered further in this work.
The chopped hemp in the plaster has a small insulating value but it also prevents the
plaster cracking. Air leakage through cracks in buildings can significantly increase
heat loss (see Chapter 5).
Research into the u-value of a straw bale wall
In 1998, Christian et al at the Oak Ridge National Laboratory, USA, tested a panel of
what was agreed to be a well constructed straw bale wall using the guarded hot box
method (earlier tests had been criticised for unrealistic, poor quality construction
such as large air gaps and wet plaster). The test wall was built with two string,
470mm bales of wheat straw laid flat. They were plastered on the outside with stucco
(cement based plaster) and two 13mm gypsum boards formed the inner cladding
(Andersen, 2004).
The test resulted in an imperial value of R27.5 which converts to a metric u-value of
0.2065 W/m2K (Appendix I). This is much higher than the u-value of 0.123 W/m2K
calculated for the Straw Bale Cabin from λ values above but
•
the hot box test included conductance and convection losses (λ calculations
consider conduction only)
35
Carol Atkinson – MSc Architecture: AEES
•
•
•
January 2008
air film surface resistances are ignored
the Cabin is clay plastered internally rather than plaster boarded. This
eliminates any air flow (and therefore convective losses) behind the inner
cladding
it seems unlikely that the test wall was compressed to minimise air gaps
between bales
In 2004, Andersen performed a guarded hot box test on a 385mm wheat straw bale
panel (reduced in width to fit the testing equipment). The bales were laid flat and
plastered both sides with stucco. A u-value of 0.208 W/m2K was measured. This is
similar to Christian’s result above but the bales were much narrower. The
improvement may be due to the use of plaster on the internal surface (reducing
airflow).
Andersen adjusted his value for full width bales and surface resistances and
concluded that the wall u-value would be 0.165 W/m2K for a 450mm straw bale
wall, plastered both sides - still worse than the λ calculated u-value of 0.123 W/m2K
for the Straw Bale Cabin. His explanation is;
•
•
•
0.165 W/m2K includes both conduction and convection losses.
The plaster is worked into both sides of the straw and therefore the first
10mm or so is actually a mixture of straw and plaster (so there is less pure
insulation)
The plaster depth at the corners of bales (where they abut each other) is
thicker because the gaps in the straw bale are filled with plaster, perhaps
40mm from both the interior and exterior, creating a thermal bridge.
The last point should not be applicable to the Cabin as care was taken to stuff all
gaps between bales with straw or straw coated thinly with clay. Amazon Nail’s
advice was to maximise insulation and minimise the use of plaster (which costs more
than straw). The second point is valid for the Cabin because the plaster was pushed
well into the straw to ensure a strong bond. It seems reasonable, therefore, to adjust
the theoretical u-value calculation for a 20mm plaster/straw mixture on both sides of
the wall (Fig 3.11).
Wall assembly
Thickness
M
Conductivity (λ)
W/mK
Resistance
M2K/W
Internal surface resistance
Earth plaster
Earth/straw bond (1)
Straw bale
Lime/straw bond (1)
Lime render
External surface resistance
0.025
0.020
0.435
0.020
0.025
-
0.800
0.430
0.060
0.465
0.870
-
0.130
0.031
0.047
7.250
0.043
0.029
0.040
Total thickness of wall
Total resistance of wall
0.525
Amended U-value (u=1/Rt)
0.132
7.570
W/m2K
Figure 3.11 U-value calculation for a straw bale wall with internal clay plaster and external lime
render amended for the plaster/straw bond.
(1) No tested conductivity could be found so it is assumed to be half way between straw and plaster.
36
Carol Atkinson – MSc Architecture: AEES
January 2008
The walls of the Cabin are handmade and their thickness does vary a little but the uvalue is assumed to be 0.132 W/m2K (Fig 3.11). The difference of 0.033 W/m2K
between this and Andersen’s test u-value of 0.165 W/m2K could be due to;
• 25mm thicker bales used for the Cabin (accounts for 0.008 W/m2K)
• convection losses (considered further below)
• limitations in testing (such as quality of the straw panel (it did have to be
reduced in size to fit the equipment), human or mechanical error)
• limitations with the hot box equipment (such as lateral transmission of heat at
the edges of the box or stratification in the box)
Thermal bridging
A thermal bridge is a part of a structure where higher thermal conductivity lowers the
overall thermal insulation of the building. They are often found at the junctions of
floors, ceilings and windows. In a brick building thermal bridges could occur at
mortar joints in aerated concrete blocks, at metal wall ties or concrete lintels over
windows for example. In a timber building bridging may occur where the timber
frame interrupts the main insulation material.
In the Straw Bale Cabin, thermal bridges occur where timber spans the entire width
of the wall – the top and bottom of the base and roof plates, the noggins that hold the
hazel spikes in place and where the roof rafters join the wall. The u-value at these
points is 1.8 times higher than where the wall is just straw. However, as the thermal
bridges make up a very small proportion (2%) of the wall, they have little impact and
the average u-value of 0.136 W/m2K (Fig 3.12) is little different to the “just straw” uvalue of 0.132 W/m2K (Fig 3.11).
Area (A)
m2
U value (U)
W/m2K
AxU
at base spikes
at wall rods
at opening posts
just straw
wall/ceiling join:
Insulation
rafter support
top plate
Top
middle (straw)
rafter support
Noggin
Bottom
base plate
Top
middle (sheep's wool)
Noggin
0.546
2.364
2.400
36.053
0.137
0.137
0.150
0.132
0.075
0.324
0.360
4.759
5.536
0.168
0.134
0.257
0.742
0.043
0.262
3.157
0.270
0.143
0.262
0.257
0.138
0.257
0.257
0.257
0.067
0.436
0.069
0.037
0.067
0.236
0.692
0.125
0.176
0.094
0.257
0.042
0.065
0.032
Total wall area (m2)
Average U-value (W/m2K)
52.214
7.118
0.136
Figure 3.12 Calculation of the average u-value for the walls of the Straw Bale Cabin.
37
Carol Atkinson – MSc Architecture: AEES
January 2008
Standards
The Straw Bale Cabin is a holiday home, the thermal performance of which is
governed by British Standards (BS EN 1647:2004). The average u-value for a grade
one holiday home must be no greater than 1.7W/m2K. The Cabin meets this
“standard” with great ease. The Cabin could also be classed as a residential park
home (BS 3632:2005) and easily meets the standards set (Fig 3.13).
The straw walls also easily meet UK building regulations and the AECB’s proposed
silver standard. In theory they also meet gold, passive house and zero heating criteria
(Fig 3.13). However, Andersen’s test throws some doubt on whether the straw walls
would achieve a u-value below 0.15 W/m2K in practice.
The Straw Bale Cabin
Park home (BS 3632:2005)
UK building regulation (1)
AECB silver standard (2)
AECB gold/passive house
BRE zero heating house (3)
Wall
(W/m2K)
0.14
0.50
0.30
0.25
0.15
0.14
Windows
(W/m2K)
1.80
2.00
1.80
1.50
0.85
1.70
Roof
(W/m2K)
0.21
0.30
0.20
0.15
0.15
0.08
Floor
(W/m2K)
0.26
0.50
0.20
0.20
0.15
0.10
Figure 3.13 U-values achieved for elements of the Straw Bale Cabin compared to a range of building
standards. Blue type highlights where the Cabin meets the various standards.
(1) building regulation u-values above are the standards required for extensions to existing
houses under 2006 Approved Document L1b (GBB, 2006 V1 page 144)
(2) accessed on 5/1/08 at www.aecb.co.uk
(3) General information report 53. Building a sustainable future: Homes for an autonomous
community. HMSO October 1998 (GBB, 2006 V2 page 98)
The table does highlight the improvements needed to the roof and floor!
Currently, standard agricultural straw bales are more than adequate to meet building
regulations. In the future, as knowledge develops, it may be necessary to design the
baling machine with construction in mind - to produce a bale of optimum width and
density.
3.3 U-value limitations
U-values are widely used to assess the level of thermal insulation in a building.
However, they only take into account heat transfer by conduction and as we have
seen in section 3.2, heat can also be transferred through a wall by convection.
In addition, u-values are primarily concerned with the thermal conductivity of new
building materials. Can the values change with the impact of weather and time?
Convection losses
Straw is an unusual insulation material in that it is structural and the adjacent layers
(plaster) are bonded to it. Most other insulation is installed in separate sheets
between an inner and outer leaf or as loose fill (Fig 3.14).
38
Carol Atkinson – MSc Architecture: AEES
Structural/bonded
Plastered straw
Hemp/lime cast wall
SIPs*
Sheet/roll
Sheep’s wool
Mineral wool
Polystyrene
January 2008
Loose fill
Warmcel (recycled paper)
Vermiculite
Polystyrene beads
Figure 3.14 Examples of insulation analysed by installation method.
The majority of new homes in the UK are built with brick and block construction
with the cavity fully or partially filled with insulation. The inner and outer layers of
the wall are usually built together with the insulation sheets placed in between as the
wall rises. It is not possible to visually assess the integrity of the insulation once the
wall is built.
Air gaps could occur in the insulation because
• the insulation is not the exact width of the cavity (full fill)
• the inside of the cavity is not flat so the insulation doesn’t fig snugly against
it
• the joints in the sheets of insulation have not been staggered
• wall ties, pipe outlets, windows and doors interrupt the insulation
• careless builders drop mortar on the insulation which is hidden when the next
sheet is placed on top
Loose fill insulation is mostly used in timber frame construction but can also be used
between bricks and blocks. When blown between the completed inner and outer leaf
it is impossible to see any blockages, gaps or settlement that occur.
These installation problems suggest that significant convection losses could occur in
cavity walls with sheet or loose fill insulation and research has shown this to be the
case;
(Little, 2005 page 51)
“Jan Lecompte’s 1990 paper “The influence of natural convection on the thermal
quality of insulated cavity construction” makes clear the substantial effect that air
passage through these gaps has on the thermal performance of insulated cavity
walls………..Lecompte measured a 193% increase in heat transfer due to a 10mm
void………………….even a 5mm void could lead to a 35% increase in heat transfer”
(Doran, 2000)
“The results of the present project, together with those of past projects by Ward,
indicate that the existing calculation procedures such as BS EN ISO 6946:1997, as
used for regulatory purposes, may often underestimate true heat losses for walls, in
some cases by more than 30%.”
With straw bale walls, the builder has the opportunity to check the integrity of the
insulation before it is plastered. However, convection currents could occur if the
bales are low density or not compressed sufficiently and if gaps are not fully stuffed.
(Wihan, 2007 page 63)
“The joints between bale stacks act as “chimneys” allowing the west sun to heat air
in the bales…..and drive it up to the top of the wall……”
39
Carol Atkinson – MSc Architecture: AEES
January 2008
Further research is required to establish what level of compression is required to
minimise air gaps between bales and courses of bales. Could bales stacked on edge
have fewer air gaps because their “softer” sides are more easily forced together?
Would the addition of a thin layer of soft material (such as sheep’s wool) at joints
and in between courses help to reduce air gaps?
Current regulations recognise, to a certain extent that convection losses occur - a
correction to u-value is required where air gaps exceed 5mm (BR443, 2006 page 13).
However, this doesn’t go far enough –there is clear evidence that significant heat can
be lost by convection in smaller gaps and in most types of wall.
If walls are to meet the increasingly high standards needed to conserve energy and
reduce carbon emissions they must actually work as intended. A greater
understanding of convection losses is required. This knowledge can then lead to
better regulation and design. Buildings should then be constructed to specification
and checks made to ensure effective heat retention.
Only when whole walls are tested can a fair comparison be made.
Degradation
Insulation materials must be able to deliver their specified thermal performance over
the lifetime of the building (or the building should be designed to enable the
insulation to be monitored, renewed or improved).
Different materials have different types of “failure risk” associated with them. More
research is urgently needed in this area (XCO2, 2002).
The failures that can arise because of air movement were outlined above. Other
failures include settlement, off gassing by blown cellular plastics, attack by vermin
or insects. However, moisture build up is probably the highest risk factor in the damp
UK climate.
(Little, 2005)
The cavity wall came about because the solid 215mm wall, popular in the inter-war
years, suffered from rain penetration. A cavity meant that moisture did not penetrate
the inner leaf. Filling the cavity with insulation reduces the air gaps but moisture can
soak into the insulation rather than draining away.
There is little research into the effect of moisture on any form of insulation. All
testing, as we have seen, is carried out in the steady, dry state. For all materials we
need to know what level of moisture could damage it, how the rate of heat loss is
affected at different moisture levels and if the damage or insulation loss is permanent
or temporary?
As far as straw and other natural materials are concerned, excessive moisture is a
known enemy. Straw bale buildings are purposely designed with large, overhanging
roofs and raised, free draining foundations. Care is taken with window details and
permeable plasters allow moisture to dry out. These measures ensure the longevity of
the building but how does the thermal performance change at various levels of
40
Carol Atkinson – MSc Architecture: AEES
January 2008
moisture? Does thermal performance change over time? Much further research is
needed.
A recent report into humidity in straw bale walls (Wihan, 2007 page 162) concluded
that “It is clear that for the extensive decomposition of straw, micro organisms need
a long term supply of liquid water, that is, an environment with relative humidity
very close to 100%.” A house on the coast of Brittany featured as a case study in the
report. Its lime plastered walls were completely saturated by two days of horizontal
driven rain in December 2005. After a few months the wall had dried to 78% relative
humidity and there was little sign of rot in the bales. The wall survived a complete
soaking because they were allowed to dry out – but what happened to the insulation
value of the wall during the wet time and how will it perform thermally in the future?
The oldest European house is still standing after 90 years – but how is it performing
thermally?
There is little information about the thermal performance of any home after
construction – whether initially to verify it met the design standard or after a period
of years to establish any change to the initial performance.
From 14 December 2007 every home for sale in England and Wales must have a
Home Information Pack which includes an Energy Performance Certificate to enable
prospective purchasers to understand the environmental impact of their new home. 16
The information provided at this early stage is very limited but perhaps in the future
it might include thermal images* of the house to ensure the insulation is still
effective.
3.4 Heat loss from a building
The walls are usually the biggest single element of the building fabric – they are 44%
of the internal surface area of the Straw Bale Cabin (Fig 3.15).
Building fabric
Straw wall only
Other wall (straw & wood)
Windows & doors
Roof
Floor
Total
Area (m2)
36.05
16.16
8.34
32.19
26.46
119.20
% of total wall
30%
14%
7%
27%
22%
Figure 3.15 Internal surface area of the Straw Bale Cabin.
The heat loss from a building is calculated using the u-value of all building elements
and the heat lost through ventilation. The winter time heat loss rate for the Straw
Bale Cabin, assuming an internal design temperature of 18oC, an external
temperature of -1oC and a ventilation rate of 1 air change per hour, is calculated in
Figure 3.16.
16
http://www.homeinformationpacks.gov.uk
41
Carol Atkinson – MSc Architecture: AEES
January 2008
In theory, approximately 1.2 kilowatts of heat must be supplied to the building to
replace the heat lost when the outdoor temperature is 19 degrees lower than the
required indoor temperature (approximately 65W for every degree drop in
temperature).
Element
Area
m
Walls
Windows
Doors
Total wall area
Roof
Rooflight
Total roof area
Floor
Total internal surface area
2
52.21
3.68
4.67
60.56
31.87
0.32
32.19
26.46
119.21
U value
2
W/m K
Temperature
difference
o
Heat loss
C
W
0.136
1.800
1.800
19
19
19
0.206
2.700
19
19
0.264
19
134.91
125.86
159.71
420.48
124.74
16.42
141.16
132.72
Fabric heat loss
Ventilation heat loss (1 air change per hour)
694.36
0.3333 x 1.0 x 81.015 x 19
513.09
Total heat loss
1,207.45
Figure 3.16 Heat loss calculation for the Straw Bale Cabin.
The ventilation rate is assumed to be 1 air change per hour (McMullan, 2002 page
94).
This calculation is widely used to size heating plant or calculate the energy demand
of a house over the heating period. However, it has serious limitations;
•
•
•
•
•
•
It is heavily dependent on realistic u-values, which as we have seen, are very
difficult to establish
As it is using u-values, it is assuming heat loss by conduction and ventilation
only. Convection and radiation losses are ignored.
It reality the internal/external temperature difference in constantly changing
Different parts of the building experience different temperature variations.
For example if the sun is shining on the south façade, the temperature here
will be markedly different to the temperature on the north side. Similarly with
wind exposed aspects of the building.
It assumes that the indoor and outdoor temperature have the same starting
point (in Chapter 4 we will see that this is not the case)
It is only 2-dimensional – as the internal surface area is used it ignores heat
loss at corners (Fig 3.17);
42
Carol Atkinson – MSc Architecture: AEES
January 2008
Figure 3.17 Diagram to show how the heat loss calculation disregards the corners of the building
(shaded pink).
In the Straw Bale Cabin, as these corners are deep and well insulated their omission
may not have a significant effect on the calculation of heat loss.
In early January 2008, electric heaters were left on at the Cabin to test the theoretical
heat loss calculation. A 400W heater was switched on for 48 hours from 5th to 7th
January and a 1500W heater was switched on for 48 hours from 7th to 9th January.
However, the heaters must have cut out because electricity meter readings showed
that only 10 kWh was used in the first 48 hours and only 23 kWh in the second 48
hours.
20
18
16
14
12
10
8
6
4
2
0
inside
outside sth
outside nth
10:40:00
07:00:00
03:20:00
23:40:00
20:00:00
16:20:00
12:40:00
09:00:00
05:20:00
01:40:00
22:00:00
18:20:00
14:40:00
theoretical
11:00:00
oC
If the heating is assumed to be even, then the meter readings suggest that the heaters
delivered 210W and 480W. Using the calculation in Figure 3.16 of 65W per oC, the
heaters should have increased the temperatures in the Cabin by 3.2oC and 7.4oC
respectively. This resultant “theoretical” indoor temperature is shown as the pale
blue line in Figure 3.18 and 3.19.
Figure 3.18 Temperature readings at the Straw Bale Cabin from 11am on 5th January to 11.30am on
7th January 2008. The “theoretical” curve is the temperature that should have resulted from the
delivered heat according to the heat loss calculation in Figure 3.16.
43
Carol Atkinson – MSc Architecture: AEES
January 2008
25
20
oC
inside
15
outside sth
10
outside nth
theoretical
5
11:15:00
07:35:00
03:55:00
00:15:00
20:35:00
16:55:00
13:15:00
09:35:00
05:55:00
02:15:00
22:35:00
18:55:00
15:15:00
11:35:00
0
Figure 3.19 Temperature readings at the Straw Bale Cabin from 11:35am on 7th January to 11:45am
on 9th January 2008. The “theoretical” curve is the temperature that should have resulted from the
delivered heat according to the heat loss calculation in Figure 3.16.
These graphs indicate that the thermal performance of the Straw Bale Cabin is much
better than theory suggests. However, further tests are required to confirm the results
and the electricity meter should be checked.
Despite its problems, the heat loss calculation does highlight the potential main paths
of heat loss;
% of
% of
fabric
fabric heat loss
Element
Walls
43.8%
19.4%
Windows
3.1%
18.1%
Doors (fully glazed)
3.9%
23.0%
Total wall area
50.8%
60.6%
Roof
27.0%
20.3%
Floor
22.2%
19.1%
Figure 3.20 Area analysis of the fabric of Straw Bale Cabin and heat loss through it.
The walls make up nearly 44% of the building fabric but less than 20% of heat is lost
through them. The windows and doors, on the other hand, account for only 7% of the
fabric but over 40% of the heat escapes through them (Fig 3.18). This highlights the
importance of the correct sizing and placing of windows and the necessity for well
insulated curtains, blinds or shutters to conserve heat after dark.
44
Carol Atkinson – MSc Architecture: AEES
January 2008
Summary of the main points from Chapter 3
•
The thermal conductivity of straw bales stacked flat is approximately 0.06
W/mK.
•
This gives a theoretical u-value of 0.132 W/m2K for a straw bale wall,
plastered both sides.
•
Danish research has shown that the actual heat loss from a straw bale wall in
situ is may be slightly worse than this due to convection currents within the
wall.
•
UK research has shown that the actual heat loss from many conventional
walls in situ is likely to be considerably worse than their theoretical u-value
indicates due to convection currents within the wall.
•
Tests on the Straw Bale Cabin suggest that the actual heat loss from the
building may be better than theory predicts but further testing is required.
•
Further research is needed on the effect moisture may have on the u-value of
straw and other insulation materials.
•
Although walls are the largest element of the building fabric, the roof and
floor must also be well insulated.
•
A high proportion of heat can be lost through glazing.
45
Carol Atkinson – MSc Architecture: AEES
January 2008
4. Thermal capacity
Thermal insulation tests are measured when then the heat flow is steady. The test
proceeds until the specified number of identical temperature readings are taken.
However, how long does it take to reach this point and how long will it take to cool
down again?
4.1 Thermal mass
The same mass of different materials can hold different quantities of heat.
The specific heat capacity of a material is the quantity of heat required to raise one
kilogram (kg) of that material by 1 degree Kelvin (K) (McMullan, 2002 page 14).
Specific heat capacity (c)
(J/kg K)
1000
1000
720
840
1200
2000
Material
Dense concrete block (1)
Earth plaster (2)
Stone (1)
Plasterboard (1)
Timber (1)
Straw (2)
Figure 4.1 Specific heat capacity of building materials. Materials vary but these are typical values
provided by (1) GBB, 2006 V1 page 279 (2) Minke, 2002 page 28
However, the same mass (kg) of these materials occupies different volumes of space,
depending on their densities. Density being the "degree of close packing", expressed
as the mass (kg) per unit volume (m3). Multiplying the specific heat capacity by the
density gives a “volume specific” heat capacity which is more applicable to
buildings.
Material
Dense concrete block (1)
Earth plaster (2)
Stone (1)
Plasterboard (1)
Softwood (1)
Straw (3)
Density (d)
(kg/m3)
2300
1900
2180
950
630
120
Volume specific
heat capacity (c x d)
(kJ/m3K)
2300
1900
1570
798
756
250
Figure 4.2 Volume specific heat capacity of building materials. Materials vary but these are typical
(3) The Straw Bale Cabin
values provided by (1) GBB, 2006 V1 page 279 (2) Minke, 2002 page 28
Therefore, heavyweight materials such as stone or concrete can store much more heat
than lightweight materials such as timber and straw in the same amount of space.
46
Carol Atkinson – MSc Architecture: AEES
January 2008
Figure 4.3 A simplified diagram to highlight the thermal capacity of heavyweight materials. These
walls have approximately the same u-value (3.4 w/m2K) so in a steady state they would conduct heat
at the same rate. However, the first 10mm of this simplified concrete block wall can store 46 Joules of
heat (2.3J x 20) but the first 10mm of the simplified softwood wall can store only 15 Joules of heat
(0.756J x 20).
A lightweight building is thermally responsive; the internal space heats quickly from
cold because less heat is absorbed by the wall when the heating is turned on. Less pre
heating is required to reach a comfortable temperature. However, a lightweight
building requires a responsive heating system to prevent over heating as the fabric
cannot absorb excessive heat.
A heavyweight building can store more heat in its inner surface to release to the
internal space when the temperature drops. This can be useful in the winter providing
free heat in the building on sunny days; solar radiation from the low winter sun
enters the building through south facing glazing, the heat is stored in thermal mass
inside the building during the day and then released back to the internal space as the
temperature falls in the evening. It can also help to stabilise the internal temperature
on hot summer days by soaking up excessive heat during the day to be purged later
by the cool night air.
(Concrete Centre, 2006)
“Dwellings with a medium to high level of thermal mass are characterised by their
inherent ability to soak up and release heat at different times of the day…………..
higher embodied impacts of concrete and masonry products can be offset in
relatively few years of operation providing that effective use is made of the thermal
mass to optimise the energy used by heating and cooling systems”
47
Carol Atkinson – MSc Architecture: AEES
January 2008
Figure 4.4 Reproduced from Thermal Mass for housing: Concrete Solutions for the Changing Climate
(Concrete Centre, 2006 page 3).
The Concrete Centre sponsored this report and the underlying research. The report
does mention other design and operational criteria that are needed for thermal mass
to work efficiently but it does not refer to other materials such as earth that may work
equally as well. It also makes no reference to the optimum thickness of the thermal
mass or how it should be distributed in the building?
(GBB, 2006 V1 page 279)
To be effective, thermal mass needs to be well connected with the space. Spreading
the mass around the surfaces is much more effective…………During a normal
diurnal cycle, 90% of the recoverable heat flow is limited to a depth of about 50mm
in dense concrete and 50% to the first 25mm. Thus for thermal storage over a 24
hour time-span there is very little to be gained from very thick concrete masses”
Is the Straw Bale Cabin a lightweight or heavy weight building?
Straw has a very low volume specific heat capacity of only 250 kJ/m3K. However, it
is coated in clay plaster (25mm) which has a high volume specific heat capacity of
approximately 1900 kJ/m3K (Fig 4.2). The clay coating is to all walls (including
internal partition walls) and ceilings. The floors are wooden and there are no carpets
or net curtains to restrict solar access to the interior surfaces.
30
25
oC
20
15
10
5
22:45:00
20:00:00
17:15:00
14:30:00
11:45:00
09:00:00
06:15:00
03:30:00
00:45:00
22:00:00
19:15:00
16:30:00
13:45:00
11:00:00
08:15:00
05:30:00
02:45:00
00:00:00
0
Figure 4.5 Temperature inside the unoccupied Straw Bale Cabin (blue line) and outside the
Cabin (pink line) on 8th and 9th August 2007.
48
Carol Atkinson – MSc Architecture: AEES
January 2008
Comparing the graph from the Cabin (Fig. 4.5) and the graph in the Concrete Centre
report (Fig. 4.4);
• The diurnal temperature swing is approximately 15oC in both graphs and the
internal temperature by varies only 4 or 5 degrees.
• The temperature inside the Cabin does not seem to be significantly lower than
the outside temperature as in the Concrete Centre graph (but it is still very
comfortable at 20oC). However, the external temperature readings at the
Cabin are taken in the shade; the peak temperatures may well have been at
least 5oC higher with the full effect of solar radiation.
The Straw Bale Cabin appears to behave like a heavy weight building.
(McMullan, 2002 page 77)
“…..heavyweight structures have smaller temperature swings than lightweight
structures.”
Figure 4.6 Thermal response in lightweight and heavyweight buildings
(reproduced from McMullan, 2002 page 77).
McMullan’s graphs include the average temperature but in the absence of a scale,
there is no indication of the time of year. The internal average temperature and
external average temperature appear to be the same.
To compare McMullan’s graph (Fig 4.6) to the Cabin, five unoccupied and unheated
days have been selected, approximately two months apart. In all graphs (Fig 4.7), the
blue line represents the inside temperature, the pink line the outside temperature and
the green line the outside average temperature;
49
Carol Atkinson – MSc Architecture: AEES
January 2008
Figure 4.7 Inside and outside temperatures at the Straw Bale Cabin during
five unoccupied and unheated days in 2007.
Again, the Straw Bale Cabin appears to behave like a heavy weight building in that
the internal temperature does not vary greatly. However, the temperature indoors is
always above the average outdoor temperature (green line). This is a significant
difference with McMullan’s graph for a heavy weight building – not only is the
temperature moderated, but it is also raised.
Only in June and August is it warmer outside than inside at peak daytime
temperatures – this suggests that the cabin stays cool in the summer. However, this is
probably due to the shade provided by the porch along the south façade.
50
Carol Atkinson – MSc Architecture: AEES
Month
April
June
August
October
December
January 2008
Difference between mean internal
and mean external temperature
+ 6.2oC
+ 4.5oC
+ 2.7oC
+ 5.4oC
+ 3.9oC
Figure 4.8 From the data graphed in figure 4.5 – indoor and outdoor temperatures
recorded at the Straw Bale Cabin in 2007.
Solar radiation entering the building from the lower spring/autumn sun probably
accounts for the higher internal average temperatures in April and October (Fig 4.8).
These tests indicate that the Straw Bale Cabin may have energy saving potential.
Further results are discussed in Chapter 6.
A data logger was also recording temperatures inside an unoccupied static caravan
during August (see Appendix E). The temperature in the caravan fluctuated around
the mean temperature, similar to McMullan’s graph for a lightweight building.
Figure 4.9 Temperature outside the Straw Bale Cabin and inside a nearby
static caravan on 8th/9th August 2007
The caravan is lightweight (no materials with thermal mass) but it is also poorly
insulated. Is it the high level of insulation or the thermal capacity of the clay plaster
that is moderating the internal temperature in the Straw Bale Cabin? Is it a
combination of both or could there be other factors?
4.2 Thickness
If a straw bale wall was not plastered it would fit the definition of a lightweight
building perfectly yet during the construction of the Cabin in summer 2006, it felt
refreshingly cool inside the un-plastered building. Was this due to the high insulation
value of the straw? Could the thickness of the wall also be a consideration?
(King, 2006 page 188/9)
Due to time lag, the actual thermal performance of straw bale walls in a climate with
diurnal temperature swings is significantly better than the u-value indicates;
51
Carol Atkinson – MSc Architecture: AEES
January 2008
“For most standard construction systems, it takes anywhere from 20 minutes to a
couple of hours to reach steady state heat flow conditions. For plastered straw bale
walls it can literally take weeks…………….this effect will make a building wall with
a good R-value act effectively like one with a much higher R-value. Both thickness
and thermal mass of the wall act to buffer diurnal temperature swings inside”
King cites research in California that determined the thermal lag (the time it takes for
a “pulse” of heat to travel through wall) to be 12 hours.
In the guarded hot box thermal conductivity test at Oak Ridge National Laboratory
the straw bale wall took two weeks to a reach steady state. 17
12:10:00
10:50:00
09:30:00
08:10:00
06:50:00
05:30:00
04:10:00
02:50:00
01:30:00
00:10:00
22:50:00
21:30:00
20:10:00
18:50:00
17:30:00
16:10:00
14:50:00
20
18
16
14
12
10
8
6
4
2
0
13:30:00
oC
In January 2008 a crude heat pulse test was carried out at the Straw Bale Cabin. The
building was heated for just under two hours with 2 electric heaters amounting to 3.5
kW. The temperature rose by 8oC from 10.5oC to 18.5oC. It took nearly 19 hours for
the temperature inside the Cabin to fall back down to the original temperature of
10.5oC (Fig 4.10).
Figure 4.10 Temperature inside the unoccupied Straw Bale Cabin (blue line) and outside the
Cabin (pink line) on 14th and 15th January 2008.
The windows and French doors were insulated with duvets and pillows in an attempt
to limit the heat loss through them (see photograph in Appendix G). The roof light
and main door were left uncovered. The heat will have escaped though all elements –
not just the straw walls.
The walls could be tested on their own in the future with a heat flux meter.
Conclusion
There is no doubt that the temperature inside the Straw Bale Cabin is greatly
dampened compared to outside diurnal temperature swings. However, it is not clear
whether this is due to
• a high level of insulation,
17
http://www.ornl.gov/sci/roofs+walls/AWT/HotboxTest/Hybrid/StrawBale/index.htm
Accessed 14/1/08
52
Carol Atkinson – MSc Architecture: AEES
•
•
•
January 2008
very thick walls (525mm),
thermal mass (25mm) provided by clay plaster on walls and ceilings or
a combination of all of the above.
In the winter the high percentage of south facing glazing facilitates solar gain in the
Cabin. In the summer, shading on the south façade and the high ceiling help to
minimise over heating.
The lime plaster on the Cabin’s exterior has a high volume specific heat capacity. It
could also help to keep the building cool in summer by absorbing heat during the day
then releasing it to the cool night air.
This study of the Straw Bale Cabin suggests that there is no simple distinction
between heavyweight and lightweight buildings. Could it be possible to have the
benefits of both types – a building that warms up quickly and retains the heat without
over heating?
Limitations and further study
Although this is a small building there are still a great many variable elements and
the weather notes kept are not comprehensive.
Further study is required to establish the optimum
• thickness of a wall
• thickness and density of clay plaster and
• location of the thermal mass (floor, wall, ceiling, all).
Further comparison with other buildings is essential.
The requirement for thermal capacity is affected by the climate in which the building
is located and the way in which it is occupied. How easily would it be to add thermal
mass if it was required by a changing climate?
Summary of the main points from Chapter 4
•
Temperatures are moderated inside the Straw Bale Cabin suggesting that
straw bale buildings require less energy for heating and cooling.
•
Insulation, thermal capacity and wall width are all important factors but it is
difficult to isolate their individual contribution.
53
Carol Atkinson – MSc Architecture: AEES
January 2008
5. Air flow
To minimise the loss of heat from a building by convection, air flow must be
controlled. In Chapter 3 we learnt how air gaps within the wall can impinge on its
thermal performance. In this Chapter the emphasis is more on air gaps in the
structure – whether a result of poor design, unsuitable materials, poor construction or
settlement or alteration over time. More heat can be lost through air gaps in “leaky”
buildings than through the fabric itself.
Infiltration is the unwanted movement of air through cracks and gaps in the building
envelope. The air movement is driven by pressure differences created by the wind
and the buoyancy of warm air (GBB, 2006 V2 page 116). Occupants feel
uncomfortable draughts and the incoming cold air must be heated, thereby increasing
the energy demand of the building.
Adequate ventilation, however, through purpose built openings is needed to provide
a healthy living environment.
Energy efficient buildings should be “built tight and ventilated right”.
5.1 Air tightness
Air tight buildings keep the warm air in and the cold air out. UK building regulations
require new buildings to have a maximum air permeability of 10 m3/h/m2. The
AECB silver standard demands a maximum of 3 m3/h/m2 in naturally ventilated
buildings (GBB, 2006 V1 page 119).
When measuring air tightness, a calibrated fan is mounted in an open doorway and a
series of steady state pressure differences are applied using the fan. Once steady state
conditions are achieved, the air flow measured through the fan equals the sum of the
air leaking through all the gaps and cracks in the building envelope (GBB, 2006 V1
page 119).
During the test all other external doors and windows are closed and intentional
natural and mechanical ventilation openings are sealed with plastic. Internal doors
are wedged open (CIBSE, 1999 page 210). The result is usually referred to as the
ventilation rate but it is infiltration that is being measured.
The Straw Bale Cabin
On 11 May 2007, Paul Teather 18 carried out an air tightness test at the Cabin in
accordance with British standard EN 13829:2001.
Figure 5.1 Calibrated fan mounted in the doorway of the Straw Bale Cabin.
18
www.thehealthyhome.co.uk
54
Carol Atkinson – MSc Architecture: AEES
January 2008
The ventilation (infiltration really) rate was found to be 1.56 m3/h/m2 at 50 pascals over six times better than building regulations require. Air flow in the Cabin is
satisfactorily controlled – significant amounts of heat will not be lost through cracks
and gaps in the fabric.
This result seems reasonable for a building with “solid” walls like the Cabin. In
addition, extra care was taken during construction to
• Apply plaster directly to walls and ceiling (to minimise air flow behind the
final finish)
• Minimise the differential movement between materials with strips of Hessian
• Include mesh in one plaster coat over the wood fibre boards (to prevent
cracking on the ceiling)
• Push strands of sheep’s wool into all gaps, however small, between window
and door frames and their fixing points in the wall
• Fit draught strips and strong ironmongery to all windows and doors
• Apply acrylic sealant behind skirting boards
However, the odd gentle stream of air coming in during the test highlighted where
improvements could be made
• under the window sills (these are the only places in the wall where the join of
wood to straw is not plastered across. Although care was taken to fully stuff
and bridge the join with Hessian, it is a tricky area to work in. More care or a
design improvement is needed in the future)
• a tiny fault in the double glazing adhesive strip on the north window (check
integrity of all strips as glazed units are fitted)
• occasional gaps where the skirting is not flush with the plaster (pay attention
to achieving a flat finish at skirting level)
• gap in the French doors (improve design)
• wall/ceiling join (a continuous plaster coat from wall round to ceiling would
solve this problem)
Traditionally, plastered buildings are less leaky than timber frame or dry lined*
buildings (Borer, 2005 page 182) but cracks can appear over time.
It is over a year since the Cabin was completed and the internal clay walls have
developed no cracks at all. There are, however, a number of hairline cracks in the
external lime plaster, mainly from the corners of windows. These cracks appeared
within 6 months and were present when the air tightness test was carried out. They
have been brushed over with lime wash in an attempt to seal them but they are still
visible. As the cracks are very narrow (a needle can’t be poked in) it is very difficult
to assess how deep they are.
Recently cracks have developed in the bedroom ceiling where the wood fibre boards
were not taped with Hessian at the gables and apex. Taping was done in the
lounge/kitchen and no cracks have appeared there. The air tightness of the Cabin will
be compromised if these cracks are not filled. Maintenance will be as important for
straw bale as for any other building.
The air tightness of the Cabin should be tested again (perhaps at 5 year intervals) to
monitor performance over time. As testing is new to 2006 regulations, there is very
55
Carol Atkinson – MSc Architecture: AEES
January 2008
little data available about the air tightness of any form of new building and no data
about how the result changes over the life of the building. Anecdotal evidence
suggests that results will worsen with age; settlement cracks in plaster, tears in
vapour barriers, alterations and degradation of materials (for example less flexible
silicon or draught strips dropping off).
5.2 Air quality
Ventilation provides fresh air to breathe. The common standard rate per person is 8
litres per second or 30 m3 per hour. Building regulations generally require 1 air
change per hour in domestic rooms (McMullan, 2002 page 95).
Ventilation should also expel carbon dioxide, odours and excessive water vapour.
However, a certain amount of moisture in the air is needed. For comfort, humans
need a fairly balanced relative humidity* of roughly between 40% and 65%. Below
40% allergies or respiratory infections may result and above 65% fungal growth and
mites are likely (GBB, 2006 V1 page 114/115). Recent research recommends
maintaining internal relative humidity below 60% to ensure that the house dust
mite’s critical equilibrium humidity will not be achieved (Howieson, 2005).
Clay is a hygroscopic* material that can passively regulate internal humidity.
(Morton et al, 2005 page 17)
“The earth materials demonstrated a clear ability to absorb and desorb atmospheric
moisture………..it was clear that the 15mm clay plaster surface strongly regulated
short term peaks……..air relative humidity was generally regulated to the target
range of 40 – 60%”
This report was the result of a 2 year research programme to monitor and evaluate
earth bricks, mortars and plasters. It looked at a house from design to occupation.
The building was essentially a clay box sitting inside a timber box with a 200mm
cavity filled with cellulose recycled newspaper insulation.
The Straw Bale Cabin
Very little energy is used to provide ventilation in the Cabin. It is provided by
• five windows that fully open (at least 1 in each room)
• trickle vents* in all windows
• a roof light than can be opened (providing high level (stack*) ventilation)
• a small fan (low wattage) in the bathroom
Provision was made for an extract fan in the kitchen but this has not yet been fitted.
Sources of moisture in the Cabin are the breath of occupants, the kettle, cooking and
showering.
The Cabin walls (including partition walls) and ceilings are clay plastered to a depth
of approximately 25mm. They were primed and painted with three coats of clay
paint.
The lascar data loggers (see Appendix D) automatically record relative humidity at
the same time as temperature. A brief review of three months relative humidity data
follows;
56
Carol Atkinson – MSc Architecture: AEES
January 2008
21 September – 21 December
A data logger was suspended internally from the beam (2.5m high) in the
lounge/kitchen area of the Cabin and another was suspended under the roof overhang
on the south side of the building. The Cabin was occupied for 57 of the 91 nights in
the period (63%).
Over 50,000 relative humidity (RH) readings were collected by the loggers in the
three months from 21st September 2007 to 21st December 2007 (Fig 5.2).
120
relative humidity %
100
80
60
40
20
0
Figure 5.2 Relative humidity recorded at the Straw Bale Cabin between 11:25am on 21st September
2007 and the same time on 21st December 2007. The blue line represents the indoor relative humidity
and the pink line is the outdoor relative humidity.
Logger position
Inside
Outside
Average temperature
17oC
9oC
Average relative humidity
58%
84%
Figure 5.3 Mean average temperature and relative humidity from the 3 month data logged at the
Straw Bale Cabin between 11:25am on 21st September 2007 and the same time on 21st December
2007.
Inside data logger
Time/explanation
Lowest RH
35%
when the
temperature
was
30-31oC
Highest RH
75-77%
when the
temperature
was
21-23oC
Lowest temperature
6.5oC
56-60%
Highest temperature
31oC
when RH
was
when RH
was
35%
Around midnight on
occupied nights in mid
November (presumably
after long evenings with
the heating on)
Around 2pm on 12
October and 7pm on 27
October (presumably
when cooking meals on
the hob)
On unoccupied nights of
24 & 26 November
As for lowest humidity
reading above
Figure 5.4 Analysis of data from the logger inside the Straw Bale Cabin from11:25am on 21st
September 2007 and the same time on 21st December 2007.
57
Carol Atkinson – MSc Architecture: AEES
January 2008
Outside data logger
Time/explanation
o
Lowest RH
39-43%
when the
temperature
was
20.5 C
Highest RH
97%
when the
temperature
was
5.5oC
Lowest temperature
o
-3 C
when RH
was
92-96%
Highest temperature
24-25oC
when RH
was
67%
2pm – 4pm on 19
October (the weather
diary noted a sunny day
following a frosty start)
9am on 27 November
(weather noted as
gloomy)
Midnight on 16
December and 8am on 15
December (weather noted
as gloomy)
Early afternoons of 7 &
12 October (weather
noted as warm, dry &
sunny)
Figure 5.5 Analysis of data from the logger outside the Straw Bale Cabin from11:25am on 21st
September 2007 and the same time on 21st December 2007.
The average humidity indoors (Fig 5.3) is well within the comfort range discussed
earlier and indeed, during the three month period, 93% of relative humidity
readings in the Straw Bale Cabin were in the comfortable range of 40% to 65%
and 75% of readings were in the range of 40% to 60%.
As, on average, it was warmer inside the Cabin than outside, lower relative humidity
should be expected indoors. However, there was a wide variation of temperature
inside the Cabin – when it was unoccupied (and unheated) the temperature fell to
6.5oC but on one occasion during occupation the internal temperature reached 31oC,
presumably after the heater had been left on late into the evening (Fig 5.4). As the
logger was positioned high in the room (2.5m) it will have recorded temperatures
higher than those experienced by the occupants at settee level (1.0m).
There was a greater range of relative humidity readings outdoors but a greater range
of temperature readings indoors, suggesting that the comfortable range of indoor
humidity is not due to higher temperature alone.
When the temperature in the Cabin dropped to its lowest point of 6.5oC, the relative
humidity indoors was approximately 60%. When the outside temperature was at
6.5oC, the humidity outdoors was mostly in the range 86% to 96%. There is
obviously more moisture present in outdoor air but it is difficult to analyse the data in
great detail as there is no information about whether the occupants were in or out,
when they cooked, showered, opened the windows or switched the heating on and
there is no precise weather data available for the site.
18 November 2007
By focussing on a shorter period it may be possible to reduce the variable factors. On
the evening that the highest indoor temperature was recorded (17th 18th November
2007) it seems reasonable to assume that the two occupants turned off the heating
and retired to bed around midnight. During the next seven hours there would be no
further moisture added to the Cabin other than from the breathing of the occupants in
the room next door and from the infiltration of fresh air (which was found to be low
58
Carol Atkinson – MSc Architecture: AEES
January 2008
in the air tightness test in section 5.1). Given the data, it is very unlikely that the
occupants opened the windows during the night.
60
50
%
40
30
20
10
06:50:00
06:25:00
06:00:00
05:35:00
05:10:00
04:45:00
04:20:00
03:55:00
03:30:00
03:05:00
02:40:00
02:15:00
01:50:00
01:25:00
01:00:00
00:35:00
00:10:00
0
Figure 5.6 Relative humidity (pink line) and temperature (blue line) recorded in the Straw Bale Cabin
in the early hours of 18th November 2007.
At midnight the indoor temperature was 31oC and the relative humidity was 35%.
The changes thereafter are practically a mirror image; after one hour the indoor
temperature dropped 8.5oC to 22.5oC and the relative humidity increased 8% to 43%,
after two hours the temperature had dropped a further 2oC and the humidity had
increased a further 2.5% and the hourly changes after that reduced to one point or
less (Fig 5.7).
Knowing the temperature and relative humidity, the moisture content of the air in the
Cabin that night can be read off a psychometric chart* (McMullan, 2002 page 105);
Change
+8%
+2.5%
+1%
+1%
+0.5%
RH
35%
43%
45.5%
46.5%
47.5%
47.5%
47.5%
48%
Time
Temperature
00:10
01:10
02:10
03:10
04:10
05:10
06:10
07:10
o
31 C
22.5oC
20.5oC
19.5oC
18.5oC
18oC
17oC
16.5oC
Change
o
-8.5 C
-2oC
-1oC
-1oC
-1oC
-0.5oC
-1oC
-0.5oC
Moisture
content
Kg/kg dry air
-
0.0077
0.0069
0.0066
0.0064
0.0062
0.0055
0.0054
Figure 5.7 Relative humidity and temperature recorded in the Straw Bale Cabin in the early hours of
18 November 2007. The moisture content in the final column is estimated from the psychometric
chart on page 105 of McMullan, 2002.
As the temperature falls, the relative humidity is expected to rise – colder air holds
less moisture, so if the amount of moisture remains the same, the degree of saturation
will rise. However, if the moisture content of the air in the Cabin remained at 0.0077
kg/kg (as it was at 01:10am) then the relative humidity at 07:10am should have been
59
Carol Atkinson – MSc Architecture: AEES
January 2008
over 60% (reading from the chart) but in the Cabin it is only 48%. The moisture
seems to be going somewhere - the clay on the walls and ceiling could be the store.
However, the findings in this test could be limited by the accuracy
• of the data loggers
• in reading off the psychometric chart
• of the chart itself or the reproduction of it
Further analysis, repeat tests and greater knowledge of variables is required before
reaching firm conclusions.
15-17 October 2007
The graphs (Fig 5.8) and table (Fig 5.9) that follow include data captured in a similar
sized structure – a static caravan (Appendix E) – also unoccupied at the time.
25
temperature oC
20
cabin
15
outside
10
caravan
5
0
relative humidity %
100
90
80
cabin
70
outside
60
caravan
50
20:15:00
15:00:00
09:45:00
04:30:00
23:15:00
18:00:00
12:45:00
07:30:00
02:15:00
21:00:00
15:45:00
10:30:00
05:15:00
00:00:00
40
Figure 5.8 Temperature and relative humidity readings from inside the unoccupied Straw Bale Cabin,
outside the Cabin and inside an unoccupied static caravan for 72 hours on 15th, 16th and 17th October
2007.
In both the Cabin and the caravan, temperature and humidity are linked – when the
temperature rises, the relative humidity falls (Fig 5.8). As there are no occupants
adding moisture to the air, the same amount of moisture causes higher humidity at
60
Carol Atkinson – MSc Architecture: AEES
January 2008
lower temperatures. However, the graphs (Fig 5.8) illustrate how both temperature
and humidity are moderated in the Cabin compared to the caravan.
Temperature
18.5oC
17.0oC
15.0oC
Relative humidity
Straw Bale Cabin
63%
65%
64%
Relative humidity
Outside
63%
68%
73%
Relative humidity
Caravan
70%
70%
75%
Figure 5.9 Comparison of relative humidity readings from inside the unoccupied Straw Bale Cabin,
outside the Cabin and inside an unoccupied static caravan on 15th, 16th and 17th October 2007 at a
temperatures experienced in all three places.
At the same temperatures the relative humidity in the Cabin is 5 – 11% lower than in
the caravan (Fig 5.9). Again, this suggests that the clay surfaces inside the Cabin may
be regulating the humidity levels.
The caravan walls are made from non hygroscopic materials and anecdotal evidence
suggests that caravans are notorious for condensation* problems.
Limitations and further work
The findings in the Cabin seem to agree with Morton’s research as quoted at the
beginning of this section – “ ….earth materials demonstrate an ability to absorb and
desorb atmospheric moisture……”
However, further work is needed before any firm conclusions can be drawn. In
addition to the requirement for more detailed weather and occupancy information
mentioned earlier, the research would benefit from
• comparison over a much longer period
• additional loggers at different heights and in different parts of the buildings
• comparison with a wider range of buildings and internal finishes
• measurements in the walls themselves (to monitor moisture being absorbed
and desorbed)
• an analysis of the effect, if any, of the primer and paint used on the Cabin
walls
Absorbent clay walls may save energy if they reduce the need for mechanical fans
but further research here must be linked to that needed on the effect of moisture on
the insulation value of the straw bale wall (Chapter 3).
Summary of main points from Chapter 5
•
Heat loss can be significant from “leaky” buildings but the Straw Bale Cabin
is a relatively air tight building and little heat is lost this way.
•
Clay plaster on straw bale walls appears to regulate indoor humidity levels to
provide a healthy indoor environment.
•
Further research is needed to establish whether or not the ability of clay to
absorb moisture should be used to reduce the energy requirement for
mechanical ventilation.
61
Carol Atkinson – MSc Architecture: AEES
January 2008
6. Further data
The three previous chapters have concentrated on the separate theories behind heat
retention in buildings, illustrated where appropriate by data from the Straw Bale
Cabin.
An enormous amount of data was collected during the monitoring period. Time and
space restraints mean it is impossible to include everything but further studies are
presented in this chapter and also in Appendices F, G, H and I.
Information about the Lascar data loggers, their calibration and how they were
positioned for this research can be found in Appendix D.
6.1 Autumn/Winter
(a) October 13 – 18
The Cabin was unoccupied from the morning of 13 October 2007 until the early
afternoon of the 18th. Three data loggers were placed as described in Appendix D
(Fig 6.1).
Figure 6.1 Position of data loggers on 13th to 18th October 2007.
Inside the Cabin all curtains were open, all windows were closed (but the trickle
vents were open) and both internal doors were open. All heating and other electrical
equipment was turned off and the weather was as noted in Figure 6.2.
Date
13th October 2007
14th October 2007
15th October 2007
16th October 2007
17th October 2007
18th October 2007
Weather
Warm with drizzly rain
Warm with drizzly rain
Bright and dry with a light breeze
Sunny and windy (light rain later on)
Sunny and windy
Early frost then sunny
Figure 6.2 Notes from the weather diary.
62
Carol Atkinson – MSc Architecture: AEES
January 2008
25
oC
20
A - inside
15
B - outside (nth)
10
C - outside (sth)
5
04:45:00
18:30:00
08:15:00
22:00:00
11:45:00
01:30:00
15:15:00
05:00:00
18:45:00
08:30:00
22:15:00
12:00:00
0
Figure 6.3 Temperatures recorded at five minute intervals from noon on 13th October to 14:30pm on
the 18th October 2007.
Looking at Figure 6.3 above, the sunny days are highlighted where the yellow south
curve (C) exceeds the pink north curve (B). Cloud cover on the first 2 days (13th &
14th) kept the north and south external temperatures mostly between 11.5oC and
16oC. On the sunny days, however, the south logger (C) recorded temperatures up to
18oC when the north logger (B) was only reached 13oC. Both plunged to a low of
1oC during the last night.
As there was no solar gain on the 13th and 14th, the temperature inside the cabin
dropped 4oC at a relatively even rate from 20.5oC at 1pm on the 13th to 16.5oC at
9am on the 15th. The average external temperature over this 44 hour period was
13.5oC.
Solar gain on 15th October raised the internal temperature by 2oC. The gain on the
16th was 1.5oC and 3.5oC on the 17th when, looking at the steep rise of the yellow
curve, the sun came out much earlier in the day and stayed out for longer than on the
previous two days.
On the 17th, it took 4 hours for the temperature inside the Cabin to rise by 3.5oC
(approximately 8am – 12 noon). The peak temperature (17.5oc) was then maintained
for 4.5 hours. The solar heat gain was lost over the next 7 hours as external
temperatures plunged during the night. If the Cabin was in use, the occupants could
have closed the curtains to retain the solar gains for longer.
The temperature drops overnight are analysed in Figure 6.4 below. A similar diagram
can be found in Appendix G where overnight temperature drops were slower when
heat loss through the windows was reduced.
63
Carol Atkinson – MSc Architecture: AEES
January 2008
Figure 6.4 Diagrammatic representation of the heat loss from the Straw Bale Cabin on 16th – 17th
October 2007. The peak external temperature on 16th October was 16oC at 2pm. Over the next 17
hours the temperature dropped to 5oC at 7am on 17th October. The internal temperature over this time
dropped only 2.5oC degrees from 17.5oC to 15oC. The peak external temperature on 17th October was
16oC at 3:30 pm. Over the next 16.5 hours the temperature dropped 14.5oC to 1.5oC at 8am on 18th
October. The internal temperature over this time dropped 4.5 degrees from 17.5oC to 13oC.
Summary of observations from this study
• The indoor temperature was always higher than the outdoor temperature.
• Heat loss from the cabin was gradual despite rapid temperature drops
outdoors.
• Heat loss would have been less if the building was occupied and the
occupants took measures to prevent the greatest heat loss (ie. close the
curtains).
• Solar gain on sunny days in October raised the internal temperature by 1.5 to
3.5oC.
• The external temperature on the north of the building could be significantly
different to that on the south of the building when the sun was shining.
.
(b) October 18 - 19
The occupancy pattern of The Straw Bale Cabin was monitored from 2pm on
Thursday 18th October until 4pm on Friday 19th October. The weather on both days
was sunny following a frosty start. Seven data loggers were set up; three in the usual
positions (Fig 6.1) plus two extra directly below the logger suspended 2.5m from the
lounge beam at 1.5m and 0.5m from floor level. A further logger was hung at 1.5m
from the floor level but closer to the sough facing French doors and a logger was
hung in the bedroom at 2.0m above floor level in the centre of the room.
64
Carol Atkinson – MSc Architecture: AEES
January 2008
Figure 6.5 Temperatures recorded at five minute intervals from 3:15pm on 18th October to 6:00pm on
the 19th October 2007.
The following table (Fig 6.6) explains the temperatures in relation to the occupant’s
diary. The letter in the first column relates to the points highlighted on the graph
above (Fig 6.5).
A
B
C
D
E
F
G
The logger 0.5m from the floor rose sharply to 20oC as the sun shone in through
the west window and warmed this part of the room. Looking over to the right of
the graph, exactly the same thing happened at the same time on the following
day, except that the temperature rose to 25oC (yellow curve – Fig 6.5).
The occupant went into the Cabin at 5.30pm when the 1.5m temperature was
15oC. She closed the curtains, put the lights on, cooked poached eggs on toast,
washed up and was sat at her laptop by 6.30pm when the 1.5m temperature had
risen to 16.5oC due to heat generated mainly by cooking.
At 7:20pm when the occupant turned on a 2 kW convector heater, the 1.5m
temperature had risen to 17oC.
At 9:20pm the heater was turned off. The 1.5m temperature was 26oC. A 2kW
heater for 2 hours (4 kWh) had increased the temperature by 9oC. Most of the
time the loggers at different heights have recorded very similar temperatures but
at the higher temperatures caused by heating, thermal stratification was more
evident – the 2.5m temperature reached 29oC and the 0.5m recorded only 21oC.
At midnight the occupant opened the bedroom door and climbed into bed,
leaving the door open. This caused the temperature in the bedroom to rise from
14oC to 16oC and the rate of temperature decline in the living room increased
slightly as the heat escaped to the bedroom.
It was 3:50am before the 1.5m temperature fell below the pre heating
temperature of the night before. The 9oC temperature rise created by 2 hours
heating was lost in 6.5 hours (but the indoor space was “increased” with the
opening of the bedroom door).
When the occupant got out of bed at 7:50am the internal temperatures recorded
by all internal loggers were all 15-16oC. The outside north logger read 0oC and
the outside south logger read 1oC. Opening the curtains revealed a bright sunny
65
Carol Atkinson – MSc Architecture: AEES
January 2008
morning with frost on the grass.
H
In an hour, the external south temperature had risen to 5.5oC. In the next hour
(9:50am) it had risen to 10.5oC and so on until its peak of 20.5oC at 13:15pm. At
10am the occupant had noticed the sun reaching the back wall of the Cabin. At
13:15pm the 1.5m logger had reached 20oC (the 1.5m logger nearer the French
doors read 21oC – presumably it was benefiting from ) The external logger on
the north side of the building peaked at 13oC.
Figure 6.6 Explanation of the temperature changes graphed in Figure 6.5.
Summary of observations from this study
• Stratification was evident at high temperatures but below 20oC the
temperature was similar at all levels. This could be a very useful design
feature – excess heat rises out of the reach of occupants but “comfortable”
heat does not.
• No heating was required during the day – 15-20oC was comfortable for this
seated occupant.
6.2 Spring
Comparing buildings is difficult. They are different sizes, made of different
materials, located in different places and occupied in different ways. There is added
difficulty in the heating season as each building is likely to have different heating
equipment delivering different levels of heat to different parts of the building.
In spring 2007, a series of short studies was set up to compare the heating of the
Cabin and two other holiday homes; the static caravan (Appendix E) and Eco Lodge
- a timber framed holiday unit built with renewable materials.
Further details of Eco Lodge and the study can be found in Appendix F. As Eco
Lodge is twice the size of the Cabin and the caravan it is difficult to make
meaningful comparisons. However, as expected the Straw Bale Cabin retained heat
for longer than both other buildings.
Studies such as this are useful for observing the behaviour of the individual building
and its different features.
Summary of observations from this study
• It can be difficult to make comparisons between buildings
• The Cabin had superior heat retention properties
6.3 Summer
The use of air conditioning is rising 8% annually in the UK. Sales of domestic air
conditioning units have risen 27% since 1996 (Concrete Centre, 2006 introduction).
66
Carol Atkinson – MSc Architecture: AEES
January 2008
If climate change results in rising temperatures this increasing energy problem could
be exacerbated if buildings are not designed to stay cool without the need for air
conditioning. Can buildings that stay warm in winter also stay cool in summer?
(a) Unoccupied days
From 7th to 10th August, both the Straw Bale Cabin and the static caravan (Appendix
E) were unoccupied. Data loggers were placed over the summer as follows;
A - Inside the Cabin from the lounge beam (2.5m above floor level)
B - Inside the caravan from a light fitting (2m above floor level)
C - Underneath the north west corner of the Cabin
D - Under the Cabin’s south facing porch
All windows were closed. The curtains in the Cabin were open and those in the
caravan were pulled across.
The weather diary noted that all 3 unoccupied days were sunny, dry and warm.
Readings were taken at 5 minute intervals. The 865 readings taken from noon on 7th
August to noon on 10th August are graphed below in Figure 6.7.
Unoccupied 7- 10 August 2007
35
30
25
20
15
10
5
0
A - straw
B - caravan
C -outside nth
07:10:00
02:00:00
20:50:00
15:40:00
10:30:00
05:20:00
00:10:00
19:00:00
13:50:00
08:40:00
03:30:00
22:20:00
17:10:00
12:00:00
D - outside sth
Figure 6.7 Temperature readings (oC) at the Straw Bale Cabin and the static caravan on 7th – 10th
August 2007.
The curves for the two outside loggers (Fig 6.7) illustrate the typical UK diurnal
temperature swings; warm during the day and cool in the evening.
Outside
Both outside loggers (C and D) were shaded from direct sunlight but D recorded
higher temperatures. The sun will have warmed the south facing air, walls and
decking under logger D. The maximum temperature recorded by logger D over the 3
days was 28.5oC at approximately 1pm on 9th August. Logger C at this point
recorded a temperature 11 degrees cooler due to the shade provided by the building.
The highest daytime temperature recorded by logger C underneath the Straw Bale
Cabin was 20oC at around 5pm as the north west corner of the Cabin was warmed by
67
Carol Atkinson – MSc Architecture: AEES
January 2008
the late afternoon sun. Each day from about 7pm until 7am, both outside loggers
recorded very similar temperatures. Lowest temperatures were 8 to 9oC recorded
from about 5am to 6am daily.
Caravan
The temperature curve inside the static caravan (logger B) follows a similar pattern
to that of the south facing outside logger (D), although higher temperatures were
reached inside the caravan. The highest temperature recorded by logger B on 7th
August was 29.5oC, just after 4pm. The highest temperature outside the Cabin (D)
earlier that day was 24.5oC – 5 degrees cooler. On the afternoon of 9th August,
temperatures inside the caravan peaked at 33oC (B) where as outside temperatures
had peaked at 28oC – again 5 degrees cooler.
The south facing logger (D) is only shaded from the sun’s rays by the thin wooden
roof of the porch but it is well ventilated to the east, west and south so that any warm
air gathered there can quickly dissipate. The caravan, however, receives the full glare
of the high summer sun. Heat passes in through the thin fabric of the unoccupied,
closed caravan from where it cannot escape until the outside air temperature falls in
the evening. At peak temperature on 9th August, it was over 11 degrees cooler
underneath the Straw Bale Cabin than it was inside the static caravan.
Throughout the night the caravan stays a relatively constant 5 degrees warmer than
the outside air temperature. There seems to be a time lag of approximately 5 hours; at
7pm on 9th August the outside temperature has dropped to 19oC. The internal
temperature drops to 19oC just after midnight on 10th August.
The temperature inside the caravan (B) did not exceed the outside temperature (D) on
8th August. Perhaps there was localised cloud cover at the caravan (8 miles from the
Cabin) on that day?
Straw Bale Cabin
The temperature curve for the Cabin (A) is much flatter than the other three. The
extremes of warm and cool have been moderated. There is only a 5oC temperature
swing experienced inside the Cabin as opposed to 21oC in the caravan and outside (D
- south) and 12.5oC outside (B - north).
Highest temperature
Lowest temperature
Temperature difference
Average temperature
Straw Cabin
A
23
18
5
20
Caravan
B
33
12
21
20
Outside (nth)
C
20
7.5
12.5
14
Outside (sth)
D
28.5
8
20.5
17
Figure 6.8 Highest lowest and average temperatures recorded on 7th – 10th August 2007.
Although a porch shades the entire south façade, there is some degree of solar gain
into the building through the lower parts of the fully glazed doors and through the
un-shaded west facing window. The small temperature rise inside the cabin during
the day is attributable to these gains.
68
Carol Atkinson – MSc Architecture: AEES
January 2008
At peak temperature, it is 3 degrees cooler under the north west corner of the Cabin
than inside the building - probably due to shading and ventilation underneath.
However, the night time temperatures are over 10 degrees warmer inside the cabin.
Over this three day period the temperatures inside the Cabin did not pass outside the
comfort zone of 18 to 25oC (Appendix A). However, temperatures in the caravan
were outside of the zone for 65% of the time.
However, a building is a human shelter - it is not meant to be empty. Presumably,
when the caravan is occupied, the residents will take measures to make the
temperatures more comfortable – such as opening doors and windows to allow a
cooling breeze. The above data is useful for comparing the performance of the
building fabric because the occupancy patterns are identical (both empty). What
happens during periods of occupancy?
Summary of observations from this study
• The temperatures inside the Cabin are moderated and lower than those
recorded outdoors on the south of the building, helped by the shade of the
porch
(b) Occupied days
Data from an occupied period at the Cabin and the caravan are presented in
Appendix H.
When the caravan is occupied the temperature can be kept similar to the outside
temperature in the heat of the day by opening doors and windows. At no point was it
cooler inside than outside.
(c) Other buildings
The caravan and the Cabin have been compared because they are similar sized
structures in a similar geographical location. Data loggers were also placed in Eco
Lodge (Appendix F) and the farmhouse at Village Farm (location of the Cabin) over
the summer.
The farm house is of solid brick construction with small single glazed windows. The
room where the logger was suspended is south facing.
These buildings are all occupied to some extent and no detailed occupancy records
were kept. The loggers had to be placed discreetly – not always in ideal places for
accurate recording.
The week from 3rd to 10th August has been selected for closer examination as the
buildings were all mostly unoccupied during this period.
With the exception of the caravan, all buildings seemed to keep relatively cool over
the summer. There could be a number of explanations for this;
69
Carol Atkinson – MSc Architecture: AEES
Building
Straw Bale Cabin
Village Farm house
Eco lodge
January 2008
Strategy
Shading porch over south facing glazing
High level of insulation
Sufficient thermal mass
Thick walls
High level ceiling
Small south facing window
Infiltration (single glazing, no draught proofing)
No insulation
Thermal mass (except that the room is wall papered and
carpeted)
Small south facing double glazed window
Built in woodland so shaded by trees
Medium level of insulation
Figure 6.9 Cooling strategies in a variety of buildings.
There are a great many limitations to this study such as
•
•
•
not enough loggers to measure external temperatures at each location (but
they vary considerably at different points at the same location anyway),
no detailed construction information, and
no occupancy records
but again, the valuable snap shot of information provides useful indicators.
Summer 2007 in East Yorkshire was not particularly hot. If global warming results in
much higher temperatures, it would be hard to predict from the data obtained, how
these buildings might perform. If high temperatures over a prolonged period became
the norm, again there is insufficient data here to make accurate predictions. In 2007
all buildings except the caravan dealt adequately with summer temperatures.
It seems that materials (providing mass and/or insulation) may not be enough on their
own and other strategies (blinds/shades/shutters/ventilation/high ceiling) are required
to protect from summer overheating. A building can only stay cool in summer if
ventilation and direct solar gain are controlled.
The less insulated buildings such as the caravan and the old farmhouse are
automatically cooler at night without any need for ventilation but they more heat in
winter and have a longer heating season.
Limitations
• The buildings compared were not always on same site and will have
experienced differing micro climates (though not too far apart)
• The buildings are not orientated the same (although caravan is better
positioned to avoid summer gains but still performed poorly)
• The buildings were very different (fabric, ceiling height, shading,
furnishings)
• Occupancy patterns were unknown (but that is often the case with post
occupancy studies – life goes on around research)
• Internal loggers positions could not be uniform
70
Carol Atkinson – MSc Architecture: AEES
•
•
January 2008
There were limited external measurements (full sun/full weather data/micro
climates)
Equipment failed
Main summary of points from Chapter 6
•
The temperatures in the Cabin are moderated in winter and summer. It
appears to retain heat in winter and keep cool in summer, although the
shading of the porch plays a large part in keeping summer temperatures low.
•
It is very difficult to compare buildings – no two are alike making it difficult
to isolate the effect of different factors and the behaviour of occupants.
Variation can be found in the fabric (walls, floors, windows, roof – and
variations in each of these), the climate (macro, micro, internal, orientation,
future changes) and in occupancy and comfort preferences.
71
Carol Atkinson – MSc Architecture: AEES
January 2008
7. Conclusion
Energy
“Peak oil” will inevitably drive up energy prices and the need for increased energy
efficiency will become obvious to all. Reducing carbon emissions to mitigate climate
change is a more urgent problem but not so obvious to people in their every day
lives. Whilst energy is predominantly fossil fuel derived, any efficiency in its use
will also reduce carbon emissions.
More efficient buildings are needed to reduce the high energy demand for space
heating. Straw bale walls could be an important part of those buildings – they tick all
the energy saving boxes;
•
Insulation
Standard agricultural straw bales provide a level of insulation twice that
required by current building regulations – an immediate 50% energy saving.
Further development could result in greater savings.
•
Heat storage
When plastered with clay or lime, straw bale walls can reduce the need for
heating and cooling by moderating indoor temperatures. Currently there are
no regulations regarding thermal capacity.
•
Air tightness
Plastered straw bale walls are inherently air tight – the Straw Bale Cabin is
six times more air tight than current building regulations require – over 80%
less heat is escaping through unwanted cracks and gaps.
At the same time, straw bales can significantly reduce the amount of energy required
to build a wall – perhaps by as much as 90%.
Most of these savings are well in excess of the Government’s call for a 60% cut in
emissions. The added bonus is that every 10kg of straw absorbs nearly 14kg of
carbon dioxide as it grows, sequestering it in the walls for the lifetime of the
building.
There may still be some way to go before straw is widely accepted as a construction
material but the energy and carbon saving facts are compelling.
A level playing field
What has become very clear from this research is that there must be a greater
understanding of the thermal performance of all building materials – both as
individual materials and, more importantly, as part of a finished wall. A new
assessment of heat retention is needed that can encompass the combined effect of
insulation (covering all mechanisms of heat transfer), thermal capacity, thickness and
air tightness on the overall performance of the wall. Until this happens, there is no
fair comparison and large energy savings can not be made.
72
Carol Atkinson – MSc Architecture: AEES
January 2008
Straw bales tick all the energy saving boxes only when combined with plaster. The
plaster reduces air movement in and through the wall and acts as a temporary store of
heat. Straw bale researchers tested their wall complete and found that reality didn’t
quite match theory because theory does not take account of convection losses.
Research into other walls found that most of them suffered significant convection
losses. However, even though this research is twenty years old, standards still do not
reflect this.
Research from the Straw Bale Cabin suggests that its actual thermal performance is
better than theory suggests but further research is required to verify this.
A huge culture shift is needed. Climate change and an end to cheap and abundant
energy supplies mean that it is important to understand the dynamic thermal
performance of the building fabric.
More than just walls
Whilst the straw bale walls of the Cabin far exceed current building regulations, the
roof and floor do not, highlighting the need for care in all elements of the building.
Location and orientation are also important; the Cabin is located in a sheltered spot,
protected from prevailing winds. In facing south it receives the full benefit of the
sun’s warmth whilst being protected from excessive summer heat by the porch.
The Cabin also illustrates perfectly how, as thermal performance improves, heating
systems must be increasingly responsive. They must efficiently deliver only the heat
required. Calculation methods must be greatly improved so that heating plant can be
appropriately sized. Currently calculations are based on the flawed theories discussed
above.
The long term
A future proofed building is difficult to achieve when it is uncertain whether the
climate will be warmer, colder, wetter, windier or any combination of these. Well
designed and executed to meet the worst case scenario, a straw bale home would
have long lasting financial and environmental benefits. High levels of insulation
would ensure it could be warm in winter and cool in summer. Appropriately
designed it may withstand high winds, driving rain and flooding as well as any other
building.
In other countries straw bale buildings have proved to be durable. Whether they will
in the UK climate remains to be seen. However, as they require very little energy to
build and harmlessly biodegrade at the end of their useful life, the environment has
nothing to loose.
The Straw Bale Cabin is a perfect example of pre-fabrication; a home that can be
built to exacting standards and transported to site. Much more environmental damage
will be caused while building regulations reach the standard necessary to
significantly reduce energy demand. In the meantime perhaps every new home
should be built with straw bales!
73
Carol Atkinson – MSc Architecture: AEES
January 2008
Further research
All forms of research have their drawbacks; individual theories ignore the many
factors that come into play in a real life situation, computer simulations are often
based on these theories and suffer similar limitations, real buildings (even small
ones) have so many variable factors that it is difficult to isolate the impact of each.
The data collected from the Straw Bale Cabin could now be used to inform and
refine a computer model to gain a greater understanding of how insulation, heat
capacity and air tightness combine to make a building thermally efficient.
Data collection will continue at the Cabin to verify the results obtained in 2007 and
additional testing techniques such as heat flux meters could be deployed to increase
the range of data.
74
Carol Atkinson – MSc Architecture: AEES
January 2008
Appendix A
Basic Principals of heat transfer
In the UK the tendency for heat transfer is illustrated in Figure A.1.
Higher temperatures
Winter
Inside buildings (from heating)
Summer
Surroundings (from solar
radiation)
Heat transfer
Lower temperatures
Surroundings
Inside buildings
Figure A.1 Heat transfer in the UK.
The main principles are that;
•
•
heat flows from objects at high temperature to objects at low temperature
until they reach the same temperature
heat energy will tend to the path of least resistance
Heat transfers mainly by means of
•
•
•
Conduction
Convection
Radiation
Conduction is the transfer of heat energy through a material (solid, liquid or gas)
without the molecules of the material changing their basic positions. Different
materials conduct heat at different rates. Thermal conductivity (λ) is measured as the
heat flow, in watts (W), across a thickness of 1 metre of material for a temperature of
1 degree Kelvin and a surface area of 1 m2 (McMullan, 2002).
Metals are the best conductors of heat because the free electrons they possess can
move from one molecule to the next. The thermal conductivity of copper for example
is 160 W/mK. Materials with low thermal conductivity are good insulators and are
used to control heat loss by conduction through the fabric of a building. For example,
timber has a thermal conductivity of 0.13 W/mK and sheep’s wool 0.039 W/mK. 19
In theory straw is a good insulator because it contains trapped, still air which has a
very low thermal conductivity of 0.02 W/mK.
Conduction is the dominant mechanism of heat loss in a well sealed building.
Convection is the transfer of heat through a liquid or a gas (never solids) by the
movement of particles. When air or water is warmed it expands, becomes less dense
and rises above the cooler fluid (McMullan, 2002).
19
www.secondnature.co.uk
75
Carol Atkinson – MSc Architecture: AEES
January 2008
Air and moisture are present in straw bale walls and if allowed to move, could
transfer heat by convection. Movement may occur if the walls are not plastered,
densely baled or tightly stacked with all joints well stuffed.
Convection is the dominant mechanism of heat loss in a draughty building.
Radiation is the transfer of heat through space, by electromagnetic waves. It occurs
when the thermal energy of surface atoms in a material generates electromagnetic
waves in the infra-red range. Rough surfaces present a larger total area and absorb or
emit more heat than smooth, polished surfaces. Dark surfaces absorb or emit more
heat than shiny silver ones (McMullan, 2002 page 23).
High temperature bodies (such as the sun) emit a larger proportion of shorter
wavelengths and these have a better penetration than longer wavelengths (McMullan,
2002 page 24).
The surface of building materials absorb high temperature solar (short wave)
radiation and emit lower temperature (long wave) radiation. Absorption and emission
factors are measured relative to the perfect adsorber and emitter – the “black body”.
The colour of a building has an important effect on the heat absorbed by the building
from the sun (high temperature radiation) but has little effect on the heat emitted
from buildings (low temperature radiation) (McMullan, 2002 page 42).
Evaporation
Heat can also be transferred by evaporation when latent heat is absorbed by a vapour
in one place and released on condensation elsewhere (McMullan, 2002 page 22).
Basic principals of thermal comfort
Thermal comfort is defined by ISO 7730 as “that state of mind which expresses
satisfaction with the thermal environment”. It varies from person to person. In any
particular thermal environment it is difficult to get more than 50% of people to agree
that the conditions are comfortable (McMullan, 2002 page 65).
The human body constantly produces heat from the food energy it consumes. This
heat needs to be dissipated at an appropriate rate to maintain the body at a constant
37oC – by convection, radiation and evaporation (perspiration and respiration). Heat
produced varies according to
•
•
•
age (rate of heat emission decreases with age)
sex (adult females generally output 85% less heat than males)
activity (an adult male can emit 70W while sleeping, 140W working in an
office and 440W while doing lifting work)
Thermal comfort can also be affected by clothing, air temperature, surface
temperature, air movement and humidity.
It is possible to adapt to surrounding conditions such as lower temperatures in winter.
76
Carol Atkinson – MSc Architecture: AEES
January 2008
The World Health Organisation specifies a comfortable temperature range of 18oC to
21oC. CIBSE Guide A specifies a winter temperature range of 22oC to 23oC for
living rooms and 23oC to 25oC in summer (CIBSE, 1999).
The English Housing survey reported that the average internal air temperature rose
from 13.8oC to 18oC from 1970 to 2000 (Teather, 2004 page 11).
The Association of EnvironmentConscious Builders 20 recommend an indoor design
temperature of 18oC as opposed 20oC as an energy saving measure.
It seems reasonable to conclude that thermal comfort can be achieved in the range of
18oC to 25oC.
20
www.aecb.co.uk
77
Carol Atkinson – MSc Architecture: AEES
January 2008
Appendix B
DRAWN BY:
Sam
Atkinson
DATE:
DES
07.09.07
Figure B.1 East and west elevations of the Straw Bale Cabin.
78
Carol Atkinson – MSc Architecture: AEES
DRAWNBY:
Sam
Atkinson
DATE:
DRAWNBY:
Sam
Atkinson
DATE:
16.10.07
22.10.07
January 2008
DESCRIPTION:
Completedstrawbalecabin.
NorthElevation
DRAWINGNo:
DESCRIPTION:
Completedstrawbalecabin.
SouthElevation
DRAWINGNo:
3
4
SCALE:
1: 75
SCALE:
1: 70
Figure B.2 North and south elevations of the Straw Bale Cabin.
79
Carol Atkinson – MSc Architecture: AEES
January 2008
Appendix C
Chapter 2 examines the embodied energy in the Straw Bale Cabin and a crude
comparison is made with the energy that would be required if the walls were
constructed with materials more commonly used to build UK homes. Figure C.1 and
C.2 calculate the embodied energy in brick and block homes with alternative
insulation materials. The values of energy embodied in the individual materials are
taken from (GBB, 2006 V1 page 277).
Brick & block walls
Bricks
Foamed glass insulation
Lightweight blocks
Gypsum plaster
Total
Average kWh/m3
kWh/m3
m3
Embodied energy
1462
751
600
900
2.50
28.49
1.30
0.20
32.49
3,655.00
21,395.99
780.00
180.00
26,010.99
801
Figure C.1 Embodied energy in the wall of the Straw Bale Cabin if it had been made of bricks and
blocks with foamed glass insulation (ignoring mortar, wall ties and stronger foundations).
Brick & block walls
Bricks
Mineral fibre
Lightweight blocks
Gypsum plaster
Total
Average kWh/m3
kWh/m3
m3
Embodied energy
1462
230
600
900
2.50
28.49
1.30
0.20
32.49
3,655.00
6,552.70
780.00
180.00
11,167.70
344
Figure C.2 Embodied energy in the wall of the Straw Bale Cabin if it had been made of bricks and
blocks with mineral fibre insulation (ignoring mortar, wall ties and stronger foundations).
80
Carol Atkinson – MSc Architecture: AEES
January 2008
Appendix D
Data loggers
The Lascar EL-USB-2 data loggers measure and record temperature, relative
humidity and dew point. They can store 16,382 temperature readings (-35oC to
+80oC) and 16,382 relative humidity (0 - 100%) readings and can be plugged directly
into the USB port of a computer for set up and data download (Lascar, 2007). Data
can be graphed, printed or exported to other applications. The manufacturers 21 state
accuracy of
• Temperature +0.5oC to +2oC
• Relative humidity +3% (20-80% rh)
Throughout this research, each data logger has been set up to measure and record
data at 5 minute intervals, always on the hour and at five minute intervals thereafter.
This allows direct comparison of the loggers at different locations. The loggers can
record at 10 second or 1 minute intervals but this level of accuracy would not add to
the results and would require more frequent data download. They can also record at
30 minute or 1 hour settings but this would not be often enough to record the
temperature changes.
Calibration
The first batch of loggers were calibrated to each other on 4th/5th February 2007.
They were hung for 7 hours in the same place and the recordings were compared.
Most readings were identical but there were occasional ½ degree differences.
Two of these loggers were destroyed through rain ingress and further loggers were
obtained. The new loggers were calibrated to each other on 3rd/5th July 2007 for 47
hours. Again, most readings were identical but there were occasional ½ degree
differences.
At the end of testing for this research, all loggers were calibrated again to ensure that
they were still measuring similarly. In the previous two tests the loggers were hung
indoors where temperatures were stable at around 20oC. This time the loggers were
all hung indoors and outdoors to be tested over wider range of temperatures. The
results remained similar.
Positioning data loggers
Inside
Ideally, the internal temperature would be recorded in the centre of a room – away
from the effects of walls, windows, height variation and heat sources. However, as
the Straw Bale Cabin was mostly in use as holiday accommodation the indoor data
logger had to be placed discreetly out of the way of occupants.
Unless otherwise stated the indoor logger at the Cabin was hung from the beam in
the lounge/kitchen area at a height of 2.5m above floor level (Fig D.1). As warm air
21
www.lascarelectronics.com
81
Carol Atkinson – MSc Architecture: AEES
January 2008
rises, this could have resulted in higher temperature readings than for the lower,
occupied space.
Occasionally loggers were placed centrally and at a variety of heights when specific
tests were being carried out. The temperature readings at other locations did not vary
significantly from those recorded from the beam. Stratification* did not appear to
occur at temperatures below 20oC.
Figure D.1 Lascar data logger in its usual position suspended from the
beam in the lounge/kitchen area of the Straw Bale Cabin.
Outside
In order to establish the most suitable place to position the external logger, a test was
carried out on 21st – 23rd April 2007. Three data loggers were hung outside the
Cabin; one under the south facing porch, one in the open to the south of the Cabin
and one, clear of the building to the north of the Cabin.
30
oC
25
20
open north
15
open south
10
shaded south
5
07:30:00
03:45:00
00:00:00
20:15:00
16:30:00
12:45:00
09:00:00
05:15:00
01:30:00
21:45:00
18:00:00
14:15:00
10:30:00
0
Figure D.2 Temperature recordings outside the Straw Bale Cabin on 21st – 23rd April 2007
During the night the temperatures recorded at all three positions were very similar.
During the day, however, temperatures varied by up to 5.5oC depending on solar
exposure (Fig D.2).
As there were such marked differences in temperature, it was decided (logger failure
permitting) to always use two external loggers – north and south. Ideally these
loggers would have been located in the open but when two unsheltered loggers were
irrecoverably damaged by rain the south logger was kept under the shade of the
82
Carol Atkinson – MSc Architecture: AEES
January 2008
porch (Fig D.4) and the north logger was moved to the protection of the roof
overhang or underneath the Cabin.
30
25
oC
20
shaded north
15
shaded south
10
5
21:00:00
16:15:00
11:30:00
06:45:00
02:00:00
21:15:00
16:30:00
11:45:00
07:00:00
02:15:00
21:30:00
16:45:00
12:00:00
0
Figure D.3 Temperature recordings outside the Straw Bale Cabin on 7th – 9th August 2007.
It is assumed that the temperature rises and falls during the day occur as the sun goes
in and out behind the clouds (Fig D.3).
Having both the north and south temperatures is useful – even though shaded, the
south logger still highlights sunny days. However, as shown in Figure D.2, the
shaded south logger is not recording the full impact of the sun on the building. It may
be 5oc warmer under the porch than on the north of the building (Fig. D.3) but it
could be at least 5oC warmer again in full sun.
Figure D.4 Lascar data logger in its usual position suspended under the
porch along the south facade of the Straw Bale Cabin.
83
Carol Atkinson – MSc Architecture: AEES
January 2008
Appendix E
For comparative purposes, data loggers were placed in a static caravan located on a
holiday park 8 miles away from the Straw Bale Cabin.
There is little information about the construction of this caravan. Despite repeated
requests by the owners, the manufacturers did not forward a copy of the missing
owner’s manual.
Figure E.1 North elevation of the Willerby Lyndhurst static caravan in which data logging was
carried out.
The caravan is a “Willerby Lyndhurst” manufactured in nearby Hull in 2000. Its
external dimensions are approximately 11.3m by 3.7m. The internal dimensions are
approximately 11.2m by 3.6m and the internal floor to ceiling height is 2.4m at the
centre of the caravan and 2.1 m at the edges.
The accommodation comprises 2 bedrooms, bathroom, lounge/kitchen area and
entrance hall.
The lounge window shown in Figure E.1 is north facing. The caravan is adjoined by
another caravan to the east (visible on the left of Fig E.1), a fence and caravan to the
west and farm buildings to the south.
The caravan has gas central heating and is occupied as weekend and holiday
accommodation by the same family from 1st March to 31st December. During
unoccupied periods the heating thermostat is set at 5oC to prevent frost damage to the
plumbing.
84
Carol Atkinson – MSc Architecture: AEES
January 2008
Appendix F
In spring 2007, a series of short studies was set up to compare the heating of the
Cabin and two other holiday homes; the static caravan (Appendix E) and Eco Lodge
(Fig F.1) - a timber framed holiday lodge built by environment aware owners with
renewable materials to a higher than usual standard for holiday accommodation.
Figure F.1 Eco Lodge (Oak Cabin) at Flaxton, near York. The photograph on the left is the east
elevation and the photograph on the right is the south elevation.
Eco lodge is a timber framed, timber clad building with natural fibre insulation and
plasterboard and timber internal finishes. The internal dimensions are approximately
10m by 6m and the internal floor to ceiling height is 3.2m at the centre of the caravan
and 2.1m at the edges.
There are four Eco lodges bordering a fishing lake in woodland. The accommodation
comprises 2 bedrooms, 2 bathrooms, and lounge/kitchen area. It gas central heating
and is let as holiday accommodation throughout the year. 22
In March and April 2007, a series of short studies was set up to examine the thermal
performance of the three buildings when occupancy and delivered heat were known.
The following factors were the same for each study;
• Heater placed in centre of the room
• Heater on full power (1.5kW) for 2 hours (3 kWh delivered heat)
• Same occupant sits at lap top 1m from the heater
• Wooden stand for logger placed 1m from heater (logger suspended at 2m)
• Curtains open
• No solar gains as test carried put after dark
The approximate building dimensions are tabled in Figure F.2 below.
3
Total internal volume (m )
Highest point (m)
Glazing in test area (m2)
Straw Bale Cabin
81
3.3
3.8
Static caravan
86
2.4
7.5
Eco Lodge
154
3.2
9.1
Figure F.2 The approximate dimensions of the holiday homes.
22
http://www.hoseasons.co.uk/ProductSite.aspx?siteCode=GREA
85
Carol Atkinson – MSc Architecture: AEES
Logging start date
Logging end date
Set up time
Logger start time
Internal temperature
External temperature
Heating start time
Internal temperature
External temperature
After 1 hour
Internal temperature
External temperature
After 2 hours
Internal temperature
External temperature
At 6am next day
Internal temperature
External temperature
At 10am next day
Internal temperature
External temperature
January 2008
Straw Bale Cabin
29th March 2007
30th March 2007
15:50pm
16:30pm
14.5oC
7oC
18:35pm
14oC
6.5oC
Static caravan
28th March 2007
29th March 2007
16:20pm
16:30pm
18oC
17.5oC
19:10pm
14.5oC
6.5oC
Eco Lodge
30th April 2007
1st May 2007
18:00pm
18:00pm
19oC
20oC
19:10pm
16oC
13oC
18oC (+4oC)
6.5oC
18.5oC (+4oC)
5.5oC (-1oC)
18.5oC (+2.5oC)
10.5oC (-2.5oC)
19oC (+1oC)
5oC (-1.5oC)
18.5oC
4 C (-1.5oC)
19oC (+0.5oC)
8.5oC (-2oC)
13.5oC (-5.5oC)
6.5oC (+1.5oC)
7oC (-11.5oC)
4.5oC (+0.5oC)
15oC (-4oC)
5oC (-3.5oC)
13.5oC
7.5oC (+1oC)
7oC
6.5oC (+2oC)
17oC (+2oC)
15.5oC (+10.5oC)
o
Figure F.3 Dates, times and temperatures at the short heating studies in March and April 2007.
The temperature in the Cabin at the start of the heating period was 14oC. It took 9
hours 40 minutes for the temperature to drop back down to this temperature. In the
Eco Lodge it took 6 hours and in the caravan it took 2 hours 45 minutes. The caravan
lost the heat over 3 times faster than the Cabin which retained the heat for the longest
(Fig F.3).
25
20
eco
straw
15
oC
caravan
e-outside
10
s-outside
c-outside
5
480
440
400
360
320
280
240
200
160
120
80
40
0
0
Figure F.4 The first 515 minutes of each study; Eco Lodge on 30th April (eco is the indoor
temperature and e-outside is the outdoor temperature), similarly the Straw Bale Cabin on 29th March
and the caravan on 28th April 2007.
86
Carol Atkinson – MSc Architecture: AEES
January 2008
The Cabin and caravan warmed up at a similar rate; 4oC in first hour. Eco Lodge
only warmed 2.5oC in first hour but a larger space was being heated.
The second hour of heating couldn’t raise the temperature in the caravan any higher.
It was probably losing heat at the same rate as the heating input.
The Eco Lodge had warmed 2oC by 10am in the morning due to solar gain through
its fully glazed east elevation.
The Straw Bale Cabin warmed up at the same rate as the caravan but retained the
heat for considerably longer (7 hours). As the buildings are a similar size and the
external conditions were similar, this can probably attributed to the superior heat
retention qualities of the Cabin
20
18
16
14
12
10
8
6
4
2
0
eco
80
12
0
16
0
20
0
24
0
28
0
32
0
36
0
40
0
44
0
48
0
straw
40
0
oC
Comparing the data from the Straw Bale Cabin and Eco Lodge only (and starting the
Eco Lodge curve at same point) the graph at Figure F.5 results;
Figure F.5 The first 515 minutes from the study of the Straw Bale Cabin and Eco Lodge adjusted to
the same start temperature.
Figure F.5 illustrates how the Straw Bale Cabin warmed up quicker and more (2oC)
than Eco Lodge but as Eco Lodge has twice the internal volume, this is not
surprising. As the Cabin also took longer (over 3 hours) to return to the original
temperature, however, this suggests that the Cabin again has superior heat retention
qualities.
87
Carol Atkinson – MSc Architecture: AEES
January 2008
Appendix G
November 19 – 23
The last of a solid month of visitors left The Straw Bale Cabin before noon on 19
November 2007. The data loggers were set up in their usual places (Fig E.1).
Figure G.1 Loggers in position
In an attempt to eliminate the effect of the windows (solar gain and greatest heat
loss) they were insulated with a crude assembly of duvets, sleeping bags, pillows,
sheets and towels (Fig G.2) but it was not possible to similarly cover the roof light
and main door.
Figure G.2 Window insulated with a duvet to minimise heat loss
The weather over these four days was cold and dismal;
Date
Weather
19th November
20th November
21st November
22nd November
Rain & wind
Dull, rain later
Dull & damp
Dull & damp
Highest temperature
o
C
7
9
10
8.5
Lowest
temperature
o
C
5.5
7.5
5.5
2.5
Figure G.3 Weather and external temperatures on 19th – 22nd November 2007.
88
Carol Atkinson – MSc Architecture: AEES
January 2008
The Cabin was unoccupied throughout and all electrical equipment was turned off.
Therefore there were no solar, casual* or heating gains in the building.
The graph below shows the temperatures recorded from 2pm on 19th November to
7am on 23rd November (1,069 readings at 5 minute intervals).
18
16
14
12
10
8
6
4
2
0
A - inside
C - outside nth
05:00:00
21:45:00
14:30:00
07:15:00
00:00:00
16:45:00
09:30:00
02:15:00
19:00:00
11:45:00
04:30:00
21:15:00
B - outside sth
14:00:00
oC
As there was very little sunshine, the temperature curve for the south external logger
(B) is very similar to that for the north logger (C).
Figure G.4 Temperatures recorded at five minute intervals from 2pm on 19th November to 7am on
22nd November 2007.
Overall review:
At the start the internal temperature was 16.5oC and the external temperature was
7oC – a difference of 9.5oC.
At the end the internal temperature was 8oC and the external temperature was 0.5oC –
a difference of 7.5oC.
Overall, the internal temperature dropped 2 degrees more than the external
temperature but the external temperature had risen during the day.
o
Internal temperature C
External temperature oC
Difference
0 hours
16.5
7
9.5
89 hours
8
0.5
7.5
Temperature drop
8.5
6.5
Over the 89 hours of this test, the average temperature was 12oC inside and 6.5oC
outside – a difference of 5.5oC.
Middle section:
The heat loss from the building shown on the graph by the blue line (A) was fairly
constant except for a 23 hour period from 00:35am until 11:35pm on 21st November
when the internal temperature stabilised at 12oC. The average external temperature at
this time was 8.6oC.
89
Carol Atkinson – MSc Architecture: AEES
January 2008
Two cold nights:
The peak external temperature on 21st November was 10oC at 13:15pm. Over the
next 18 hours the temperature dropped 7.5 degrees to 2.5oC at 7:15 am. The internal
temperature over this time dropped only 1 degree from 12oC to 11oC
The peak external temperature on 22nd November was 8.5oC at 2pm. Over the next
17 hours the temperature dropped 8.5 degrees to 0oC at 7am. The internal
temperature over this time dropped 1.5 degrees from 10.5oC to 8oC.
Figure G.5 Diagrammatic representation of the heat loss from the Straw Bale Cabin on 21st – 23rd
November 2007.
At 4:10am on 23rd November the internal temperature had dropped down to the
external peak of 10.5oC the previous day – a 14 hour time lag.
90
Carol Atkinson – MSc Architecture: AEES
January 2008
Appendix H
The graph below plots the temperatures inside the static caravan and the outside
temperature on the south façade at the Straw Bale Cabin. The caravan’s owners
visited with family for the week beginning 26th August. The weather diary states that
all days were warm and sunny except the 20th – 23rd August which were cool.
Caravan 20 Aug - 1 Sept
40
35
30
25
20
15
10
5
0
outside sth
05:10:00
09:05:00
13:00:00
16:55:00
20:50:00
00:45:00
04:40:00
08:35:00
12:30:00
16:25:00
20:20:00
00:15:00
04:10:00
08:05:00
12:00:00
caravan
Figure H.1 Inside temperatures (oC) at the static caravan (pink line) and outside temperatures under
the south facing porch at the Straw Bale Cabin (blue line).
The temperature inside the caravan follows the outside temperature. The main
differences are
• On days 2, 4 and 5 the peak temperatures inside and out are similar. This is
probably because the doors and windows were open
• On days 3 and 6 the family probably went out for the day.
• Small temperature rises in the early morning are probably due to heating or
cooking. The small rises in the late evening are probably due to lighting,
heating or cooking.
So, it seems that when the caravan is occupied the temperature can be kept similar to
the outside temperature in the heat of the day by opening doors and windows. At no
point was it cooler inside than outside.
The batteries in the data logger in the Straw Bale Cabin failed on 26 August. The
data available is graphed below;
91
Carol Atkinson – MSc Architecture: AEES
January 2008
sbc - 20 - 26 August
35
30
25
20
15
10
5
0
outside sth
09:10:00
23:05:00
13:00:00
02:55:00
16:50:00
06:45:00
20:40:00
10:35:00
00:30:00
14:25:00
04:20:00
18:15:00
08:10:00
22:05:00
12:00:00
straw
Figure H.2 Inside temperatures (oC) at the Straw Bale Cabin (pink line) and outside temperatures
under the south facing porch at the Cabin (blue line).
The Cabin was occupied until the morning of the 24th.
92
Carol Atkinson – MSc Architecture: AEES
January 2008
Appendix I
Imperial and metric conversions
Imperial measurements are widely used in the United States of America. For
conversion to metric as more common in European countries;
R metric – R imperial x 0.1761
U metric = U imperial x 5.678
Therefore;
R imperial = 27.5
R= 1/U
27.5 = 1/U
U imperial = 0.03637
U metric = 0.2065 W/m2K
93
Carol Atkinson – MSc Architecture: AEES
January 2008
8. References
(Andersen, 2004)
Halmhuse, Udformning og materialeegenskaber. By JM Andersen and BM
Andersen. Danish Building and Urban Research, Statens Byggeforskningsinstitut, By
og Byg Resultater 033 Statens, Denmark. Accessed on 7/12/07 at
www.dejsign.dk/cosco/by_og_resultater_033.pdf
(Boardman, 2007)
Home Truths: A low carbon strategy to reduce UK housing emissions by 80% by
2050. By Brenda Boardman. November 2007. University of Oxford’s Environmental
Change Institute. A research report for The Co-operative Bank and Friends of the
Earth. ISBN 1-874370-43-5. Accessed on 21/12/07 at
http://www.eci.ox.ac.uk/research/energy/downloads/boardman07-hometruths.pdf
(Borer, 2005)
The Whole House Book. 2nd edition 2005 By Pat Borer and Cindy Harris. Published
by CAT Publications. ISBN 1-90217-522-0
(Boyle et al, 2003)
Energy Systems and Sustainability – Power for a Sustainable Future. Edited by
Godfrey Boyle, Bob Everett and Janet Ramage. Published by the Oxford University
Press. ISBN 0-19-926179-2
(Boyle, 2004)
Renewable Energy – Power for a Sustainable Future. 2nd edition 2004. Edited by
Godfrey Boyle. Published by the Oxford University Press. ISBN 0-19-926178-4
(BR 443, 2006)
BR 443 - Conventions for U-value Calculations. 2nd Edition 2006. By Brian
Anderson. Published by BRE Press. ISBN 1-86081-924-9
(CIBSE, 1999)
Guide A. October 1999. The Chartered Institute of Building Service Engineers.
London. ISBN 0-900953-96-9.
(Concrete Centre, 2006)
Thermal Mass for Housing: concrete Solutions for the Changing Climate. The
Concrete Centre. First published 2006. ISBN 1-904818-42-0. Accessed on 14/01/08
at
http://www.concretecentre.com/pdf/MB_Thermal%20Mass%20for%20Housing.pdf
(Doran, 2000)
Field Investigations of the thermal performance of Construction Elements as built.
DETR Framework Project Report. BRE client report no. 78132 36/8/79 cc1637.
Prepared by Sean Doran, November 2000 (revised June 2001). Accessed on 5/1/08 at
http://projects.bre.co.uk/uvalues/u-values.pdf
94
Carol Atkinson – MSc Architecture: AEES
January 2008
(DTI, 2006)
Digest of UK energy Statistics 2006. Accessed on 25/11/07 at
http://www.dti.gov.uk/energy/statistics/publications/dukes/page29812.html
(FOE, 2002)
The Good Wood Guide. Edited by Lesley Smeardon. Published by Friends of the
Earth, Fauna & Flora International 2002. ISBN 1-85750-342-2
(GBB, 2006)
The Green Building Bible. 3rd edition Volume 1 & 2. Publishing editor Keith Hall
(Contributory authors specified when relevant) Published by The Green Building
Press. ISNB 1-898130-03-05 (volume 1), 1-898130-04-3 (volume 2)
(Harris, 2005)
Environmental Impacts of Building. By Cindy Harris. Lecture notes from March
2005. MSc Architecture: Advanced Environmental and Energy Studies. University of
East London, School of Computing and Technology, Longbridge Road, Dagenham,
RM8 2AS.
(Howieson, 2005)
Housing and Asthma. By Stirling Howieson. Published in 2005 by Spon Press. ISBN
0-415-33646-5
(Jones, 2002)
Building with Straw Bales: A practical guide for the UK and Ireland. By Barbara
Jones. First published in 2002 by Green Books. ISBN – 1-903998-13-1
(Joyce, 2007)
The Heat Before the Cold. By Terrence Joyce. Woods Hole Oceanographic
Institution. Published in the New York Times. Last updated July 2007. Accessed on
29/12/07 at http://www.whoi.edu/page.do?pid=12455&tid=282&cid=10143
(King, 2006)
Design of Straw Bale Buildings, The State of the Art. 2nd edition 2006. By Bruce
King. Published by Green Building Press. ISBN 978-0-9764911-1-8
(Lascar, 2007)
EL-USB-2 Humidity, Temperatures and dew point USB data logger. Issue 6 04/2007
details accessed on 25/11/07 at
http://www.lascarelectronics.com/PRODUCTS.CFM?area=3&type=DATA%20LOG
GERS&CFID=13306057&CFTOKEN=f61a78fc9d26654d-766076F3-C297-FD96B39B275D8FC48C82
(Little, 2005)
Partial Fill Cavity walls: Have we reached the limits of Technology? By Joseph
Little. Accessed on 15/1/07 at http://www.century.ie/cent/PartialFillCavity.pdf
(McMullan, 2002)
95
Carol Atkinson – MSc Architecture: AEES
January 2008
Environmental Science in Buildings. 5th edition 2002. By Randall McMullan.
Published by Palgrave Macmillan. ISBN 0-333-94771-1
(Minke, 2004)
Building with Straw. Design and Technology of a Sustainable Architecture. 2004. By
Gernot Minke and Friedemann Mahlke. Published by Birkhauser. ISBN 3-76437171-4
(Morton et al, 2005)
Low Cost Earth Bricks in Construction. By Tom Morton, Fionn Stevenson, Bruce
Taylor and Nicholas Charlton Smith. Published by Arc-Architects. ISBN 0-95505800-7. Accessed at http://www.arc-architects.com/research/Earth-Masonry.htm on
7/11/07
(Musset, 2004)
Building with Carbon. By John Musset. Thesis. MSc Architecture: Advanced
Environmental and Energy Studies. University of East London, School of Computing
and Technology, Longbridge Road, Dagenham, RM8 2AS.
(Steen, 2000)
The Beauty of Straw Bale Homes. By Athena and Bill Steen. Published by Chelsea
Green Publishing company. ISBN 1-890132-77-2
(Teather, 2004)
A study of ceramic microsphere insulation with a consideration of the wider
implications. September 2004. By Paul Teather. Thesis. MSc Architecture:
Advanced Environmental and Energy Studies. University of East London, School of
Computing and Technology, Longbridge Road, Dagenham, RM8 2AS.
(Wihan, 2007)
Humidity in Straw Bale Walls and its effect on the Decomposition of Straw. July
2007. By Jakub Wihan. Thesis. MSc Architecture: Advanced Environmental and
Energy Studies. University of East London, School of Computing and Technology,
Longbridge Road, Dagenham, RM8 2AS. Accessed on 8/11/07 at
http://www.jakubwihan.com/pdf/thesis.pdf
(XCO2, 2002)
Insulation for Sustainability: A Guide. A Study by XCO2 Conisbee Ltd for BING
(Federation of European Rigid Polyurethane Foam Associations). Accessed on
16/1/08 at
http://www.xco2.com/downloads/Insulation%20for%20Sustainability_Report%20(fu
ll).pdf
96
Carol Atkinson – MSc Architecture: AEES
January 2008
9. Bibliography
(Haverhill, 2002)
Client report (209-717 Rev.1): Final Report on the Construction of the Hemp Houses
at Haverhill, Suffolk. BRE October 2002. Accessed on 8/11/07at
http://www.projects.bre.co.uk/hemphomes/HempHousesatHaverhillfinal.pdf
(Haverhill, 2003)
Client report (212020): Thermo-graphic Inspection of the Masonry and Hemp
Houses at Haverhill, Suffolk. BRE 17 April 2003. Accessed on 8/11/07at
http://www.suffolkhousing.org/pixs/Thermo%20report.pdf
(Stone, 2003)
Thermal Performance of Straw Bale Wall Systems. By Nehemiah Stone. Oak Ridge
National Laboratories. Accessed on 8/11/07 at
http://www.ecobuildnetwork.org/pdfs/Thermal_properties.pdf
(Straube, 2004)
Monitoring the Hygrothermal Performance of Strawbale Walls. By John Straube and
Chris Schumacher. Accessed on 8/11/07 at
http://www.ecobuildnetwork.org/pdfs/Winery_Monitoring.pdf
(Walker, 2004)
Compression Load Testing Straw Bale walls. By Peter Walker, May 2004. Dept. of
Architecture & Civil Engineering, University of Bath.
97
Carol Atkinson – MSc Architecture: AEES
January 2008
10. Glossary of Terms
Chapter 1
Greenhouse gases
The principal greenhouse gases are water vapour, carbon dioxide and methane. The
natural “greenhouse effect” allows solar radiation to warm the surface of the earth
but inhibits the outflow of infra red radiation, keeping the earth at a habitable,
equilibrium temperature. However, anthropogenic (human induced) greenhouse
gases from the combustion of fossil fuels are trapping more infra red radiation,
causing the mean surface temperature of the earth to rise.
Gulf Stream
The Gulf Stream is one of the strongest ocean currents in the world. It is driven by
surface wind patterns and differences in water density. Surface water in the north
Atlantic is cooled by winds from the Artic. It becomes increasingly salty and dense
as it sinks to the ocean floor. The cold water then moves towards the equator where it
warms slowly. To replace the cold equator-bound water, the Gulf Stream moves
warm water from the Gulf of Mexico north into the Atlantic. The Gulf Stream brings
warmth to the UK raising the temperature about 9oC above the average for this
latitude. (www.bbc.co.uk/climate/impact/gulf_stream.shtml)
Intergovernmental Panel on Climate Change (IPCC)
A scientific intergovernmental body set up by the World Meteorological
Organisation and the United Nations Environment Programme. Hundreds of
scientists from all over the world contribute to the IPCC as authors, contributors and
reviewers. Governments of member countries decide on work programmes and
accept and adopt reports. (www.ipcc.ch)
Peak Oil
Oil is a finite resource. It is not yet running out but it is reaching the point where the
global production rate will “peak” and begin to decline. At this point supplies of oil
will not be able to meet worldwide demand.
Chapter 2
Load bearing method
Also known as the Nebraskan method. The bales themselves take the weight of the
roof, as opposed to the infill method where a timber frame support the roof and the
straw bales provide insulation only.
Bales laid flat
When bales are laid flat their strings are in the centre of the wall. Their longest
dimension is parallel to the wall and their shortest dimension is vertical. The strings
are exposed when bales are stacked on edge and the wall is narrower because the
shortest dimension is horizontal.
98
Carol Atkinson – MSc Architecture: AEES
January 2008
Cut side
Many baling machines are trailed to the side of the tractor so that the driver can see
the straw being fed into the baler. The bales made by these machines are packed
from the side. They have a folded side where the straw is pushed into the bale
chamber and a denser cut side where the sharp knives trim the straw to size.
Baling needle
A large metal needle that can be threaded with baler twine and pushed through straw
bales. One end is bent over to form a handle and the other end is flat and pointed
with two holes so that 2 strings can be pushed through together when a bale is to be
retied and split in half. The needle must be wider than the bales being customised.
Hessian
Strong, coarse fabric made from jute (rough, long fibre made from plants grown
mainly in Bangladesh, India and China).
Low-e
“Low emittance”. A metallic oxide coating applied to the inner face of a double or
triple glazed unit which reduces heat loss through the glass.
Breathable membrane
Thin material used to prevent water entry into a construction while allowing water
vapour to escape.
OSB
Oriented strand board. Engineered sheet material made from long strands of wood
(often spruce or pine) oriented in a random fashion to give maximum strength and
glued together under heat and pressure.
Marmoleum
Floor covering made from linseed and cork with jute backing
Photosynthesis
The process by which green plants use sunlight to form nutrients from carbon
dioxide and water.
Hempcrete
Hemp shiv* is mixed with hydraulic lime to form a bio-composite material which
can be cast in situ for infill walls (Borer, 2005 page 129).
Shiv
Non fibrous inner core of the hemp plant
Chapter 3
Chaff
Husks of grain separated from the seed during harvesting.
99
Carol Atkinson – MSc Architecture: AEES
January 2008
SIP (Structural insulated panel)
An inner and outer layer of sheathing board (typically OSB) is bonded to a core of
rigid insulation such as expanded polystyrene, polyurethane or urethane. Core
thicknesses range from 100mm upwards. SIPs are factory made and delivered to site
as building sections. (Green Building Magazine vol 17 no 1 page 34).
Thermal imaging
A camera sensitive to infra red radiation (heat) produces an image of a wall made up
of different shades. The lighter the shade, the greater the heat loss (GBB, 2006 V2
page 101)
Chapter 5
Relative humidity
Warm air can hold more moisture than cold air. The relative humidity of a sample of
air compares the actual amount of moisture in the air with the maximum amount of
moisture the air can contain at that temperature. A RH of 100% represents fully
saturated air (McMullan, 2002 page 103).
Hygroscopic
Materials that naturally absorb and desorb moisture
Trickle vent
A duct in the head of the window fitted with a plastic damper which can be adjusted
to regulate fresh air entering the building.
Stack ventilation
Ventilation caused by temperature and height differences; less dense, warm air inside
a building rises and leaves the building at high level. This creates a low air pressure
compared to outside and cold fresh air is drawn into the building.
Psychometric chart
A set of graphs combined so that they plot the relationships between the different
variables used to specify humidity (McMullan, 2002 page 105).
Condensation
If moist air comes into contact with a cold surface, the air will be cooled to its dew
point. The air is then saturated, can no longer contain the same amount of water
vapour as before and the excess water vapour condenses to liquid (McMullan, 2002
page 107).
100