Multi-foil Insulation
BD2768
www.communities.gov.uk
community, opportunity, prosperity
Multi-foil Insulation
BD2768
Philip C Eames
June 2009
Department for Communities and Local Government
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Contents | 3
Contents
Executive Summary
5
Chapter 1
Introduction
7
Chapter 2
BR443 conventions for U-value calculations
8
Chapter 3The current scientific opinion of suitable test methods for
thermal conductivity of wall elements
9
Chapter 4
The consultancy responses
13
Chapter 5
The thermal behaviour of multi-foil films
17
Chapter 6
Conclusions
26
Chapter 7
Biography of the author
27
Executive Summary | 5
Executive Summary
Following the consultation1 on proposals for amending the Building Regulations and
Approved Document L Communities and Local Government (CLG) commissioned an
expert review by Professor Philip Eames of Loughborough University of the current
scientific opinion on test methods for determining the thermal performance of building
materials and structures was undertaken. The consultation responses were subsequently
reviewed to establish if they provided new evidence in support of a particular test method
or replayed the earlier debate. A theoretical heat transfer model was developed to enable
predictions to be made of multi-foil and standard insulation performance in an inclined
roof application.
From the review of test methods it was clear that currently two steady state techniques
are widely used and generally accepted for the determination of the thermal performance
of insulation products, the guarded hot plate method, BS EN 12664:2001, BS EN
12667:2001, ASTM C177 and the calibrated and guarded hot box BS EN ISO 8990:1996,
ASTM C1363 – 05. The development of a test protocol for in-situ thermal testing of
building materials is ongoing; however the test protocol is not yet proven or widely
accepted.
An analysis of the consultation responses reveal that:
i) Twelve of the respondents consider that the inclusion of reference to BR443 in
its current form is appropriate since hot box testing is the accepted international
standard, is appropriate for all insulation materials and is essential to maintain
and drive up standards of insulation in buildings, and
ii) Ten believe that testing using hot box techniques is not appropriate for all
building materials and that other test methods for example in-situ test methods
should be allowed. The information presented in the responses appeared to
replay the earlier debate.
Predictions were made with the developed heat transfer model for a typical roof
construction with multi-foil insulation with an unventilated air space either side of it and
with various thicknesses of mineral fibre insulation. The simulations showed that for the
roof constructions simulated the thermal performance of the structure achieved with
a multi-foil insulation with aluminium reflective surfaces, (emittance 0.07) and a core
thermal conductivity of 0.04 was not equivalent to 100mm of mineral fibre insulation.
If the multi-foil insulation had silver reflective films with emittance of 0.02 and aerogel
blanket cores with thermal conductivity of 0.012 W/mK the predicted performance
achieved of the roof structure was better than that with 100mm of mineral fibre insulation
but less than that for 200mm of mineral fibre insulation.
Part L of the Building Regulations – Proposed new editions of the Approved Documents L – Consultation – Communities and Local
Government – June 2008
1
6 | Multi-foil Insulation – BD2768
If an in-situ test methodology can be shown to provide repeatable robust results and
achieve European or other recognised international approval it could be a valuable addition
to current methods. However, based on the analysis of the consultation responses and the
heat transfer model predictions in this report, no new substantive evidence on robustness,
accuracy, and repeatability to support the move from hot box characterisation to in-situ
performance testing is presented.
Chapter 1 Introduction | 7
Chapter 1
Introduction
1.1 The following report was commissioned by the Department for Communities
and Local Government under project contract BD2768 Multi-foil Insulation. The
objectives of the project were to:
• Undertake a literature review of scientific opinion/test methods pre-consultation
and assess whether there is any evidence to counter this in the relevant
consultation responses
• Develop a theoretical heat transfer model to enable the comparison of the
thermal performance of multi-foil insulation with other ‘traditional’ insulation
products
• Review the consultation responses to establish whether they provide any new
evidence or simply replay the pre-consultation debate with no new evidence to
support comparative testing and/or hot box testing.
8 | Multi-foil Insulation – BD2768
Chapter 2
BR443 conventions for U-value
calculations
2.1 BR443 provides guidance on the use of the calculation methods for determining
U-values of building elements based on the standards developed in the European
Committee for Standardisation (CEN) and the International Standards Organisation
(ISO) and published as British Standards.
2.2 Of particular relevance to this report is section 3.10 which deals with the thermal
properties of reflective foil products, in particular 3.10.2 which considers multi-foil
insulation. The specified method for determining product performance is either
from measurement of the thermal resistance of the core according to BS EN 12664
or BS EN 12667 together with the emissivity of the surfaces or in a hot box apparatus
conforming to BS EN ISO 8990. Importantly in the case of the hot box it is further
specified that the test should be undertaken with airspace on either side of the
product, and that the quoted result should be the thermal resistance of the assembly
consisting of the product and two air spaces. This value can then be included in a
calculation according to BS EN ISO 6946 with the other components of the structure
to determine the full structure U-value.
Chapter 3 The current scientific opinion of suitable test methods for thermal conductivity of wall elements | 9
Chapter 3
The current scientific opinion of suitable
test methods for thermal conductivity of
wall elements
3.1 When undertaking tests to determine the thermal conductivity of building materials
it is essential that the procedure followed is robust, controllable and returns results
that are both accurate and repeatable. Currently two techniques are widely used and
generally accepted for the determination of the thermal performance of insulation
products, the guarded hot plate method, BS EN 12664:2001, BS EN 12667:2001,
ASTM C177 and the calibrated and guarded hot box BS EN ISO 8990:1996, ASTM
C1363 – 05.
3.2 Both methods require that steady state conditions are achieved. The guarded hot
plate is most suited to measure the thermal conductivity of flat surfaced materials
of uniform thickness which can be mounted in the guarded hot plate apparatus.
For materials that are not flat surfaced or of uniform thickness the inclusion of air
spaces resulting from poor contact between the insulation material and the guarded
hot plate apparatus would introduce an error in the measurement. A review of
the guarded hot plate technique and its history was published by the Institue of
Physics (IOP) in their journal Measurement Science and Technology. The guarded
hot box can be used for measuring the thermal performance of individual materials
and importantly inhomogeneous building envelope assemblies. In recognition of
the effects of inclination on convective heat transfer within façade structures the
rotatable guarded hot box is now being used in some laboratories. Some researchers
are now undertaking transient analysis with the hot box, after steady state conditions
are achieved, the temperature in one chamber is increased in a controlled way to a
new set temperature, the test continues until steady state conditions at the new set
temperature are achieved.
3.3 The guarded hot box technique is well proven and accepted to give robust reliable
steady state measurements of the thermal performance of building envelope
assemblies with many years of successful application in laboratories and test houses
around the world. Numerous references in the scientific literature to their application
for building component characterisation from wall materials and constructions to
doors incorporating vacuum insulation panels, (2-8) and glazing systems in which
the IR (infra red) transfer is reduced by the use of low emittance films (9-12), indicate
that these two methods are both widely used and accepted global standards. The
10 | Multi-foil Insulation – BD2768
wide spread use and acceptance of the test procedure for characterising systems
that use low-emittance coatings to reduce IR heat transfer, for example windows,
is a clear indication that the scientific community consider that the test procedure is
appropriate for such systems.
3.4 In-situ thermal testing seeks to determine the performance of a real building or part
of a building structure by monitoring over an extended period of time. The exterior
of the test structure is exposed to real weather conditions and thus any tests are
transient in nature, thermal mass effects can be considerable and data measurement
and analysis more complicated. Measurements reflect the performance of the full
structure, reflecting both materials and construction method (including any defects)
and not the performance of a single constituent part. It is thus difficult to obtain
absolute values of the thermal characteristics of any individual component that are
accurate, robust, repeatable, and transferable between different constructions and
geographic locations.
3.5 The development of a test protocol for in-situ thermal testing of building materials
is ongoing, however until the test protocol is proven and accepted by national and
international standards organisations, in-situ testing of the thermal performance of
building materials will in all probability remain an adjunct to the accepted hot box
and hot plate methods for the foreseeable future.
3.6 The hot box is suitable for testing both individual materials and wall constructions,
BR443 section 3.10.2, recommends that multi-foil insulation should be tested within
a unit that includes the two adjacent air spaces, the low emittance reflective films on
the exterior surfaces of the multi-foil insulation will thus be interior to the unit and
perform with a similar effectiveness to when installed in-situ. To provide the two air
spaces exterior layers of material are required, these could be for example plywood
sheets. The heat transfer modes from the metering box of the hot box through a
test structure (consisting of a plywood sheet, airspace, multi-foil insulation, airspace,
plywood sheet) to the environment chamber would be:
• convective and radiative heat transfer from the metering box to the exterior of
the first plywood sheet
• heat conduction through the plywood sheet
• heat transfer from the interior surface of the first plywood sheet by a mixture
of convection and radiation to the exterior low emittance reflective film of the
multi-foil insulation
• heat transfer through the insulation by conduction if the interior is opaque to
long wave radiation or by conduction and radiation if the interior is transparent
to long wave radiation
Chapter 3 The current scientific opinion of suitable test methods for thermal conductivity of wall elements | 11
• heat transfer from the low emittance reflective surface by a mixture of
convection and radiation to the interior surface of the second plywood sheet
• heat conduction through the plywood sheet
• convective and radiative heat transfer from the exterior surface of the plywood
sheet to the environment chamber.
3.7 The surfaces of the multi-foil insulation do not directly view the surfaces of the
guard and environment chambers of the hot box and do not have any radiative heat
exchange with them. The radiative heat transfer reduction due to the low emittance
reflective surfaces is interior to the structure and guarded hot box testing will enable
the heat transfer through the full test structure to be determined.
References
1)
Thermal conductivity of insulations using guarded hot plates, including recent
developments and sources of reference materials. D. Salmon Measurement Science
and Technology. 12 (2001) 89–98.
2)
Thermal properties of a variable cavity wall, D.P. Aviram, A.N. Fried, J.J. Roberts,
Building and Environment 36 (2001) 1057–1072.
3)
U-value of a dried wall made of perforated porous clay bricks. Hot box measurement
versus numerical analysis. K. Ghazi Wakili, Ch. Tanner, Energy and Buildings 35
(2003) 675–680.
4)
Experimental and numerical thermal analysis of a balcony board with integrated glass
fibre reinforced polymer GFRP elements. K Ghazi Wakili, H Simmler, T Frank, Energy
and Buildings 39 (2007) 76–81.
5)
Steady-state heat transfer in an insulated, reinforced concrete wall: theory, numerical
simulations, and experiments, G.F. Jones, R.W. Jones, Energy and Buildings 29
(1999). 293–305.
6)
Thermal analysis of a wooden door system with integrated vacuum insulation panels,
T. Nussbaumer, R. Bundi, Ch. Tanner, H. Muehlebach, Energy and Buildings 37 (2005)
1107–1113.
7)
A new model based on experimental results for the thermal characterization of
Bricks, J Vivancos, J Soto, I Perez, J V. Ros-Lis, R M-Manez, Building and Environment
44 (2009) 1047–1052.
12 | Multi-foil Insulation – BD2768
8)
Experimental and numerical investigation of the thermal performance of a protected
vacuum insulation system applied to a concrete wall, T. Nussbaumer, K. Ghazi Wakili,
Ch. Tanner l, Applied Energy 83 (2006) 841–855.
9)
Energy labelling of glazing and windows in Denmark: calculated and measured
values, K Duer, S Svendsen, M M, Morgensen and J B, Laustsen, Solar Energy Vol. 73,
No. 1, pp. 23–31, (2002).
10) Low emittance coatings and the thermal performance of vacuum glazing, Yueping
Fang , Philip C. Eames, Brian Norton, Trevor J. Hyde, Junfu Zhao, Jinlei Wang, Ye
Huang, Solar Energy 81 (2007) 8–12.
11) Characterization of the thermal insulating properties of vacuum glazing, N. Ng, R.E.
Collins, L. So, Materials Science and Engineering B 138 (2007) 128–134.
12) Heat transfer across corrugated sheets and honeycomb transparent insulation. H.
Suehrcke, D. Daldehog, J.A. Harris, R.W. Lowe, Solar Energy 76 (2004) 351–358.
Chapter 4 The consultancy responses | 13
Chapter 4
The consultancy responses
4.1 The responses obtained varied significantly in length and detail. This analysis of the
consultancy responses is limited to the technical responses and does not attempt
to include procedural issues raised in the response of the European Multi-foil
Manufacturers.
4.2 The following groups the consultancy responses into nine categories and presents a
brief summary of the most salient points raised.
A)
Those that are in agreement with the appropriateness of the
specification of BR443 for testing all insulation materials
1) BBA
2) BRE
3) BRUFMA
4) CIBSE
5) Construction Products Association
6) Eurisol mineral wool insulation association
7) Kingspan Insulation
8) KNAUF
9) Barry Turner LABC
10) Recticel Insulation
11) WEB DYNAMICS
12) Confidential.
4.3 The key points that are raised in the responses that fall into category A are that
the test specified in BR443 is at present the only internationally accepted way of
determining material thermal performance, that the test is well proven providing
robust accurate repeatable measurement and that the test is appropriate for all
materials. The responses of WEB DYNAMICS details that they are currently using
hot box methods to determine the thermal performance of multi-foil products and
the response of the CPA states that some multi-foil manufacturers are using hot box
techniques to characterise their products. The consensus of these responses is that
if it is desired to improve the standard of building thermal performance all materials
14 | Multi-foil Insulation – BD2768
used should be characterised using the hot box or hot plate method, failure to do so
would lead to the deployment of materials with greatly exaggerated performance
claims. The confidential response additionally indicated that the dynamic in-situ test
methodology is not developed sufficiently to provide meaningful results.
B)
Those that think that the test methods specified in BR443 are not
appropriate for testing multi-foil insulation products and that the
requirement for hot box testing should be replaced
1) Tony Reid, The Confederation of Multi-foil Manufacturers Ltd.
2) Jon Harper.
4.4 The key points that are raised in the responses that fall into category B are that the
tests specified in BR443 are not suitable for testing multi-foil insulation products and
that reference to it should be removed and that other methods should be allowed,
for example in situ comparative testing.
C)
Those that think BR443 is not appropriate for thermal block products
and does not necessarily reflect in situ performance
1) Chris Weller Dover District Council Building Control – Personal capacity.
4.5 The key points of this response are that the tests specified in BR443 are suitable for
determining the thermal conductivities of standard insulation products but do not
really provide details of how “thermal block products work” or how a product will
perform when installed in a building. This response supports the use of in-situ testing.
D)
Those that think that the performance of all buildings should be
experimentally determined when built
1) Alba Building Sciences.
4.6 The key point that is raised in this response is that buildings perform differently to the
way predicted at the design stage and therefore every buildings thermal performance
must be experimentally verified after construction.
E)
Those that state alternative in-situ test methods are being developed
1) Loucas Gourtosyannis, normapme.
2) Ludovic Lauwers Chairman of Workshop 36.
Chapter 4 The consultancy responses | 15
4.7 The key point that is raised in these responses is that a new standard is being
developed to specify the conditions required to undertake in-situ testing and
evaluation of materials.
F)
Those that think that the hot box test does not adequately measure the
performance of multi-foils and that additional guidance is required
1) Boulder Developments.
4.8 The results of comparative performance test methods should not be used to make
claims with regard to R and U values.
G)
Those that believe that the document in its present form, although
appearing to specify that BR443 is the only acceptable test method, it
does not acknowledge that scope exists for using other test methods
1) TRADA.
2) Trada Technology.
4.9 Both responses state that scope exists for using other test methods although the
Approved Document in its present form appears to specify that BR443 is the only
acceptable test method. Reliance on U-values measured in a hot box is not the right
way to proceed in future and that they believe that part/whole building testing
should also be acceptable.
H)
Those that did not deal with the multi-foil question
1) VELUX.
4.10 The response of VELUX did not deal with the multi-foil issue.
I)
Those that think that the test methods specified in BR443 are not
appropriate for testing multi-foil insulation products, or if a measure
of real performance in buildings is desired, any insulation products
and believe that the requirement for hot box testing should be either
replaced with or at the least informed by in-situ testing. In-situ testing
should be considered to be a more appropriate way of determining
building insulation products real performance. The specification of
BR443 with no allowance for in-situ testing is likely to have significant
negative impact on progress towards achieving the desired levels of
energy efficiency in buildings.
1) European Multi-foil Manufacturers, Laurent Thierry.
16 | Multi-foil Insulation – BD2768
4.11 A long and detailed response dealing with both technical and procedural issues
supported by 18 annexes is presented. Only technical issues were considered in
this analysis. The specification of BR 443 is the result of a limited understanding of
the applicability of BR 443 to multi-foil products, and the implications of specifying
BR443 on the move to obtaining better performing buildings. Significant progress
is being made with regard to obtaining a European Technical Approval, ETA and
this should be recognised and alternative test methods allowed. In-situ test results
indicate how a product really performs when installed in a real building and thus
provide a more realistic assessment of their value and performance compared to
static laboratory tests. The Department is starting to recognise the value of in-situ
testing, how can it thus require that hot box testing is the only acceptable method?
In-situ testing is required to drive standards forward and all insulation products
should be evaluated in this way. In-situ testing is essential since many buildings have
been shown to perform significantly poorer than predicted based on calculations
using laboratory determined U-values.
4.12 The European Multi-foil Manufacturers make a series of recommendations for
amendments to the Approved Document L that would address their concerns.
Summary of the responses
4.13 From the above it is clear that there are two distinct groupings:
• Category A responses that consider that the inclusion of BR443 in its current
form is appropriate since hot box testing is the accepted international standard,
is appropriate for all insulation materials and is essential to maintain and drive up
standards of insulation in buildings; and
• Category B-G and I responses that believe that testing using hot box techniques
is not appropriate for all building materials and that other test methods for
example in-situ test methods should be allowed. Response I further indicates
that in-situ testing is a better way of evaluating the performance of all
insulation systems and that it is urgent that such test procedures are developed
and implemented rapidly if building standards are to be improved and the
desired energy efficiency levels achieved. Response I suggests that laboratory
characterisation followed by simple model prediction is not adequate to reflect
insulation’s true performance which will be affected by air movement etc.
and should be replaced by a more holistic analysis technique that can capture
these effects.
4.14 The responses appear to essentially replay the pre-consultation debate with no
significant new evidence to support comparative testing and/or hot box testing
presented.
Chapter 5 The thermal behaviour of multi-foil films | 17
Chapter 5
The thermal behaviour of multi-foil films
Conventional insulation, aerogels and vacuum
insulation panels
5.1 The basic definition of heat transfer from Incropera and DeWitt (1996)2 is that “Heat
transfer is energy in transit due to a temperature difference”. The three modes
of heat transfer are conduction, convection, and radiation. Conduction refers to
the heat transfer that occurs when there is a temperature gradient in a stationary
medium. Convection is the heat transfer that occurs between a surface and a moving
fluid when they are at different temperatures. Thermal radiation heat transfer occurs
between two surfaces at different temperatures in the absence of an intervening
medium.
5.2 Conventional mineral fibre or bead based insulation materials commonly used
in buildings, work by effectively providing a low thermal conductivity matrix
in which stationary air is contained, expanded polystyrene foam and similar
insulation materials work by having a foamed bubble matrix in which stationary
gas is enclosed, since the matrix voids are large in comparison to the path length
between intermolecular collisions of the gas molecules, this effectively limits the
minimum thermal conductivity to that of the still gas, in the case of air, approximately
0.026W/mK, in the case of carbon dioxide 0.016W/mK. The materials used to form
conventional insulation materials are in general opaque to long-wave radiation.
5.3 To obtain lower heat transfer/thermal conductivity two options are available; reduce
the matrix voids to a size that is less than the mean free path of gas molecules or to
evacuate the gas from the matrix. To date only two systems have been developed
that have demonstrated lower levels of thermal conductivity than standard insulation
materials when tested using standard test apparatus; Aerogels and Vacuum
Insulation Panels, (VIP).
5.4 The structure of an aerogel consists of 2-5nm diameter particles which cluster
together to form a highly porous three dimensional structure with pores in the 1 to
100 nm range. When based on silica the aerogel produced is effectively transparent,
although some light scattering occurs. The heat transfer through an aerogel is
comprised of the gaseous conduction in the pores, conduction within the chains of
particles that form the aerogel matrix and Infra Red radiation. Because the average
Incropera and DeWitt Fundamentals of Heat and mass Transfer, John Wiley & Sons, Inc. 1996
2
18 | Multi-foil Insulation – BD2768
pore size, 1-100 nm is less than the mean free path of air molecules, (70nm at
normal pressure and temperature) and the silica matrix thermal conductivity is very
low, silica based aerogels can achieve thermal conductivities of less than that of still
air. To reduce long-wave radiative heat transfer through silica aerogels it is essential
to introduce an opacifier during manufacture, this is often in the form of carbon
particles. Thermal conductivities of silica based aerogels of down to 0.005W/mK
have been achieved compared to 0.026W/mK of still air. By using reinforcing fibres
silica aerogels have been incorporated into commercially available flexible insulation
blankets suitable for building applications with a reported thermal conductivity of
0.012W/mK.
Multi-foil films
5.5 Multi-foil films consist of a series of reflective layers interspersed with layers of
wadding and foam. The major claim that they make for their apparent high thermal
performance is based on their ability to reflect long-wave radiative energy thus
achieving low levels of heat transfer. To better understand this a one dimensional
model was developed specifically for analysis of the heat transfer through building
elements incorporating multi-foil films, calculating heat transfer by conduction,
convection and radiation. The basic methodology adopted was to specify a heat flux
through the wall/roof section and perform energy balances to determine the heat
transfer by each mode and resultant temperature distribution within the structure.
Keeping the internal room air temperature at a constant level of 25ºC and increasing
the heat flux through the wall element allows the decrease in external temperature
required to achieve a given flux to be determined. The U-value of the structure can be
calculated at the different flux levels revealing any non-linear effects resulting from
radiative and convective heat transfer. The model is one dimensional and simulates
the heat transfer through the roof element and insulation when installed between
rafters with no external air infiltration.
The system structure
5.6 The multi-foil structure modelled consists of 19 layers, (nodes 4 to 22 in Table 1)
the exact physical properties of the foam, wadding layers and reflective foil are not
known therefore the following simulations have been made:
1) The thermal conductivity of each of the foam and wadding layers was assumed
to be that of i) mineral wool (0.04W/mK) still air (0.026W/mK) and an aerogel
based blanket (0.012W/mK). The use of adjacent double foam layers appears to
be to take advantage of the thermal contact resistance due to the presence of still
air; it is thus probable that the foam thermal conductivity is greater than that of
still air.
Chapter 5 The thermal behaviour of multi-foil films | 19
2) The reflective layer will be very narrow and have a higher thermal conductivity
than that of the foam or wadding thus essentially providing negligible thermal
conduction resistance and be of uniform temperature.
3) Convection in the air spaces either side of the multi-foil insulation can be
approximated by correlations from the literature. For determination of the
convective heat transfer coefficient of the cavity it was assumed to be 25mm
wide, inclined at 45º with a width to height aspect ratio of 20:1.
4) The values for the long-wave emittance of the reflective films simulated was
0.07 corresponding to bright Aluminium Foil and 0.02 corresponding to
polished silver.
5.7 To determine the limits on the multi-foil system performance with regard to internal
radiative heat transfer it was simulated with two assumptions i) that the wadding
and foam were transparent to IR radiation and ii) that they were completely opaque
to IR radiation. The emittance of both the plaster board and the vapour permeable
membrane were taken to be 0.9. The air spaces between the plaster board and
the insulation and the insulation and the vapour membranes were not ventilated,
the air space between the vapour membrane and the roof tiles was assumed to be
ventilated. The thermal resistance from the room air to the plaster board and from
the vapour membrane to the ventilated cavity were set at 0.13 and 0.1 respectively.
5.8 The system simulated with emittance of reflective films of 0.02 and thermal
conductivity of foam layers of 0.012W/mK corresponding to polished silver and
aerogel blankets effectively represents the best system performance that can hoped
to be achieved using the very best materials currently available.
5.9 To enable comparison with standard insulation materials simulations were
undertaken for roof constructions using mineral fibre insulation (thermal conductivity
0.04W/mK) of thickness 100 and 200mm with and without a reflective backing,
detailed in Table 2.
20 | Multi-foil Insulation – BD2768
Table 1: The construction details of the simulated multi-foil insulated roof
Node Material
Thickness
Heat
transfer
Surface
emissivity
Thermal
conductivity
W/mK
0.9
0.22
1
Room air
2
Plaster Board
0.012
Conduction
3
Air Space
0.025
Convection
Radiation
4
Reflective film
–
5
Wadding
0.005
6
Reflective film
–
7
Foam
0.004
8
Foam
9
Reflective film
–
10
Foam
0.004
11
Foam
12
Reflective film
–
13
Wadding
0.005
14
Reflective film
–
15
Foam
0.004
16
Foam
17
Reflective film
–
18
Foam
0.004
19
Foam
20
Reflective film
–
21
Wadding
0.005
22
Reflective film
–
23
Airspace
0.025
24
Vapour
permeable
membrane
25
Ventilated air
space
0.07/0.02
Conduction
Radiation
0.04/0.026/
0.012
0.07/0.02
Conduction
Radiation
0.04/0.026/
0.012
0.07/0.02
Conduction
Radiation
0.04/0.026/
0.012
0.07/0.02
Conduction
Radiation
0.04/0.026/
0.012
0.07/0.02
Conduction
Radiation
0.04/0.026/
0.012
0.07/0.02
Conduction
Radiation
0.04/0.026/
0.012
0.07/0.02
Conduction
Radiation
0.04/0.026/
0.012
0.07/0.02
Convection
Radiation
0.9
Chapter 5 The thermal behaviour of multi-foil films | 21
Table 2: The construction details of the mineral wool insulated roofs
Node Material
Thickness
(mm)
Heat
transfer
Surface
emissivity
Thermal
conductivity
W/mK
0.9
0.22
1
Room air
2
Plaster Board
0.012
Conduction
3
Mineral wool
0.100/200
Conduction
4
Reflective film
–
5
Airspace
0.025
6
Vapour
permeable
membrane
7
Ventilated air
space
0.04
0.9/0.07
Convection
Radiation
0.9
Predicted performance
5.10 Figure 1 presents the predicted U value for the four mineral wool based insulated
roofs and the multi-foil systems with emissivity of 0.07 and foam core thermal
conductivity of 0.04W/mK. Multi-foil 1 is the case for which the foam is considered
to be opaque to IR while multi-foil 2 is the case for which the foam is considered to be
transparent to IR. It is evident that for this simulation the multi-foil system performs
poorly in comparison to the mineral wool insulation systems. The predicted increase
in U value with increasing heat flux are for multi-foil 1, multi-foil 2 and mineral 100R
due to a predicted increase in the rate of convective heat transfer in the air spaces due
to increased temperature differences.
5.11 The heat transfer between the multi-foil and the plaster board and vapour
membrane is a combination of convection and radiation. Due to the low surface
emittance of the reflective layer the radiative heat transfer is low and the effect of the
onset of convection in these air layers and its increase with temperature difference
can be clearly seen in the increasing U-value of the structure. This is a clear indication
that the effectiveness of the multi-foil insulation is strongly dependant on the
insulating effect of the air layers either side of it. The slight decrease in the predicted
U value for the mineral 100 system is due to a reduction in long-wave radiative
heat transfer between the mineral wool outer surface and the vapour membrane
that results due to the reduction in temperatures of these surfaces and the quartic
behaviour of radiative heat transfer. The required temperature differences between
the internal room air at 25ºC and the air in the ventilated air space to obtain the set
22 | Multi-foil Insulation – BD2768
rates of heat flux through the structure are shown in Figure 2. It can be seen from this
graph that much larger temperature differences are required to achieve the same
levels of heat flux for the better performing mineral wool based insulation systems.
5.12 Figure 3 presents the predicted U values for the four mineral wool insulated roof
elements and the multi-foil systems with reflective surface emittance of 0.07 and
foam core thermal conductivities of 0.026W/mK (still air). It is clear that a significant
improvement in performance occurs for the multi-foil systems however they still fail
to perform at a level that is comparable to the 100mm thickness of mineral wool
insulated roof.
5.13 Figure 4 presents the predicted U values for the four mineral wool insulated roof
elements and the multi-foil systems with reflective surface emittance of 0.07 and
foam core thermal conductivities of 0.012W/mK (aerogel blanket). The multi-foil
based systems now achieve a performance that is between that of the 100 and
200mm thickness of mineral fibre.
5.14 Figure 5 presents the predicted U values for the four mineral wool insulated roof
elements and the multi-foil systems with reflective surface emittance of 0.02 and
foam core thermal conductivities of 0.012W/mK (aerogel blanket). Although the
multi-foil based systems performance improves it still does not achieve the same level
of performance that is achieved by the 200mm thick mineral wool insulation system.
Figure 1: Predicted U values of the mineral wool and multi-foil insulation
systems for given flux levels. The thermal conductivity of the foam core is
0.04W/mK and the emittance of reflective surfaces is 0.07
0.5
multi-foil 1
multi-foil 2
Mineral 100
Mineral 200
Mineral 100R
Mineral 200R
U value (Wm-2 K-1)
0.4
0.3
0.2
0.1
0
0
2
4
6
Flux (Wm )
-2
8
10
Chapter 5 The thermal behaviour of multi-foil films | 23
Figure 2: Predicted temperature differences required to achieve given flux
levels for the mineral wool and multi-foil insulation systems. The thermal
conductivity of the foam core is 0.04W/mK and the emittance of reflective
surfaces is 0.07
60
multi-foil 1
multi-foil 2
Mineral 100
Mineral 200
Mineral 100R
Mineral 200R
Temperature Difference (K)
50
40
30
20
10
0
0
2
4
6
8
10
Flux (Wm-2)
Figure 3: Predicted U values of the mineral wool and multi-foil insulation
systems for given flux levels. The thermal conductivity of the foam core is
0.026W/mK (still air) and the emittance of reflective surfaces is 0.07
0.5
multi-foil 1
multi-foil 2
Mineral 100
Mineral 200
Mineral 100R
Mineral 200R
U value (Wm-2 K-1)
0.4
0.3
0.2
0.1
0
0
2
4
6
Flux (Wm )
-2
8
10
24 | Multi-foil Insulation – BD2768
Figure 4: Predicted U values of the mineral wool and multi-foil insulation
systems for given flux levels. The thermal conductivity of the foam core is
0.012W/mK (aerogel blanket) and the emittance of reflective surfaces is 0.07
0.5
multi-foil 1
multi-foil 2
Mineral 100
Mineral 200
Mineral 100R
Mineral 200R
U value (Wm-2 K-1)
0.4
0.3
0.2
0.1
0
0
2
4
6
8
10
Flux (Wm )
-2
Figure 5: Predicted U values of the mineral wool and multi-foil insulation
systems for given flux levels. The thermal conductivity of the foam core is
0.012W/mK (aerogel blanket) and the emittance of reflective surfaces is 0.02
0.5
multi-foil 1
multi-foil 2
Mineral 100
Mineral 200
Mineral 100R
Mineral 200R
U value (Wm-2 K-1)
0.4
0.3
0.2
0.1
0
0
2
4
6
Flux (Wm )
-2
8
10
Chapter 5 The thermal behaviour of multi-foil films | 25
5.15 It is clear from the above presented figures that the multi-foil systems even if using
the best materials available cannot out perform 200mm of mineral wool insulation
for the roof structure simulated. It is important to note that the multi-foil system if
installed correctly would provide an excellent barrier to unwanted air movement in
addition to its role in providing insulation, the potential for unwanted air movement
through the mineral fibre insulation should also be minimised since this could reduce
its thermal performance.
26 | Multi-foil Insulation – BD2768
Chapter 6
Conclusions
6.1 The internationally accepted methods for determining the thermal performance of
building material and building component structures are the well know hot plate and
hot box techniques which are used due to their ability to provide absolute values that
are both accurate and repeatable.
6.2 The consultation responses are divided into two categories, those that think the
standard test methods of BR443 are appropriate for all building materials and
those that think that other test methods, for example in-situ testing should also be
allowed. Several reports indicate that buildings do tend to perform less well than
predicted using simple models and the laboratory measured thermal characteristics
of materials. If an in-situ test methodology can be shown to provide repeatable
robust results and achieve European or other recognised international approval
it could be a valuable addition to current methods. To date no new substantive
evidence on robustness, accuracy and repeatability to support the move from hot box
characterisation to in-situ performance testing is presented.
6.3 A model developed for the prediction of multi-foil system thermal behaviour
indicates that the level of performance achieved even if the best possible materials
were used in its construction, silver coatings with an emittance of 0.02 and foam core
materials with a thermal conductivity equivalent to aerogel blankets 0.012W/mK,
they do not obtain a lower U value than 200mm of mineral wool insulation.
Chapter 7 Biography of the author | 27
Chapter 7
Biography of the author
Professor Philip Eames
Following his PhD at Cranfield University in which he developed a unified numerical
model for optics and heat transfer within line-axis concentrating solar energy collectors
he worked for two years on an S.E.R.C. funded project at the University of Ulster. In this
early work at Cranfield and Ulster he developed and experimentally validated original
computational fluid dynamics codes to predict conductive, convective, and radiative heat
transfer in solar energy collectors and to investigate mixing due to laminar jets in thermally
stratified stores. After this he was appointed Lecturer in Building Services Engineering
with subsequent promotion to Reader in Solar Energy and Advanced Fenestration and in
November 2000 Professor of Solar Energy Applications. During this time he first directed
the centre for Performance Research on the Built Environment and then the Centre for
Sustainable Technologies. Prior to moving to Warwick University in October 2006 to
take up the post of Professor of Energy Efficiency and Conservation he directed the Built
Environment Research Institute at the University of Ulster for two years. In October 2008
he moved to Loughborough University to take up the post of Professor of Renewable
Energy in the Centre for Renewable Energy Systems Technology in the Department of
Electronic and Electrical Engineering. He has an internationally recognised track record for
undertaking high quality research in the field of energy saving and solar energy harnessing
building fabric components, thermal energy storage systems and life cycle analysis.
Research awards of in excess of £4 million have been awarded to support his research from
funding bodies including the EPSRC, EU, and TSB. He has published over 170 journal and
conference papers, regularly undertakes reviews for a range of journals, and serves on
technical and organising committees of International Conferences.
To date Professor Eames has supervised 12 PhD students through to successful completion.
He has assessed funding applications from both the UK and overseas and examined PhD
students in the UK, Australia, and Spain.
ISBN 978-1-4098-1512-9
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ISBN: 978-1-4098-1512-9
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