The Carbon Footprint of Multi-storey Buildings Using Different
Construction Materials.
Stephen John, Andy Buchanan, Nicolas Perez.
University of Canterbury, Christchurch, New Zealand.
Abstract.
This paper develops the concept of carbon footprinting for multi-storey buildings by
extending the results from a recent New Zealand Ministry of Agriculture and Forestry
report. Life cycle assessment is used to quantify the green house gas emissions
derived both from the production of the buildings’ materials (cradle-to-gate), as well
as the total emissions over the full 60-year lifetime of the buildings (cradle-to-grave).
The buildings use either timber or steel or concrete as the main structural material.
The importance of considering the end-of-life disposal of building materials is
highlighted. Using more timber in the construction of a multi-storey building can
reduce the carbon footprint of that building. If the building materials provide a
permanent, net removal of carbon from the atmosphere, then timber multi-storey
buildings can have a significantly lower carbon footprint than equivalent steel or
concrete buildings.
Key Words.
Carbon footprinting; buildings; timber; LCA; GWP.
1.
Introduction.
A report recently made available by the New Zealand Ministry of Agriculture and
Forestry (MAF) - Environmental Impacts of Multi-storey Buildings Using Different
Construction Materials (John et al., 2009) - shows that using more timber materials in
the construction of a multi-storey building, largely through replacing traditional
concrete or steel structural components, reduces the net carbon emissions associated
with that building.
The above statement, standing alone, could raise many more questions than it
answers, one of which, fairly, could be ‘Is this based on good science and does it
provide a balanced opinion?’ This paper will discuss some of the supporting science
and background information and put the statement in context by explaining what is
meant by a building’s carbon footprint and, in particular, what is the effect of using
different structural building materials on a building’s carbon footprint.
2.
Carbon footprinting.
The MAF Greenhouse Gas Footprinting Strategy for the Land-Based Primary Sector
helps to position NZ’s land-based primary sector to respond to significant and
increasing pressure by key export markets for information on the greenhouse gas
(GHG) intensity for primary products, such as harvested logs and wood-based
products.
Carbon footprinting is a sub-set of a broader measure, the ecological footprint, which
itself is a measure of the human demand on the Earth’s ecosystem and compares that
demand to the Earth’s ecological capacity to regenerate resources and provide
services. Carbon footprinting calculates ‘the amount of GHG emissions caused by a
particular activity or entity’ (BSI, 2008). This is commonly also referred to as global
warming potential (GWP) and is measured in tonnes (or kilograms) of carbon dioxide
equivalent (CO2eq.).
The technique of carbon footprinting of whole buildings is an extension of the
footprinting of individual products and activities to provide an aggregated impact for
all the materials that are used to construct a building or, extended further, to include
the full lifetime impact of a building, encompassing its full operational phase and endof-life. A comparison of the carbon footprint of one building with another can give an
indication of the benefit of using different building materials with regard to climate
change
The boundaries defining the footprint must clearly specify what is included, what is
excluded and importantly, the time-frame over which the footprint applies – for
example, for a material, does it include the whole life-cycle of the product, cradle -tograve or only part of the life-cycle, such as cradle-to-gate (the production process) or
only the in-use (operational) phase.
Calculating the carbon footprint of a building takes the above concepts and calculates
the CO2 equivalent (CO2eq.) emissions associated with that building, carefully
specifying whether the footprint refers just to the materials used in that building’s
construction, or more completely to the full lifetime construction and use (operation)
of the building. The full lifetime would include at least the initial embodied CO2eq.
emissions for the production of the materials, and all emissions associated with
transport of materials beyond the factory gate, on-going building maintenance, the
building’s operation over the defined lifetime of the building, and a stated end-of-life
scenario for the building and its de-constructed components. What is important is that
the boundaries and scope are clearly stated, which can then allow apples-for-apples
comparisons to be made.
Approximately 50% of dry timber is elemental carbon; thus, 1 kg of wood contains
approximately 0.5 kg of carbon, which equates to1.83 kg of CO2. When calculating a
carbon footprint, whether to include this stored carbon in timber (and to a far lesser
extent small amounts of stored carbon in other building materials) is a contentious and
much debated issue. If stored carbon is not included, this is a gross footprint; if stored
carbon is included, it is a net footprint. What is clear is that if a carbon footprint
calculation does account for stored carbon (net), then there must be a defined end-oflife scenario for all materials and any release of GHGs at end-of-life must be
accounted for completely and correctly. If carbon can be shown to be removed from
the atmosphere permanently, this can form a significant and enduring offset to
emissions from other parts of the life cycle.
The carbon footprint of a building can be presented as a per square metre figure by
dividing total net emissions by the useable floor area of the building (CO2eq.
tonnes/m2).
3.
Life Cycle Assessment (LCA).
Life cycle assessment is a rigorous, systems-perspective methodology for the
investigation and evaluation of the environmental impacts of a given product or
service caused or necessitated by its existence over its whole life cycle (the ISO
14040 (2006) and ISO 14044 (2006) publications provide the principles and
framework, requirements and guidelines for LCA). Global warming potential – or
CO2 equivalent emissions - is just one environmental impact assessed by LCA, of
particular relevance when considering the materials that make up a building and the
current emphasis on limiting GHG emissions to the atmosphere.
Employing LCA to make an accurate calculation of the GWP of a building can be a
complex, costly and time-consuming exercise. However, in simplified form, to
establish the initial embodied CO2eq. emissions of a building’s materials (cradle-togate) requires ;
• The collection of accurate data on the quantities of materials in the building
(information often available as-built from the Quantity Surveyor), from
necessity making some judgement calls on what materials to include (either
because the material is present in large quantity or because the material’s
manufacture produces large CO2 emissions)
• Utilising Life Cycle Inventory (LCI) data to determine a suitable, accurate
dataset of GWP coefficients. A data set covering most building materials in NZ
– and utilising NZ specific datasets for the provision of energy - has recently
been developed as part of the project Life Cycle Assessment; Adopting and
Adapting Overseas LCA Data and Methodologies for Building Materials in New
Zealand (Nebel et al., 2009). Where NZ-specific data is not available, other
datasets, such as those from Ecoinvent or GaBi (Gabi, 2006) may be applied.
Considerable care should be taken over the selection and use of an appropriate
data set and the possible limitation of that data-set when presenting results.
• A simple spreadsheet can then be used to multiply material quantities by the
appropriate GWP coefficient, followed by summation to give the total GWP of
the materials in a building.
To extend footprinting to cover the full life-cycle of the building requires
considerably more information. A full assessment must include the operational in-use
phase of the building – undertaken either through collection of real, in-use data or
through complex computer modelling and analysis. Then the GWP of all associated
transport, on-going building maintenance and the chosen end-of-life scenario must be
fully accounted for. The whole operation most often requires a detailed energy audit
and modelling and the use of sophisticated LCA software packages such as GaBi 4.3
(Gabi, 2006).
Providing a carbon footprint for a building through to the initial just-built point is a lot
simpler than a footprint for a building over its full life cycle.
4.
LCA of modelled multi-story buildings.
The report Environmental Impacts of Multi-storey Buildings Using Different
Construction Materials (John et al., 2009) presented the results of a full and detailed
LCA for GWP and energy use of four similar commercial, open-plan building designs
– Concrete, Steel, Timber and TimberPlus – all based on an actual six-storey 4,200 m2
building, the new Biological Sciences building at the University of Canterbury. All
four buildings were designed for a 60-year lifetime, with very similar low operational
energy consumption. The Concrete and Steel buildings employed conventional
structural design and construction methods, whilst the Timber buildings were
designed with an innovative, post-tensioned timber structure using laminated veneer
lumber (LVL). The TimberPlus design further increased the use of timber in
architectural features such as exterior cladding, windows and ceilings. Predicted
construction times for all four buildings are similar. Figure 1 shows a schematic of
the TimberPlus building.
Figure 1: TimberPlus building, North-east and South-west perspective views.
The report presents the GWP and primary energy use for each building, considering
different end-of-life scenarios. Figure 2 shows the GWP for each stage of the lifecycle of all the building types, where all waste and demolition materials are disposed
in land-fill – that is a full cradle-to-grave LCA..
8000
7000
GWP (tonnes CO2 eq.)
6000
5000
4000
2
3000
2000
1000
0
-1000
-2000
Concrete
Steel
Timber
Timber+
Building type
Initial Embodied
End of Life
Maintenance
Transport
Operational
CO2 storage
Figure 2: GWP (tonnes CO2 equivalent) for each stage of the life cycle of all the building types
Figure 2 shows that the 60-year life cycle of the building is dominated by emissions
due to the operation of the building during occupancy. Providing each building has
similar performance characteristics, the GWP of the materials themselves does not
directly influence the GWP of the building during this operational phase – and the
greatest reduction in each building’s carbon footprint can be brought about by
reducing the emissions associated with the activities of this operational phase, such as
through better overall building design, passive heating and cooling, the use of phasechange materials, energy efficient lighting, etc.. Current research at the University of
Canterbury is showing promising results that timber buildings can be designed to
offer low-operational energy consumption to rival either steel or concrete buildings.
4.1
Materials only – Cradle-to-gate.
The initial embodied emissions are also significant and as continuing improvements
in the operational energy efficiency (and associated emissions) of buildings are made,
the relative significance of embodied emissions increases as these form a higher
proportion of the total emissions over the lifetime of a building.
In figure 2, the initial embodied emissions are gross emissions and do not include any
offset for carbon stored in the timber materials. However, there is significant CO2
stored in the timber in the buildings – appearing below the graph – very small in the
Concrete and Steel buildings but over 1,150 tonnes in the TimberPlus design.
Table 1 shows the cradle-to-gate emissions for each building design, both gross (not
including carbon storage in timber) and net (including carbon storage). The result of
using more timber gives the TimberPlus design a gross footprint of 0.16 tonnes/m2
and a net footprint of -0.17 tonnes/m2.
This net negative footprint for the TimberPlus building means that the carbon stored
by the building materials more than cancels out all the GHG emitted in the
manufacture of all the other building materials. For as long as the timber materials
remain in existence, this net removal of carbon from the atmosphere remains.
Table 1. Cradle-to-gate (embodied) emissions (CO2eq.tonnes) for the materials in the four
building designs with the associated carbon footprint (CO2eq. tonnes/m2).
Cradle-to-gate
(gross)
Building
Design
Concrete
Steel
Timber
TimberPlus
Gross
CO2eq.
emissions
(tonnes)
1,576
1,615
971
566
Carbon
footprint
2
(tonnes/m )
0.45
0.46
0.27
0.16
Cradle-to-gate
(net)
CO2eq.
sequestered
in building
materials
(tonnes)
32
31
846
1,162
Net
CO2eq.
emissions
(tonnes)
1,544
1,584
125
-596
Carbon
footprint
2
(tonnes/m )
0.44
0.45
0.04
-0.17
The carbon footprint reflects the benefit of using more timber in a building and
particularly the reduction in net emissions when timber replaces significant quantities
of both concrete and steel. A practical example of this comparative footprinting
technique can be found on the NZWood website (http://www.nzwood.co.nz) where
the building calculator allows the design of some standard structures using either
concrete or steel or timber as the main structural component.
4.2
Full building life-cycle – cradle-to-grave.
In the above land-fill scenario shown in Figure 2, some of the CO2 stored in the
timber materials is released back in to the atmosphere at the end-of-life and during the
following years – in Figure 2, the end-of-life contribution is shown at the top of each
column in the graph – which reduces the effective (net) CO2 storage.
Landfill generated methane (CH4) is the real problem – methane is a far more potent
GHG than CO2 and contributes a disproportionately large share of GWP to the end-oflife. Ximenes et al. (2008) demonstrated that 18% of carbon in wooden materials
placed in landfills decomposes within 19 - 46 years following the initial disposal but
after this period no further significant amount of carbon is released. From the
proportion of carbon released, 50% of that will form into CO2 and 50% into CH4
(IPCC, 2006). A recent figure, based on physical data (MfE, 2009) showing 42%
capture of CH4, has been taken into account. Despite the emission of some methane,
the Timber buildings both still demonstrate lower GWP – and a smaller carbon
footprint - than either the Concrete or Steel buildings due to lower embodied CO2 in
the materials and some permanent carbon storage in the landfill. The greater use of
timber in the TimberPlus building gives the lowest overall footprint.
Table 2. Cradle-to-gate emissions compared to cradle-to-grave emissions (CO2eq. tonnes) for the
four building designs with the associated carbon footprint (CO2eq. tonnes/m2).
Cradle-to-gate*
Building
Design
CO2eq.
emissions
(tonnes)
Carbon
footprint
2
(tonnes/m )
Cradle-to-grave**
CO2eq.
emissions
(tonnes)
Carbon
footprint
2
(tonnes/m )
Concrete
1,576
0.45
6,794
1.92
Steel
1,615
0.46
6,883
1.95
Timber
971
0.27
5,982
1.69
TimberPlus
566
0.16
5,276
1.49
* This does not include any carbon storage in the building materials.
** This includes both carbon storage and all emissions at end-of-life.
Table 2 provides a comparison of the carbon footprint of the building materials only
(cradle-to-gate) to the footprint when the full operational life of the building and its
disposal at the end-of-life is also included (cradle-to-grave). Whilst operating the
building for 60 years has significantly increased the overall emissions of all buildings,
the TimberPlus building clearly still has the lowest whole lifetime footprint, 20% less
than the equivalent Concrete or Steel building.
An end-of-life material reutilisation scenario, where timber was combusted for energy
recovery and steel and concrete were recycled (instead of all material going to landfill) demonstrated a similar trend with the smallest footprint for the TimberPlus
building (John et al., 2009). Recycling of steel and concrete is somewhat more
beneficial than landfilling these materials because recycling displaces the need to use
new primary materials with high initial embodied GWP.
5.
Permanent storage of carbon in timber building materials.
Now consider a scenario where all the carbon stored in the building materials is
permanently removed from the atmosphere. I
Is this a valid scenario? 100% permanent carbon storage may not be realistic (for
instance, some leakage will always occur) but the following options could all
contribute to effectively near-permanent storage of nearly all the carbon in the timber
products;
• Landfilling of all timber products with minimised subsequent GHG release
(future landfills may achieve this through being permanently sealed).
• Re-using all timber products in other new buildings (and acknowledging that
eventually the timber will have to be disposed).
• Replacement of any deconstructed timber building with a new building
containing at least the same amount of wood. However, the net storage of
carbon in the building materials can only be counted once.
• Landfilling with any methane being collected for energy production (where
CO2 is released back to the atmosphere but displaces equivalent emissions
which would have resulted from the use of other carbon-based fossil fuels).
• Efficient burning of all waste and demolition timber for energy production
(thus displacing other fossil fuel use, as above).
Some of the above scenarios require an advance in technologies and/or policy changes
within New Zealand. However, a recent example of progress towards this permanent
storage scenario is occurring now in Christchurch where the old Burwood Landfill is
generating methane gas which is being collected and used to displace the use of other
carbon-based fossil fuels in the heating of the QEII pool complex.
The underlying consideration is that as long as the timber products exist, they are
storing carbon (or displacing fossil fuel use). This approach does not assume any
particular end-of-life scenario; it simply says that timber products - that exist and are
being utilised or prevented from decomposing and releasing GHGs back into the
atmosphere - store carbon and there are mechanisms for retaining this beneficial
storage over the very long term.
Figure 3 shows GWP emissions for the materials in the four buildings, assuming
permanent storage of carbon in wood products. The net GWP of the materials in the
Timber building is just 5% of that from the Concrete and Steel buildings. For the
TimberPlus building, GWP is again negative with the potential long-term storage of
over 630 tonnes of CO2eq.
Figure 4 shows the situation for the full life cycle of the buildings with permanent
carbon storage. The net GWP of the TimberPlus building over 60 years of operation
and subsequent deconstruction is around only 65% of the Steel building – a
significant reduction in CO2eq. emissions.
Wood
Other
Aluminium
Steel
lu
s
Ti
m
be
rP
St
ee
l
Ti
m
be
r
Concrete
C
on
cr
et
e
tonnes CO
1800
1600
1400
1200
1000
800
600
400
200
0
-200
-400
-600
-800
-1000
-1200
-1400
Figure 3: GWP emissions (CO2eq. tonnes) for the materials in the four buildings, assuming
permanent storage of carbon in wood products.
Figure 4 shows the situation for the full life cycle of the buildings with permanent
carbon storage. The net GWP of the TimberPlus building over 60 years of operation
and subsequent deconstruction is around only 65% of the Steel building – a
significant reduction in CO2eq. emissions.
8000
7000
tonnes CO2
6000
5000
4000
3000
2000
1000
0
Concrete
Steel
Timber
TimberPlus
Figure 4. Net life cycle GWP emissions (CO2eq. tonnes) for the four buildings, assuming
permanent storage of carbon in wood products.
. Table 3. Cradle-to-gate emissions compared to cradle-to-grave emissions (CO2eq. tonnes) for
the four building designs with the associated carbon footprint (CO2eq. tonnes/m2),
assuming permanent storage of carbon in timber products.
Cradle-to-gate
Cradle-to-grave
Building
Design
Concrete
Steel
Timber
TimberPlus
Net CO2eq.
emissions
(tonnes)
1,499
1,519
82
- 633
Carbon
footprint
2
(tonnes/m )
0.42
0.43
0.02
-0.18
Net
CO2eq.
emissions
(tonnes)
6,580
6,723
5,414
4,558
Carbon
footprint
2
(tonnes/m )
1.86
1.90
1.53
1.29
Table 3 compares the cradle-to-gate to cradle-to-grave emissions and carbon footprint
for all four buildings assuming permanent storage of carbon in timber products.
Again, this demonstrates that the footprint of a building is significantly reduced by
using more timber, with the cradle-to-grave footprint of the TimberPlus building over
30% less than the Concrete and Steel buildings.
6.
Discussion and conclusions.
In answer to the question posed in the Introduction, this paper shows that the careful
collection and use of appropriate data, consistent methodology and solid scientific
principles allow the calculation of a simple carbon footprint for a building. To make
valid comparisons of buildings using different construction materials, each building
must provide clear end-of-life disposal options for all the building materials.
Using more timber materials in the construction of multi-storey buildings reduces the
net carbon emissions associated both with the initial embodied emissions of the
building materials and also with the total GHG emissions of those buildings over their
full life-cycle. This is demonstrated by comparing the carbon footprint of each
building. It is anticipated that increasing the amount of wood – particularly through
displacing concrete and steel - in smaller constructions, such as houses, will have the
same effect.
Care needs to be taken when making a comparison between buildings using different
construction materials to ensure that each building offers the same functionality (that
is usage, covering both services and occupancy) and that the emissions associated
with the operation of each building are equivalent (for instance, overall, the same
heating and cooling, etc., otherwise benefits achieved through the choice of materials
may be sacrificed through increased emissions during use). A building’s footprint is
dependent on the end-of-life deconstruction and disposal of that building and when
considering the long-term impact of a building over its full lifetime, end-of-life must
not be ignored. Any assumptions about the future development of new technologies
for disposal and/or reuse of all building materials must be clearly stated. An end-oflife scenario which envisages permanent carbon storage is not currently practical but
more options will be available in the next decade or two.
Relatively simple calculations can give a cradle-to-gate comparison of the materials in
different buildings – leading to either a gross carbon footprint (does not include any
carbon storage) or a net carbon footprint (includes carbon storage). Over the full
lifetime of a building, much more data, combined with robust LCA is needed to offer
a fair and valid comparison. In all cases, care must be taken to ensure that the
boundaries of any study, such as the anticipated lifetime of the building, are the same,
that the LCI data and GWP coefficients used are up-to-date and applicable and all
assumptions are clearly stated.
Current research into innovative, commercial multi-storey timber buildings offers the
building industry an alternative to traditional concrete and steel construction. The
advantages of using more timber materials with lower embodied GWP, embodied
carbon and realistic end-of-life disposal options positions timber as the material with
the lowest carbon footprint. More information on these timber buildings can be
obtained by contacting the Structural Timber Innovation Company (STIC) at
http://www.STIC.co.nz/.
It should be noted that very few buildings are made entirely of a single material.
Good, sensible building construction should combine the use of appropriate materials
and technology, where carbon footprinting can then be a useful tool to demonstrate
the effect of using different building materials on GWP.
7.
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goods and services, British Standards Institute.
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IPCC (2006). NGGIP Publication 2006 IPCC Guidelines for National
Greenhouse Gas Inventories
ISO 14040 (2006): Environmental management – Life cycle assessment –
Principles and Framework. International Organisation for Standardisation.
ISO 14044 (2006): Environmental management – Life cycle assessment –
Requirements and guidelines. International Organisation for Standardisation.
John, S.M., Nebel, B., Perez, N. and Buchanan, A. (2009). Environmental
Impacts of Multi-storey Buildings Using Different Construction Materials.
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(http://www.maf.govt.nz/forestry/publications/lca-materials.pdf )
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Adopting and Adapting Overseas LCA Data and Methodologies for Building
Materials In New Zealand. ScionResearch, Rotorua, N.Z.
Ximenes, F.; Gardner, W.D.; Cowie, A.L. (2008): The decomposition of wood
products in landfills in Sydney, Australia. Waste Management, In Press,
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