The Carbon Footprint of Multi-storey Buildings Using Different Construction Materials.

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1 The 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 ing 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 derived both from the production of the buildings materials (cradle-to-gate), as well as the total 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 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 than equivalent steel or concrete buildings. Key Words. ing; 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 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 and, in particular, what is the effect of using different structural building materials on a building s carbon. 2. ing. 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. ing is a sub-set of a broader measure, the ecological, 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. ing calculates the amount of GHG caused by a particular activity or entity (BSI, 2008). This is commonly also referred to as global

2 warming potential (GWP) and is measured in tonnes (or kilograms) of carbon dioxide equivalent (). The technique of carbon ing of whole buildings is an extension of the ing 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 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 must clearly specify what is included, what is excluded and importantly, the time-frame over which the 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 of a building takes the above concepts and calculates the CO 2 equivalent () associated with that building, carefully specifying whether the 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 for the production of the materials, and all 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 CO 2. When calculating a carbon, 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 ; if stored carbon is included, it is a net. What is clear is that if a carbon 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 from other parts of the life cycle. The carbon of a building can be presented as a per square metre figure by dividing total net by the useable floor area of the building ( tonnes/m 2 ). 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 (2006) and ISO (2006) publications provide the principles and

3 framework, requirements and guidelines for LCA). Global warming potential or CO 2 equivalent - 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 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 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 CO 2 ) 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 ing 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 for a building through to the initial just-built point is a lot simpler than a 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 m 2 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

4 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 GWP (tonnes CO2 eq.) Concrete Steel Timber Timber+ Building type Initial Embodied Maintenance Transport Operational End of Life CO2 storage Figure 2: GWP (tonnes CO 2 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 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 can be brought about by

5 reducing the 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 are also significant and as continuing improvements in the operational energy efficiency (and associated ) of buildings are made, the relative significance of embodied increases as these form a higher proportion of the total over the lifetime of a building. In figure 2, the initial embodied are gross and do not include any offset for carbon stored in the timber materials. However, there is significant CO 2 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 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 of 0.16 tonnes/m 2 and a net of tonnes/m 2. This net negative 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) (tonnes) for the materials in the four building designs with the associated carbon ( tonnes/m 2 ). Building Design Cradle-to-gate (gross) Gross sequestered in building materials Cradle-to-gate (net) Net Concrete 1, , Steel 1, , Timber TimberPlus , The carbon reflects the benefit of using more timber in a building and particularly the reduction in net when timber replaces significant quantities of both concrete and steel. A practical example of this comparative ing technique can be found on the NZWood website ( where the building calculator allows the design of some standard structures using either concrete or steel or timber as the main structural component.

6 4.2 Full building life-cycle cradle-to-grave. In the above land-fill scenario shown in Figure 2, some of the CO 2 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) CO 2 storage. Landfill generated methane (CH 4 ) is the real problem methane is a far more potent GHG than CO 2 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 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 CO 2 and 50% into CH 4 (IPCC, 2006). A recent figure, based on physical data (MfE, 2009) showing 42% capture of CH 4, has been taken into account. Despite the emission of some methane, the Timber buildings both still demonstrate lower GWP and a smaller carbon - than either the Concrete or Steel buildings due to lower embodied CO 2 in the materials and some permanent carbon storage in the landfill. The greater use of timber in the TimberPlus building gives the lowest overall. Table 2. Cradle-to-gate compared to cradle-to-grave ( tonnes) for the four building designs with the associated carbon ( tonnes/m 2 ). Cradle-to-gate* Cradle-to-grave** Building Design Concrete 1, , Steel 1, , Timber , TimberPlus , * This does not include any carbon storage in the building materials. ** This includes both carbon storage and all at end-of-life. Table 2 provides a comparison of the carbon of the building materials only (cradle-to-gate) to the 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 of all buildings, the TimberPlus building clearly still has the lowest whole lifetime, 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 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

7 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 CO 2 is released back to the atmosphere but displaces equivalent 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 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 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.

8 tonnes CO Concrete Steel Timber TimberPlus Wood Other Aluminium Steel Concrete Figure 3: GWP ( 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 tonnes CO Concrete Steel Timber TimberPlus Figure 4. Net life cycle GWP ( tonnes) for the four buildings, assuming permanent storage of carbon in wood products.. Table 3. Cradle-to-gate compared to cradle-to-grave ( tonnes) for the four building designs with the associated carbon ( tonnes/m 2 ), assuming permanent storage of carbon in timber products. Cradle-to-gate Cradle-to-grave

9 Building Design Net Net Concrete 1, , Steel 1, , Timber , TimberPlus , Table 3 compares the cradle-to-gate to cradle-to-grave and carbon for all four buildings assuming permanent storage of carbon in timber products. Again, this demonstrates that the of a building is significantly reduced by using more timber, with the cradle-to-grave 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 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 associated both with the initial embodied of the building materials and also with the total GHG of those buildings over their full life-cycle. This is demonstrated by comparing the carbon 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 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 during use). A building s 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 (does not include any carbon storage) or a net carbon (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,

10 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. More information on these timber buildings can be obtained by contacting the Structural Timber Innovation Company (STIC) at 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 ing can then be a useful tool to demonstrate the effect of using different building materials on GWP. 7. References. BSI (2008). Guide to PAS 2050:2008 How to assess the carbon of goods and services, British Standards Institute. GaBi 2006; LBP,PE; GaBi 4.3. Software-systems and Databases for Life Cycle Engineering. Copyright, TM. Stuttgart, Echterdingen. IPCC (2006). NGGIP Publication 2006 IPCC Guidelines for National Greenhouse Gas Inventories ISO (2006): Environmental management Life cycle assessment Principles and Framework. International Organisation for Standardisation. ISO (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. Research Report , University of Canterbury, New Zealand. ( ) MfE (2009): New Zealand's Greenhouse Gas Inventory Ref ME928. Ministry for the Environment, New Zealand. Nebel, B., Alcorn, A. and Wittstock, B. (2009). Life Cycle Assessment: 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, Corrected Proof, Available online 4 January 2008

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