THE CARBON FOOTPRINT OF MULTI-STOREY TIMBER BUILDINGS COMPARED WITH CONVENTIONAL MATERIALS

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1 THE CARBON FOOTPRINT OF MULTI-STOREY TIMBER BUILDINGS COMPARED WITH CONVENTIONAL MATERIALS Stephen John 1, Nicolas Perez 2, Andrew H. Buchanan 3 ABSTRACT: This paper develops the concept of carbon ing for multi-storey buildings, comparing either timber or steel or concrete as the main structural material. 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 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, over their full lifetime, timber multi-storey buildings can have a significantly lower carbon than equivalent steel or concrete buildings. KEYWORDS: ; buildings; timber; LCA; GWP 1 INTRODUCTION 123 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 [1], 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. Reduced carbon associated with timber buildings are due to the considerably lower embodied energy and associated for the manufacture of timber materials compared to those of steel and concrete, together with the effective long-term removal of carbon from the atmosphere by timber materials. Emissions associated with the operation of suitably designed, modern timber buildings over their full lifetime are similar to those for equivalent steel or concrete buildings. Thus, industry-wide efforts and continuing success at reducing the operational energy of all 1 Stephen John, Civil and Natural Resources Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. Stephen.john@canterbury.ac.nz 2 Nicolas Perez, Civil and Natural Resources Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. ; nicolas.perez@pg.canterbury.ac.nz 3 Andrew H. Buchanan, Civil and Natural Resources Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. andy.buchanan@canterbury.ac.nz buildings means that the reduced embodied energy of timber buildings will become increasingly significant. This paper examines the concept of carbon ing for buildings, the role of life cycle assessment and the importance of the deconstruction and recycling and/or disposal of the buildings. A comparison, using carbon ing to measure environmental impact, is made between a typical office-type, multi-storey building where the main structural material is either timber or concrete or steel. 2 RESEARCH OVERVIEW The New Zealand Ministry of Agriculture and Forestry commissioned the University of Canterbury (UC) to research the relative environmental impacts (energy and greenhouse gas ) of a typical, commercial multi-storey building, over its full lifetime, constructed using either timber, steel or concrete as the main structural material. The project was based around a new Biological Sciences building being constructed in concrete at the University of Canterbury in 2009 and virtual timber and steel designs for this same building. Experienced building industry professionals and consultants provided design, architectural, and building services information, as well as energy usage and life cycle assessments (LCA) and critical review at each stage of the project.

2 A further virtual design, the TimberPlus building, increased the use of timber in architectural features to investigate the effect of maximising the use of timber in such buildings. All buildings in the study were designed to have similar low operational energy over the predicted 60 year lifetime, thus allowing the research to focus on the different building materials. The end-of-life deconstruction and disposal or recycling of materials was investigated. The project was able to break new ground and extend beyond a purely theoretical comparison of the materials in multi-storey buildings due to the innovative Pres- Lam technology developed at UC and now available commercially which means that open-plan, multistorey timber buildings, particularly suited to earthquake-prone regions, can now be built. The Pres- Lam system sources raw timber products from sustainably-grown, renewable plantation forests to manufacture engineered wood products, such as laminated veneer lumber (LVL) to provide high-value, accurately prefabricated, customised building components and a total building system particularly suited to medium-rise multi-storey buildings. 3 THE BUILDINGS Studies have indicated that typically structural components account for between 16 and 65 per cent of initial embodied energy [2-5]. Hence, when considering the environmental impacts of building materials, the structural components used in a building are of significant importance. This research emphasises alternative structural designs where the predominant structural material is either timber, steel or concrete. The structural design considers all the main structural components - the skeleton - including the foundation, the beams, columns, external walls, as well as the internal flooring system and roof structure. The architectural design provides the external cladding including windows and louvres, internal walls, insulation, etc.. In some areas, the distinction between structural and architectural design is not clear cut and there can be considerable overlap; changes to the structure often enforce changes to the architecture and visa versa meaning any final design is therefore the result of an iterative, developmental design process. Most commercial buildings tend to make extensive use of steel, glass, and concrete materials in construction, all of which can be energy-intensive to produce and are non-renewable. However, recent developments in wood technology and engineered timber products, seismic and acoustic design, fabrication and construction techniques have enabled timber to be utilised much more extensively for the basic structure of medium-rise, multi-storey buildings, such as a typical down-town office block. More detailed information on the technology and building system is provided in Pampanin [6] and Smith [7]. The case study buildings analysed in this research are based on an actual building, a new six-storey, 4,247 m 2 (gross floor area) (3,536 m 2 net usable area), science laboratory for the School of Biological Sciences at the University of Canterbury in Christchurch, New Zealand. This template building was used to produce architectural and structural drawings for four alternative case studies in which the structures and finishes are predominantly either concrete or steel or timber. All four buildings were designed as the same simple, commercial (office type) building with open plan floor spaces. Concrete building pre-cast reinforced concrete exposed in structural frames and shear walls, with the same external fibre-cement cladding as the actual Biological Sciences building in the light weight walls in South façade. Steel building all the main structural components are steel, with the use of Eccentrically Braced Frames (EBF) resisting lateral loading in both (short and long) directions. Framing and cladding are steel. Timber building the main structural components are prefabricated laminated veneer lumber (LVL) columns and beams, the east and west walls are prefabricated solid LVL members, floors are timber / concrete composite and the external cladding is fibre-cement. TimberPlus building as for the Timber building above but with greatly increased use of timber throughout including timber external cladding and cedar windows and louvres, solid timber internal walls, timber ceilings and other features which maximise the use of timber throughout the building. In each design, the objective was to use as much of the target material as reasonably possible, both in structures and finishes. However, to standardise - and adhere to good NZ design practice - for the Concrete, Steel and Timber designs, many interior and exterior finishes are as commonly found in typical NZ multistorey buildings and similar in each design. Figure 1 shows two views of the TimberPlus building design. 4 OPERATIONAL ENERGY John et al. [1] provides detailed information on the building services in each building design, operational energy simulation modelling and life cycle operational energy usage based on the energy profile for a typical multi-storey office building in Christchurch.

3 Figure 1: TimberPlus building, North-east and South-west perspective views. Of central importance to the design of this research was the requirement to have all four alternative designs displaying very similar operational energy consumption over the lifetime of the buildings. Several previous researchers have found that even when the energy efficiency of buildings being compared is codecompliant, the effects of the embodied energy of construction materials are difficult to discern and negligible in comparison to the much larger variations in operational energy between the different buildings [3, 8-10]. Comparing all four buildings with similar operational energy consumption means that the differences in the environmental impacts are determined by the differences in the embodied and recurrent (maintenance and refurbishment) energy and global warming potential (GWP) in the different materials used in each building. The energy consumption profile for the actual Concrete building was used as a benchmark energy target for the alternative Steel, Timber and TimberPlus buildings. The predicted annual operational energy consumed in the four buildings is fairly similar; the Concrete building uses 84 kwh/m 2 /yr, followed by the Steel and the TimberPlus buildings, both using 86 kwh/ m 2 /yr, and the Timber building, using 88 kwh m 2 /yr. The underlying difference between operational energy consumption between the buildings is mostly due to the amount of concrete (acting as thermal mass) involved in each building. Modifying the design to achieve similar operational energy consumption is achieved through changes to the insulating materials, thermal mass and heating and cooling equipment in each of the four buildings The difference in energy consumption between the Timber and TimberPlus buildings is due to the influence of solid wood in the partitions, external walls and ceiling acting as thermal mass, storing and exchanging heat [11] and also because the TimberPlus building normally has higher R/values than the Timber building, due to timber external claddings and interior linings. 5 CARBON FOOTPRINTING ing calculates the amount of green house gas (GHG) caused by a particular activity or entity [12], commonly also referred to as global warming potential (GWP) and is measured in tonnes (or kilograms) of carbon dioxide equivalent (CO 2 eq.). 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 end-of-life. A comparison of the carbon of one building with another can give an indication of the potential environmental benefit of using different building materials with regard to the emission of greenhouse gases. 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-to-grave 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 (CO 2 eq.) 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 CO 2 eq. 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.

4 Approximately 50% of dry timber is elemental carbon; thus, 1 kg of wood contains approximately 0.5 kg of carbon, which equates to 1.83 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 much debated issue. If a carbon calculation does account for stored carbon, then there must be a defined end-of-life 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 (CO 2 eq. tonnes/m 2 ). In simplified form, to establish the initial carbon of a building s materials (cradle-to-gate) requires ; The collection of accurate data on the quantities of materials in the building deciding what materials to include (either because the material is present in large quantity or because the material s manufacture produces large CO 2 eq. ) Utilising life cycle inventory (LCI) data for all the building materials considered, to determine a suitable, accurate dataset of GWP coefficients. (Many countries now have local data sets covering at least the most common building materials used in a region, including both locally manufactured materials and those imported from outside the country). A simple spreadsheet which multiplies material quantities by the appropriate GWP coefficient, followed by summation to give the total GWP of the materials in a building. The rigorous, systems-perspective methodology of life cycle assessment (LCA) allows carbon ing to be extended to cover the full life-cycle of the building but 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. 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 [13]. 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. Employing LCA to make an accurate calculation of a building s GWP can be a complex, costly and timeconsuming exercise. 5.1 LCA OF MODELLED MULTI-STORY BUILDINGS The report Environmental Impacts of Multi-storey Buildings Using Different Construction Materials [1] presents the results of a full and detailed LCA (following ISO and standards [14,15]) for GWP and energy use of the buildings described in Section 3 above, 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. 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 reducing the associated with the activities of this operational phase, such as through better overall building design, passive heating and cooling, the use of phase-change materials, energy efficient lighting, etc.. Current research at the University of Canterbury [16] is showing promising results that timber buildings can be designed to offer low-operational energy consumption to rival either steel or concrete buildings. 5.2 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 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, very small in the Concrete and Steel buildings but over 1,150 tonnes in the TimberPlus design.

5 GWP (t CO2equiv.) Concrete Steel Building type Timber Timber+ Initial Embodied Maintenance Transport Operational End of Life CO2 storage Figure 2: GWP (tonnes CO 2 equivalent) for each stage of the 60-year life cycle of all the building types. 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 CO 2 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 CO 2 from the atmosphere remains. 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 (TimberPlus design). 5.3 FULL BUILDING LIFE-CYCLE CRADLE-TO-GRAVE. In the 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 a major problem methane is a far more potent GHG than CO 2 and contributes a disproportionately large share of GWP to the end-of-life. Ximenes et al. [17] 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[ [18]. A recent figure, based on physical data [97] showing 42% capture of CH 4, has been taken into account. Despite the emission of some methane, Table 1. Cradle-to-gate (embodied) (CO 2eq.tonnes) for the materials in the four building designs with the associated carbon (CO 2eq. tonnes/m 2 ). Building Design Cradle-to-gate (gross) Gross CO 2 eq. CO 2 eq. sequestered in building materials Cradle-to-gate (net) Net CO 2 eq. Concrete 1, , Steel 1, , Timber TimberPlus ,

6 Table 2. Cradle-to-gate compared to cradle-to-grave (CO 2eq. tonnes) for the four building designs with the associated carbon (CO 2eq. tonnes/m2). Cradle-to-gate* Cradle-to-grave** Building Design CO 2 eq. CO 2 eq. 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. 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 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 land-fill) demonstrated a similar trend with the smallest for the TimberPlus building [1]. 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. 6 PERMANENT STORAGE OF CARBON IN TIMBER BUILDING MATERIALS In the above land-fill scenario, at least some of the carbon stored in the timber building materials is released back into the atmosphere. Under the following end-of-life options, it is realistic to consider near-permanent storage of nearly all the carbon in timber building materials; 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). tonnes CO Concrete Steel Timber TimberPlus Wood Other Aluminium Steel Concrete Figure 3: GWP (CO 2eq. tonnes) for the materials in the four buildings, assuming permanent storage of carbon in wood products.

7 tonnes CO Concrete Steel Timber TimberPlus Figure 4: Net 60-year life cycle GWP (CO 2eq. tonnes) for the four buildings, assuming permanent storage of carbon in wood products. Table 3: Cradle-to-gate compared to cradle-to-grave (CO 2eq. tonnes) for the four building designs with the associated carbon (CO 2eq. tonnes/m2), assuming permanent storage of carbon in timber products. Building Design Cradle-to-gate Net CO 2 eq. Cradle-to-grave Net CO 2 eq. Concrete 1, , Steel 1, , Timber , TimberPlus , 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 around the world. However, there are many examples in many countries of progress towards this permanent storage scenario. 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 CO 2 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 CO 2 eq. Figure 4 shows the net 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 CO 2 eq.. 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.

8 7 DISCUSSION AND CONCLUSIONS The careful collection and use of appropriate data, consistent methodology and solid scientific principles allow the calculation of a 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 CO 2 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 multi-storey 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. In the case of the study buildings, the net negative for the TimberPlus building means that the CO 2 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. 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 its materials 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. In many places around the world, an end-of-life 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, including actual or predicted operational energy consumption, 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 multistorey 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 building material with the lowest carbon. 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. ACKNOWLEDGEMENTS The authors would like to acknowledge the support and co-operation of The New Zealand Ministry of Agriculture and Forestry (MAF) and The Structural Timber Innovation Company (STIC) Ltd. REFERENCES [1] John, S.M., Nebel, B., Perez, N. and Buchanan, A. (2009). Environmental Impacts of Multistorey Buildings Using Different Construction Materials. Research Report , University of Canterbury, New Zealand. ( a-materials.pdf ) [2] Aye, L., Bamford, N., Charters, B., & Robinson, J. (1999). Optimising embodied energy in commercial office development. RICS Foundation, [3] Cole, R. J., & Kernan, P. C. (1996). Life-cycle energy use in office buildings. Building and Environment, 31(4), (311). [4] Oppenheim, D., & Treloar, G. (1995). Embodied energy and office buildings - A case study. Proceedings of the 33rd Australian and New Zealand Solar Energy Society Conference 1, [5] Treloar, G. J., Fay, R., Ilozor, B., & Love, P. E. D. (2001). An analysis of the embodied energy of office buildings by height. Facilities, 19(5-6), (211). [6] Pampanin, S., Palermo, A., Buchanan, A., Fragicomo, M., & Deam, B. (2006). Code Provisions for Seismic Design of Multi-Storey Post-Tensioned Timber Buildings. CIB-W18 Conference Proceedings: Paper , Florence, Italy.

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