The Effect of Material Service Life on the Life Cycle Embodied Energy of Multi-Unit Residential Buildings

Transcription

1 The Effect of Material Service Life on the Life Cycle Embodied Energy of Multi-Unit Residential Buildings Rauf, A. 1 ; Crawford, R.H. 1 1 The University of Melbourne, Faculty of Architecture, Building and Planning, Melbourne, Australia Abstract: Previous analyses of energy across the various life cycle stages of a building have shown the significance of both the energy required for building operation as well as energy embodied in initial building construction. Service life and durability of materials are among the most important factors affecting the embodied energy associated with maintenance and replacement of materials over a building s life. Variations in service life of materials can potentially lead to significant variability in a building's embodied energy. The aim of this study was to investigate the effect of variations in the service life of materials on the life cycle embodied energy demand associated with a multi-unit residential building. The initial and recurring embodied energy of a case study building were calculated, with material service life values based on average figures obtained from the literature. These values were then varied to reflect the extent of service life variability likely for a selection of the main building materials and the recurring embodied energy recalculated for each scenario. The results from this initial study indicate that the service life of materials can have a significant effect on the total energy embodied in a building over its life. Keywords: material service life; recurrent embodied energy; life cycle embodied energy; multi-unit residential building Introduction Fossil fuel-based energy use is considered one of the largest contributors to environmental degradation and global warming. The main source of greenhouse gas (GHG) emissions from the building sector is energy consumption [1]. Buildings are responsible for 30-40% of energy use and associated GHG emissions both in Australia and worldwide with a significant share of this attributable to residential buildings [2]. This problem is further exacerbated with the increasing global population, expected to reach 8 billion by 2028 [3]. It is likely that urban growth and its related infrastructure will continue to impact greatly on the natural environment well into the future, particularly through the consumption of raw materials and energy. In this scenario, it is important for all stakeholders in the building industry to incorporate strategies at the feasibility and design stages of buildings to reduce the energy consumption associated with the building sector in order to reduce greenhouse gas emissions and avoid further degradation of the natural environment. Previous studies have shown the significance of the energy required for the operation of buildings as well as the energy embodied in initial building construction [4]. Few studies have 1

2 analysed the recurrent embodied energy involved in maintenance and refurbishment activities over a building s life [5, 6]. Recurrent embodied energy associated with the replacement of building materials and components is directly affected by the service life of building materials. However, the significance of material service life and recurrent embodied energy on the life cycle energy performance of a building is not well known. The aim of this study was to determine what effect variations to the service life of materials have on the life cycle embodied energy demand associated with multi-unit residential buildings. Background Recurrent embodied energy of buildings The energy associated with the production of construction materials and construction of a building, known as a building s initial embodied energy, has been quantified in numerous previous studies [inter alia 7, 8, 9]. While there is still considerable debate around the appropriate method of analysis that should be used, the principles around how embodied energy is calculated are fairly well understood. However, the quantity of energy associated with manufacturing the materials needed for maintenance and repair throughout a building s life is much less understood. There are a much more limited number of studies where this recurrent embodied energy has been calculated. Some of the reasons for this are the considerable variability that exists in the service life of materials and the lack of building material and component service life data. A study by Treloar et al. [10] shows that embodied energy associated with the replacement of building materials can represent up to 32% of the initial embodied energy of a building. Another study by Crawford on residential construction assemblies shows that the energy embodied in material replacement can represent between 7 and 110% of their initial embodied energy [5]. In a study of office buildings, Cole and Kernan [11] have also shown that recurrent embodied energy can be significant, representing 1.3, 3.2 and 7.3 times the initial embodied energy value for an assumed building lifespan of 35, 50 and 100 years, respectively. In another study of residential buildings, embodied energy was increased due to the maintenance and replacement of materials from 14.1 GJ/m 2 to 23.5 GJ/m 2 over 50 years and to GJ/m 2 over 100 years [12]. Service life of building materials A material s service life is the amount of time that it can be expected to be serviceable. While predictions of service life will often be based on previous experience or warranty periods, a number of key factors will determine the actual service life of a material in use. These factors include material quality, design and detailing, quality of workmanship, maintenance regime and levels, material durability and exposure to deteriorating effects associated with the local climate and environment [13]. The service life of a material affects the number of times it will be replaced over the life of a building. The lower the service life of a material, the greater the quantity of material required for ongoing maintenance and repair and therefore the greater the embodied energy demand associated with manufacturing and installing replacement materials throughout a building s 2

3 life. As it is typically fossil fuel-based, this additional demand for energy may result in a considerable ongoing burden on the environment. Case study building In Australia, medium density housing is the most rapidly growing dwelling type. Flats and apartments accounted for 11% of all dwellings in [14]. A nine-storey apartment building, known as Forte, located in Melbourne, Australia and constructed by Lend Lease in 2012 was used as the case study for this analysis (Figure 1). This building is currently the world's tallest timber building with 197 m 2 of retail space on the ground floor and 23 residential apartments with a total area of 1,558 m 2 [15]. Concrete is used for the footings and ground floor. From the first level up, the entire structure (including load bearing walls, floor slabs, stairwells and elevator cores) is composed from solid timber using Cross Laminated Timber (CLT). A 10 mm thick layer of Uniroll (manufactured using recycled foam rubber and cork) was applied to all CLT flooring [15]. External walls are clad with an additional protective rain screen made of a 4 mm thick LDPE core with two aluminium sheets of 0.5 mm thickness on either side of this core. The windows are double-glazed and aluminium-framed. Figure 1 Exterior (left) and interior view (right) of Forte building. Source: ANCR [16] Life cycle embodied energy analysis In order to determine what effect a variation in the service life of materials would have on the life cycle embodied energy demand of the case study building, it was necessary to quantify the building's total life cycle embodied energy including its initial and recurring embodied energy. A range of scenarios for the service life of some of the main construction materials were then developed and the life cycle embodied energy demand of the building recalculated. Initial embodied energy An input-output-based hybrid analysis was used to quantify the embodied energy associated with the initial construction of the case study building. Delivered quantities of materials used in the construction of the apartment building were multiplied by the embodied energy coefficient of the respective material, obtained from Treloar and Crawford (2010). Any 3

4 remaining data gaps were filled with the use of a disaggregated energy-based input-output model of the Australian economy. This accounted for non-material inputs required in the construction of the building (e.g. services). A detailed description of the hybrid approach used is provided by Crawford [17]. Recurrent embodied energy The recurrent embodied energy was calculated based on the number of times each individual material would likely be replaced during the useful life of the building. Average material service life figures from the literature were assumed for this initial analysis (see [6]). The period of analysis chosen for this study was 50 years and thus it was assumed at the end of this period the building would be at the end of its useful life, and demolished. The embodied energy associated with the materials being replaced over the life of the building was calculated as per its initial embodied energy. The delivered material quantities associated with each replacement were multiplied by the material embodied energy coefficients. Input-output data was then used to fill any remaining data gaps as for initial embodied energy. The energy embodied in each material was then multiplied by the number of replacements for that material over the life of the building, and summed to determine the total recurrent embodied. The exact number of replacements required for each material was determined by dividing the service life of the building (50 years), by the average service life of the material, subtracting 1 (representing the material used in initial construction at Year Zero) and rounding up to the nearest whole number (to reflect the fact that materials can only be replaced in whole numbers). Material service life scenarios The average material service life values used in the initial analysis of the recurrent embodied energy of the building were varied to determine the effect of material service life on its total life cycle embodied energy demand. The material service life scenarios were chosen to reflect the extent of service life variability likely for a selection of the main building materials used within the building. The life cycle embodied energy demand associated with the building was then recalculated for each scenario. Initial embodied energy was constant across each scenario as this is not affected by variations to the service life of the materials. Changes to the service life of materials will affect the recurrent embodied energy demand, however. Minimum and maximum material service life values from the available literature were used as the basis of the two different material service life scenarios chosen for the main construction materials (i.e., timber, concrete, steel, carpet, paint etc.). A list of specific material service life values used is provided in another study by the authors (see [6]). A limitation of this initial study is that it assumes that for each scenario, all selected materials will be replaced within their minimum, average or maximum service life. In reality this is unlikely, as some materials may be maintained more frequently than others and so may need replacing closer to their maximum service life while others, which may not have been subject to more frequent maintenance, may need to be replaced closer to what might be considered their minimum 4

5 service life. The purpose of this study is thus to determine the possible extremes in variability of life cycle embodied energy that may be attributed to variations in the service life of materials. Results and Discussion This section presents the results of the analysis including the initial and recurrent embodied energy associated with each material service life scenario for the case study building. Initial embodied energy The embodied energy calculated for the initial construction of the case study building was found to be 105,832 GJ (60 GJ/m 2 ). CLT panels were found responsible for the highest amount of initial embodied energy (35%) followed by steel and concrete with a share of 21% and 13%, respectively. Recurrent embodied energy The recurrent embodied energy associated with the replacement of materials for the building over a period of 50 years, based on average service life figures obtained from the literature, was found to be 46,985 GJ (27 GJ/m 2 ). For the minimum and maximum material service life scenarios, recurrent embodied energy was found to be 111,692 GJ (64 GJ/m 2 ) and 26,683 GJ (15 GJ/m 2 ), respectively. These results reflect the fact that at an increase in material service life will result in a decrease in recurrent embodied energy requirements, up to 76% in the case of the building analysed. Life cycle embodied energy Life cycle embodied energy demand for the building over 50 years was calculated by combining initial and recurring embodied energy for each scenario. Based on the average material service life figures, life cycle embodied energy was 152,817 GJ or 87 GJ/m 2. For the minimum and maximum material service life scenarios life cycle embodied energy was 217,523 GJ and 132,515 GJ, respectively. Figure 2 shows the breakdown of the embodied energy demand by life cycle stage for each material service life scenario. Variations in material service life and thus recurrent embodied energy, results in up to a 39% reduction in life cycle embodied energy demand comparing minimum and maximum material service life results. Compared to the average material service life scenario, the total possible reduction in life cycle embodied energy demand by extending the service life of materials is up to 13%. As Figure 3 shows, the initial embodied energy of the building represents 49%, 69% and 80% of its life cycle embodied energy demand, for minimum, average and maximum material service life scenarios, respectively. This shows that when materials are poorly maintained and/or require greater frequency of replacement, the recurrent embodied energy of a building may become as significant as the embodied energy associated with its initial construction. This proportion will increase even further for a building with a service life longer than 50 years. 5

6 Figure 2 Life cycle embodied energy of the case study building based on minimum, average and maximum material service life scenarios 51% 31% 20% 49% 69% 80% REE IEE MSL (min.) MSL (av.) MSL (max.) Figure 3 Life cycle embodied energy of the case study building based on minimum, average and maximum service life scenarios, by life cycle stage proportion Conclusion The aim of this study was to determine what effect a variation in the service life of materials would have on the life cycle embodied energy of a multi-unit residential building. A case study apartment building located in Melbourne, Australia was used for this analysis. The initial and recurring embodied energy of the case study building were calculated using a comprehensive hybrid assessment approach, with material service life values based on average figures obtained from the literature. These service life values were then varied to reflect the extent of service life variability (minimum and maximum) likely for a selection of the main building materials and the recurring embodied energy and life cycle energy recalculated for each scenario. 6

7 The study has shown that a variation in the service life of materials can significantly affect the recurrent embodied energy of a building. While an increase in the service life of materials was shown to result in a reduction in recurrent embodied energy demand of up to 76% for the apartment building analysed, in terms of the total life cycle embodied energy demand of the building, the reduction was found to be in the order of up to 39%. This study has also shown that the recurrent embodied energy associated with the maintenance and replacement of materials can be significant. In fact, the recurrent embodied energy requirement for material replacement may be as significant as the initial embodied energy of a building over 50 years and likely to be more significant for buildings with a service life beyond this. This demonstrates that in an attempt to reduce the life cycle energy demand of buildings and minimise the associated environmental impacts, it is important that the service life and durability of materials is taken into account. This also suggests that the use of more durable materials may be a preferred option. However, as more durable materials may be more energy intensive in some cases, the service life of buildings should be considered when specifying these types of materials to ensure any unnecessary energy demand associated with over specification is avoided. Consideration of material choice in the context of a building s thermal performance is also important so that the selection of materials to minimise embodied energy across the building life cycle does not adversely affect a building s thermal performance and thus operational energy demand. References 1. UNEP (2009) Buildings and Climate Change - A Summary for Decision-makers, United Nations Environment Programme: France. 2. UNEP (2007) Buildings & Climate Change: status, challenges & opportunities, United Nations Environment Programme. 3. UN (1999) The World at Six Billion, United Nations Population Division: New York, USA. 4. Treloar, G.J., Love, P.E.D., and Holt, G.D. (2001) Using national input output data for embodied energy analysis of individual residential buildings. Construction Management & Economics. 19(1): p Crawford, R.H., Czerniakowski, I., and Fuller, R. (2010) A comprehensive framework for assessing the life-cycle energy of building construction assemblies. Architectural science review. 53(3): p Rauf, A. and Crawford, R.H. (2013) The relationship between material service life and the life cycle energy of contemporary residential buildings in Australia. Architectural Science Review. 56(3): p Treloar, G.J. (1998) A Comprehensive Embodied Energy Analysis Framework, in Faculty of Science and Technology Deakin University: Geelong. 8. Crawford, R.H. (2004) Using Input-Output Data in Life Cycle Inventory Analysis, Deakin University: Geelong. 9. Fay, M.R. (1999) Comparative Life Cycle Energy Studies of Typical Australian Suburban Dwellings, The University of Melbourne: Melbourne. 10. Treloar, G.J., Fay, R., Love, P.E.D., and Iyer-Raniga, U. (2000) Analysing the life-cycle energy of an Australian residential building and its householders. Building Research and Information. 28(3): p

8 11. Cole, R.J. and Kernan, P.C. (1996) Life-cycle energy use in office buildings. Building and Environment. 31(4): p Fay, R., Treloar, G., and Iyer-Raniga, U. (2000) Life-cycle energy analysis of buildings: a case study. Building Research & Information. 28(1): p ABCB (2006) Durability in Buildings-Guideline Document, The Australian Building Codes Board (ABCB): Canberra, ACT. 14. ABS (2012) Housing -Year Book Australia, 2012, Cat. No , Australian Bureau of Statistics, Canberra 15. Durlinger, B., Crossin, D.E., and Wong, D.J.P. (2013) Life Cycle Assessment of a cross laminated timber building Forest & Wood Products Australia: Melbourne, Victoria. 16. ANCR (2012) Forte Building, in National Construction Review: Victoria, Australia. 17. Crawford, R.H. (2011) Life Cycle Assessment in the Built Environment: Routledge- Taylor & Francis. 8