The Influence of Construction Materials on Life-Cycle Energy Use and Carbon Dioxide Emissions of Medium Size Commercial Buildings

Abstract

This thesis studies the influence of construction materials on the life-cycle energy consumption and carbon dioxide (CO2) emissions of medium sized low energy consumption commercial buildings. When describing buildings by materials, there is a tendency to label them according to the main structural material used. However, the vast majority of commercial buildings use a large number of materials. Hence it is not clear which materials or combinations of materials can achieve the best performance, in terms of lifecycle energy use and CO2 emissions. The buildings analysed here were based on an actual six-storey 4250m2 (gross floor area) building, with a mixed-mode ventilation system, currently under construction at the University of Canterbury in Christchurch. While the actual building is being constructed in concrete, the author has designed two further versions in which the structures and finishes are predominantly steel or timber. Despite having different structural materials, large quantities of finishes materials are common to all three buildings; large glazed curtain walls and sun louvers, stairs balustrade and most of the offices internal finishes. A fourth building was also produced in which all possible common finishes' of the timber building were replaced by timber components. This building is labelled as Timber-plus and was included to assess the difference of the three initial 'common finishes' buildings against a building that might be expected to have a low or even negative total embodied CO2 emission in structure and finishes. In order to highlight the influence of materials, each building was designed to have a similar indoor climate with roughly the same amount of operational energy for heating and cooling over its full life. Both energy use and CO2 emissions have been assessed over three main stages in the life (and potential environmental impact) of a building: initial production of the building materials (initial embodied energy and initial embodied CO2 emissions); operation of the building (mainly in terms of its energy use); and the refurbishment and maintenance of the building materials over the building's effective life (recurrent embodied energy and CO2 emissions). Calculation of embodied energy and embodied CO2 emissions are based on materials' estimates undertaken by a Quantity Surveyor. DesignBuilder software was used to estimate whole life-cycle energy used and CO2 emitted in the operation of the buildings over a period of 60 years. Two different methods for embodied energy and embodied CO2 calculation were applied to the four buildings. The first method was by multiplying the volume of each material in the schedule calculated by the Quantity Surveyor by the New Zealand specific coefficients of embodied energy and embodied CO2 produced by Andrew Alcorn (2003). The second method was analysing the same schedule of materials with GaBi professional LCA software. Materials' inventories in GaBi are average German industry data collected by PE Europe between 1996 and 2004 (Alcorn, 2003; Nebel & Love, 2008). The energy results of the thesis show that when using the Alcorn coefficients, the total embodied energy (initial plus recurrent embodied energy) averaged 23% and operating energy consumption averaged 77% of the total life-cycle energy consumption for the four buildings. Using the GaBi coefficients, total embodied energy averaged 19% and operating energy consumption averaged 81% of the total life-cycle energy consumption of the four buildings. Using the Alcorn coefficients, the difference between the highest (steel building) and lowest (timber-plus building) life-cycle energy consumption represents a 22% increment of the highest over the lowest. Using the GaBi coefficients, the difference between the lowest (timber-plus building) and the highest (timber building) life-cycle energy consumption represents a 15% increment of the highest over the lowest. The CO2 results shows that when using the Alcorn coefficients, the total embodied CO2 emissions averaged 7% and operating CO2 emissions averaged 93%. Using the GaBi coefficients, total embodied CO2 emissions averaged 16% and operating CO2 emissions averaged 84% of the life-cycle CO2 emissions of the four buildings. Using the Alcorn coefficients, the difference between the highest (steel building) and lowest (timber-plus building) life-cycle CO2 emissions represents a 27% increment of the highest over the lower. Using the GaBi coefficients, the difference between the highest (timber building) and the lowest (timber-plus building) lifecycle CO2 emissions represents a 9% increment of the highest over the lowest. While for the case of embodied energy the Alcorn results averaged 32% higher than the GaBi, in the case of embodied CO2 the Alcorn results averaged 62% lower than the GaBi. Major differences in the results produced when using the two different sets of embodied energy and CO2 coefficients are due mainly to their different approaches to the CO2 sequestration in timber materials. While the Alcorn coefficients account for the CO2 sequestration of timber materials, the GaBi coefficients do not. This is particularly noteworthy as the CO2 sequestration of timber has been neglected in previous research. It was established that embodied energy can significantly influence the life-cycle energy consumption and CO2 emissions of contemporary low energy buildings. Using the Alcorn coefficients, the steel building embodied the equivalent of 27 years of operating energy consumption and 12 years of operating CO2 emissions. At the other end of the spectrum the timber-plus building embodied the equivalent of 11 years of operating energy consumption and has stored the equivalent of 3.6 years of operating CO2 emissions. Using the GaBi coefficients, the steel building embodied the equivalent of 19 years of operating energy consumption and 14 years of operating CO2 emissions, while the timber-plus building embodied the equivalent of 8 years of operating energy consumption and 8 years of operating CO2 emissions. These findings are of significance, for example, in the assessment and weighting of the embodied energy and embodied CO2 components of building sustainable rating tools.