Life cycle energy requirements of residential buildings in New Zealand

Abstract

In New Zealand the building and construction industry makes a significant contribution to the economy. The residential sector which makes up about two thirds of the building and construction industry in New Zealand is unique due to the high rate of home ownership compared to other countries and the rapidly increasing and changing population. The need to improve performance in terms of energy use to reduce the greenhouse gas emissions attributable to the residential sector is now widely recognised. However, there is little evidence about the life cycle performance of NZ houses in terms of energy, cost and environmental impact, as these have not been studied in detail so far. This research focuses on the life cycle performance of common NZ house types in the Auckland region where one third of the New Zealand population live. Due to the current practice of changing ownership at relatively short periods of about seven years, decisions related to design and construction of NZ houses are based on short-term concerns. However, as buildings last for long periods compared to many other products it is essential to evaluate buildings based on the life cycle performance. Further, sustainable use of building materials, reduction in waste, recycling or reusing the waste materials are not concerns of those who are involved in the construction industry in New Zealand at present. The aim of this research therefore is to identify and rank the critical factors that influence the life cycle performance of New Zealand houses in the Auckland region. In order to achieve this, common construction types currently used in the construction of NZ houses are identified and an extensive literature survey is earned out to identify the theory and practice of life cycle energy, cost and environmental impact of buildings. This information is then used to generate a computer model, which could be used to examine the current construction practices and design decisions over the useful life of the houses using established methods adapted from the previous work of other researchers but based on the conditions representative of New Zealand practices. The model could be used by designers at the sketch/detail design stages to assess the life cycle performance of their designs. The results demonstrate that floor, walls and roof represent the bulk of the embodided energy and therefore preliminary energy calculations for these elements could aid in the initial screening of design and construction types appropriate for any situation. The life cycle embodied energy of NZ houses is only around 20% of the total life cycle energy based on the common construction types studied. This is comparable to the figures reported by previous research. Although heavy construction with reduced maintenance requirements over the useful life recorded lower life cycle embodied energy the reduction was not significant. The results show that of the operating energy, which is the dominant component of the life cycle energy, about a third is for space heating a third for water heating and the balance third is for other uses. The space heating energy requirement could be significantly reduced by increasing the level of insulation. Although increasing the level of insulation could increase the embodied energy, for a standard non-optimised house this alone could reduce the life cycle energy dramatically. This was clearly evident whether or not operating energy other than space heating was included. Although bigger houses and those located in the colder climates use more life cycle energy, use of additional insulation showed a dramatic reduction in life cycle energy and therefore the overall impact. Therefore, reduction in life cycle energy is not reliant on the use of thermal mass, which is less common in New Zealand. Although, additional insulation increased the life cycle cost marginally it could buffer the owner against any sudden increase in energy prices while providing improved comfort and health benefits. The space heating energy is the only component of total operating energy that depends on design and construction of the building. However, the other operating energy requirements such as water heating and lighting could also be improved at the design stage by the inclusion of measures such as, highly insulated hot water cylinders, solar hot water systems, fuel switching and energy efficient compact fluorescent light bulbs, etc. Whether these would be replaced with identical or similar products when replacement becomes necessary at the end of their useful life by the homeowners would decide the success of such measures. The results also highlighted the importance of finishes used in NZ houses. Due to shorter replacement cycles, finishes contribute significantly to the total life cycle impact in terms of energy, cost and greenhouse gas emissions. Therefore in terms of life cycle performance not only the mass of the element but also the replacement cycle is equally important. Life cycle greenhouse gas emissions of NZ houses follow a pattern similar to life cycle energy, as these emissions are mainly due to the energy use. Use of additional insulation could reduce the greenhouse gas emissions significantly. Furniture and domestic appliances although not reliant on the design and construction type contribute significantly to both life cycle energy and greenhouse gas emissions over the useful life of the building due to their relatively short lifespan. Energy used for the operation of appliances was about 20% of the total operating energy over the useful life. Therefore, these items make a significant contribution to the total performance although their replacement requirements and performance are not clearly known. Research into life cycle requirements of furniture, domestic appliances and equipment used in NZ houses and environmental impacts of New Zealand building materials is therefore essential for better understanding of the life cycle performance of NZ houses.