The cost effectiveness of housing thermal performance improvements in avoiding `CO_2-` emissions

It is widely recognised that the built environment is a significant contributor to greenhouse gas emissions worldwide. In Australia, to reduce the greenhouse gas emissions associated with new houses, energy efficiency provisions were introduced into the Building Code of Australia (BCA). The primary focus of the regulations has been on achieving thermal comfort through a reduction in the energy houses require for heating and cooling (space-conditioning energy). A star rating system is used to indicate the level of thermal performance a new house achieves. Ratings range from 0 to 10 stars and theoretically, the higher the star rating the less space-conditioning energy a house requires. Currently, all new houses built in Australia require a minimum of either a 5 or 6 star rating, depending on the state/territory, with the required minimum level expected to increase incrementally in the future. It is widely claimed that there are considerable opportunities for cost effective greenhouse gas abatement in the residential building sector. However, the claims generally neglect to take into account any increase in embodied energy (and associated emissions) that may result from implementing those opportunities. Increasing a house's thermal performance generally increases its embodied emissions. Current research findings indicate that embodied energy and its associated emissions can contribute significantly to a house's lifecycle energy and `CO_2`-emissions, with that contribution increasing the more thermally efficient a house becomes. The aim of this research was to determine and rank the cost effectiveness of a wide range of thermal performance improvements, for houses with ratings from 4 to 8 stars, taking into account their embodied emissions. To achieve this, several project homes constructed in Tasmania whose size and floor plan varied were selected. Using thermal simulation software the space-conditioning energy requirements of the thermal performance improvements were calculated. The cost of each thermal performance improvement was estimated and the resulting increase in embodied energy calculated. For each house, timber floor and slab-on-ground designs were modelled. The same thermal performance improvements were made to each house. These were ranked for their cost effectiveness in reducing space-conditioning emissions, minimising the increase in embodied emissions and reducing net emissions. For each measure of cost effectiveness, the rankings were compared to determine the effect house design and house size had on the results. The results show that the cost effectiveness of achieving pre-determined levels of thermal performance varies significantly depending on the methods and materials used. There are numerous methods that can be used to improve the thermal performance of a house to a certain level, with costs varying significantly. While generally the most and least cost effective designs in minimizing embodied emissions are the same for each house, some design differences between the houses are a significant factor in determining how cost effective improvements will be in minimizing embodied emissions. In terms of cost effectiveness in reducing net emissions the results show that for lower star band ranges (5-6 star), the most cost effective designs in reducing net emissions are also the most cost effective in saving space-conditioning emissions. However, in the higher star band ranges cost effectiveness in saving space-conditioning emissions cannot be used to predict reliably the cost effectiveness in saving net emissions. Finally, the results shows that while heating appliance type and efficiency do not affect cost effectiveness rankings, the choice of heating appliance is significant in determining whether an increase in embodied emissions outweighs a decrease in space-conditioning emissions and at what level of thermal performance that occurs.