Energy efficient houses can significantly reduce heating and cooling energy use and associated greenhouse gas emissions. However, for increasingly efficient buildings, the embodied energy required to produce additional insulation and high-performance glazing can offset operational energy savings and lead to higher overall energy use.
Our study, published in the journal Renewable and Sustainable Energy Reviews, looked at current energy efficiency regulation in Australia. We found that further increasing the energy efficiency level of houses beyond the current minimum requirement does not result in significant energy savings and can even lead to greater energy demands. It is therefore critical that we consider the entire life cycle of a building, beyond just heating and cooling.
Rating Australian homes
In Australia, the NatHERS star-rating scheme is used to provide a measure of the energy efficiency of residential buildings on a scale of one to 10 stars, with 10 being extremely energy efficient and requiring almost zero energy for heating and cooling. The current minimum rating that a house needs to achieve is six stars.
However, achieving a high star rating typically requires more materials – notably insulation, glazing and efficient window frames. These materials require a significant amount of energy in their production, known as embodied energy.
In a climate such as Melbourne’s, a six-star rated home would require less than 114 megajoules of energy per square metre each year for heating and cooling. That’s enough energy to charge 13 smartphones (2016) over a year. In Brisbane, the milder climate results in a lower energy demand for heating and cooling, with a maximum of 43 MJ/m² a year. A new six-star house in Melbourne typically uses two-thirds less energy for heating and cooling than an average home.
Going beyond six stars and towards seven, eight, nine and 10 stars further reduces the heating and cooling demand but requires additional materials for insulation and glazing.
We wanted to find out if the additional embodied energy use of these materials would be worth the heating and cooling savings they offer, as the house becomes increasingly efficient. And the answer is: not always.
Evaluating the overall energy use of energy efficient houses
We used a typical Australian detached house with a fixed floor plan and used two sets of materials to achieve the minimum six star rating in Melbourne and Brisbane, respectively. From this baseline scenario, we investigated increasing energy efficiency through:
- increasing the insulation level and installing high-performance glazing (improvement by material); and
- modifying basic design features of the house, for example, exposing a concrete slab on ground for thermal mass (improvement by design).
Through each of these two approaches, we tried to achieve increasing levels of efficiency for the house, both in Melbourne and Brisbane. We also tried a combination of both approaches to obtain a 10-star house in each location. Here is what we found.
Operational energy was modelled using the official Australian dynamic energy simulation software FirstRate 5 while embodied energy was quantified using the most advanced technique globally, called “hybrid analysis”.
Figure 1 compares the total energy use of all investigated scenarios to the baseline scenario, over 50 years, for Melbourne (top) and Brisbane (bottom). The embodied energy associated with extra insulation and high-performance windows is grouped under the thermal embodied energy category. A strong focus on pragmatic and realistic modifications was made. That meant that systems or designs that are not likely to be implemented in an Australian context – insulating the ground floor slab of the garage, triple glazed argon-filled windows, clerestory windows – were not considered. The energy use was measured over 50 years, during which material replacements were factored in and the operational energy level held constant (assuming no deterioration of components or systems). Only fibreglass insulation was used as it is the most common insulation material in Australia.
Three main observations can be made:
First, additional insulation and glazing are not the solution. In Melbourne, improvements by material yielded almost no benefits beyond the six star baseline rating. In Brisbane, improvements by material actually resulted in a significant increase to the overall energy use, doing exactly the opposite of what the scheme aims to achieve. A nine star house by material in Brisbane can actually use 262 GJ of additional energy over 50 years – enough fuel energy to drive from Adelaide to Darwin (2834 km) 52 times.
Second, improvements by design can reduce both thermal and embodied energy use. As can be observed, improvements by design yielded overall life cycle energy use reductions over 50 years, both in terms of operational and embodied energy use. This is because certain strategies, such as reverse brick veneer walls or exposing the concrete slab either do not require additional embodied energy or actually reduce the total embodied energy demand by using less materials (and not needing to replace them).
Third, the highest star rating does not necessarily result in the lowest overall energy use. As depicted in Figure 1, scenarios combining improvements by material and design achieve the highest possible star rating (10) but do not result in the lowest total energy use across all scenarios.
The current star rating system does not therefore adequately measure the overall energy performance of buildings.
For building energy efficiency regulations to achieve their goal of reducing energy use, they need to consider embodied energy on top of operational energy use. A life cycle approach is needed to ensure that additional operational performance is not offset by additional embodied energy use.
However, this would only be possible and effective by adopting the comprehensive hybrid analysis as a common quantification approach for embodied energy, developing embodied energy (and greenhouse emissions, water and other indicators) databases for construction materials, training professionals in life cycle thinking and raising awareness about embodied environmental effects in general.
In addition, better designs can yield significant life cycle energy savings. Rather than focusing on material and technological improvements, energy efficiency regulations should actively promote better building design as a means to achieve performance. This should be accompanied by better training and education of professionals.
As more stringent operational energy efficiency regulations are imposed, the significance of embodied energy demands will continue to increase and the findings of this study and other similar research will only become more relevant.
Dr Robert Crawford is a senior lecturer in construction and environmental assessment at the University of Melbourne. Dr André Stephan is a postdoctoral research fellow in the Faculty of Architecture, Building and Planning at the University of Melbourne. Christopher Jensen is a lecturer in construction management at the University of Melbourne. Erika Bartak is a tutor in environmental building systems and design at the University of Melbourne.