While the windows in our homes simply allow light inside, and keep the rain outside, they also have a large impact on the heating and cooling efforts applied to your home. With up to 40% of a home’s heating energy being lost and 87% of heat gained through the windows, improving the performance of your windows not only reduces your energy costs but also greenhouse gas emissions.
There are literally hundreds of glass and window types to choose from, but selecting the correct one is critical in improving the energy efficiency of your home, and therefore your energy costs.
With the Building Code of Australia (BCA) now stipulating regulations on energy efficiency, your window selection along with other factors determine the star rating for your home. Each state is slightly different due to their individual climate, and it’s important to understand what is best for your climate when selecting your windows.
While there are 69 climate zones defined in the BCA, essentially all of them fall into three broad categories:
Cooling climate (the largest use of energy is in cooling the home):
the energy efficient solution is to keep the sun’s rays out and the cool air in. In this climate zone (tropical, subtropical or hot arid) the best results are obtained from windows that limit solar heat gain on all orientations (low solar heat gain coefficient). Good insulation (a low U-value) is also beneficial, especially if the home is air‑conditioned.
Mixed climate (the use of energy is equal between cooling and heating the home):
requires a home that can minimise the effects of the sun’s heat during summer, then insulate and utilise solar heat gain in winter. This means different glazing solutions. In this climate zone (temperate) the best results are obtained from windows that insulate well (low U-value), admit plenty of solar energy (high solar heat gain coefficient) on the north during cooler months, but limit solar heat gain from the east and west (low solar heat gain coefficient). Ideally, northerly windows should be protected by correctly sized eaves to prevent summer time heat and glare while still allowing sun penetration in winter. Argon gas is recommended.
Heating climate (the largest use of energy is in heating the home):
the priority is to retain heat in the home and maximise the use of solar energy in winter. In this climate zone (alpine and cool temperate) the best results are obtained from windows that insulate well (low U-value) and admit plenty of free solar energy (high solar heat gain coefficient). Large west-facing windows may contribute to short-term overheating in summer, but glazing with a low solar heat gain coefficient must be used with caution on the west because of the energy penalty it can cause over the rest of the year. Argon gas is highly recommended.
There is a range of software tools that have been developed to determine the efficiency and star ratings of your home. All windows must now be rated under the Window Energy Rating Scheme (WERS) with the information verified by the Australian Fenestration Rating Council (AFRC). WERS rated residential windows have star ratings and percentage improvements to help give an easy comparison of windows, however it is specifically the U-value and the solar heat gain coefficient that are required and used in this rating tools.
Your designer or builder should use an authorised computer modelling package to explore different ways of using building materials including glass; as well as other passive energy efficiency techniques such as eaves, shading, location and orientation; while ensuring adequate ventilation and natural lighting, to meet energy efficiency requirements.
The focus on maintaining good design principles for your home, rather than the specific energy performance of any one building element, will provide you with a home that not only complies with energy regulations, but one that is comfortable for you to live in.