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Sound insulation and provisions for thermal mass in walls and floors are fabulous additions to a sustainable home, but without a well-designed window system — comprising efficient glass and framing — all your good work can literally go out the window.
Windows are the biggest culprits in the building envelope for unwanted heat gain in summer and significant loss in winter, especially as glazing accounts for up to 40 per cent of the wall area of a typical Australian home. This is because windows are a fast-transfer point for both radiant heat (sunlight) and conductive heat (air). So what’s the best way to stop the flow of heat? Choose the right glass for your climate. Richard Hamber, Window Energy Rating Scheme (WERS) manager, says there are many types of glass available and making a decision can be overwhelming.
Basically, there are two main considerations: the U-value and the solar heat-gain coefficient (SHGC). The U-value measures how well a window prevents the flow of heat (conductive), while the SHGC measures how well it blocks heat caused by sunlight (radiant).
“It does not matter where you live or what time of the year, having a high-performing, energy-efficient window system [low U-value] will ensure any heating or cooling appliance runs less by stopping the flow of thermal energy out of the building,” Hamber says.
When it comes to SHGC, it depends on where you live as to whether a high or low figure is best. “If you live in Hobart you’ll want to minimise heating but cooling is not an issue, use glazing that has the lowest U-value but a high SHGC,” says Dr Veronica Soebarto, associate professor at the School of Architecture, Landscape Architecture and Urban Design at The University of Adelaide. “If cooling is also important, because you
live in Adelaide, then do the same, but make sure the window is shaded in the summer to minimise solar heat
gain. If heating is not an issue as you live in Darwin, use glazing with a low U-value and low SHGC to minimise
solar heat gain.”
The frame has a significant impact on the performance of a window and can significantly lower the overall U-value, as it also acts as a heat transfer point. Aluminium, timber, uPVC (unplasticised polyvinyl chloride) and various composites are popular materials used in window frames.
“Thermally efficient frames, such as timber, are the first and most sustainable choice, followed by uPVC, which has good thermal qualities but is not very eco-friendly in manufacture,” says Danielle King, director of Green Moves Australia. “Aluminium is not thermally efficient at all and transfers heat quickly. If you do go for aluminium frames, ensure you have a thermal break in the frame to minimise the heat transfer.”
Timber or uPVC frames with clear or low-emissivity double glazing (which absorb, reflect and emit low levels of radiant heat) are about 50 per cent more efficient than standard aluminium single-glazed windows. As a compromise, composite frames – with a high-insulating material such as wood on the inside and durable aluminium on the outside – are a good alternative.
To simplify the process, the independent WERS star-rating system rates individual products for energy efficiency and allows consumers to select window systems on a level playing field. “WERS rates all windows as a combination of frame and glass and gives them a star and percentage rating that allows you to compare,” Hamber says.
The heating stars indicate how well the window system will retain warmth, while the cooling stars indicate how well it will protect you from summer heat. In both cases, a 5-star rating earns top billing, but you may of course focus on a particular element, depending on your climate.
Not all windows are WERS rated; to be sure you are buying a WERS-rated product check the website (www.wers.net) to find manufacturers or ask for a WERS compliance certificate from your supplier.
For truly high-performance windows, double glazing is the way to go. In these, two panes of glass are separated by a small air gap, which limits heat flow by providing an insulating barrier. Double glazing reduces heat loss by 30 per cent compared to single glazing, and also helps with noise reduction.
“There are thousands of types of glass that can be used in windows,” Hamber says. “The best glass for energy efficiency would have to be double glazing. The use of an air gap between two layers [of glass] works as a very effective insulation barrier.
“Double glazing can also incorporate a number of different technologies, such as tints, low-emissivity layers and different gap fills, such as argon, to increase the energy efficiency of the product. There is also the option to extend the double glazing to triple glazing.”
According to Sustainability Victoria, double glazing does not impede solar heat gain. This is great news for harnessing winter sun, but you will still need adequate summer shading. It can be used in most areas but is most appropriate in cold climates, skylights and large, glazed areas.
Double glazing can be retrofitted to existing windows, causing less waste and costing much less than new glazing. However, Hamber says costs have come down significantly in Victoria, Tasmania and the ACT, while the long-term energy savings make it a worthwhile investment Australia wide.