The loss of energy through windows can come at a financial cost to homeowners, and also at an environmental cost through the emission of greenhouse gases. However, a lot of this energy can be saved if homes are designed with energy efficiency in mind.

Windows are the weakest point in a building’s insulation, and energy efficient window design can make a difference to thermal comfort. According to a study carried out by Arup Façade Engineering for the Australian Glass and Glazing Association, up to 87 per cent of solar heat gain can occur through windows in summer and up to 49 per cent of heat loss in winter. While reducing the size or number of windows would greatly reduce unwanted heat transfer, this would lead to gloomy, unappealing homes. Luckily there are a number of options for more energy efficient windows.

The most basic option is a single, clear pane of glass. This will allow high transmittance of UV light, visible light and infra-red (heat) energy. There can also be added benefits depending on the type of window frame used.

There is a wide spectrum of tinted glass options available that can drastically reduce UV transmittance and also reduce visible light and heat energy, though the effectiveness and performance will depend on the colour and strength of the tint. Some are designed specifically to reduce the visible light passing through, while others are designed to allow maximum visible light transmittance while cutting out as much of the UV and infra red wavelengths as possible.

Window film is available in both non-reflective and metalised reflective varieties for around $50 to $80 per m2, with the reflective version having significant heat control properties. Rob Hamilton, president of the Window Film Association of Australia and New Zealand (WFAANZ) says “window film can be specified for new windows or it can be retrofitted to existing windows. Either way, window film will significantly improve the energy rating of the whole home.”

The next option is reflective and coated glass. By applying particles of metallic oxide to one of the surfaces of the glass, a transparent layer is formed that is effective at reducing solar transmittance. This is crucial in terms of saving on heating and cooling costs.

The most effective option in terms of reducing heat transmittance is an insulated glass unit (IGU), also called a double-glazed or twin-glazed unit. Ian Koochew, executive director of the Australian Glass and Glazing Association, says double glazed windows are far more effective than any of the single glazed options. “Using double glazed windows rather than single glazed can lead to a drop in heat loss of at least 40 per cent to 50 per cent.” He also says the insulation value can be increased by using a different gas in the vacuum between the panes. “A heavier gas like argon or xenon will provide better insulation than regular air.”

The effectiveness of an IGU can be increased even further if it is combined with a low emissivity (Low-E) coating. Emissivity refers to the amount of heat radiation that is allowed to escape from the surface. A low emissivity means more heat is retained or reflected back towards its source. G. James’ Solarplus Low E is an example of this type of product. It contains silver oxide and other metal oxides and is specifically designed for use in IGUs.

The insulation value of glass is measured as a U-Value, where the lower the number, the better the insulation value of the glass. Claire Culross, research scientist with G. James, describes it as a “measure of the air-to-air heat transmittance due to thermal conductance and the difference in indoor and outdoor temperature”. She says that a high U-Value in a single pane aluminium unit may be around six, while the lowest available in residential applications for a high performance double glazed unit could be as low as 2.2.

Dick Clarke, principal of Envirotecture and past president of the Building Designers Association, says “in a passively heated and cooled building, you'll need a glass with a low U-Value. In the cooler southern climates you're also looking for glass with a high solar heat gain coefficient (SHGC), but in the tropics it is better to use glass with both a low U-Value and SHGC.” SHGC refers to the amount of solar energy that is allowed to enter a building.

The difference in performance between a single pane aluminium window compared to a Low E, argon filled double glazed window in timber is vast. According to a 2007 report from Peter Lyons & Associates, depending on the size and type of building, the difference could be as much as 2.5 stars in the energy rating, up to four tonnes in the annual reduction of greenhouse gas emissions and a saving of $100 to $300 per year in heating and cooling costs.

With the recent push towards sustainability in all forms of buildings, the financial savings are just one consideration. Kristin Tomkins, executive director of building policy with the Australian Housing Industry Association (HIA), says “Victoria, WA, SA and the ACT already require that all new homes conform to a 5 star minimum energy rating,” while there is a chance that Tasmania, the NT (both currently four star) and possibly Queensland (currently 3.5 star), will soon be joining them. “NSW has a slightly different system, called BASIX, which requires around a 4 star energy rating.”

Regardless of which state or territory you’re in, the difference that energy efficient windows can make to a building’s overall energy performance is too big to ignore.

Chris Kennedy

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