A window and film buyers guide


Poorly performing windows can drag down the thermal performance of your home. Lance Turner looks at some solutions.

The importance of reducing heat flows through windows and doors should not be overlooked. A great deal of heat can flow through single-pane glass and an otherwise well-insulated house can suffer considerable unwanted heat loss or heat gain. In fact, a single-pane plain glass window has almost no insulating ability—around R0.2.

The Australian Window Association (AWA) estimates up to 40 per cent of a home’s heating energy can be lost through windows and up to 87 per cent of its heat gained through them. Choosing high-performing windows and placing them appropriately can reduce energy costs significantly and improve thermal comfort. The art is in knowing how different windows will interact with the design of your home.

Heat transfer

There are three main ways heat transfers through windows: radiation, conduction and air infiltration.

Firstly, heat is lost by indirect radiation. Warm objects inside the room radiate heat at long wavelengths (between 5 and 40 micrometres). This energy cannot pass directly through plain glass as it is opaque to such long-wavelength radiation. However, some radiant energy is absorbed by the glass and this is conducted through the glass to the outside. In summer, the reverse occurs, with longwave radiant heat (radiated by hot air and hot surfaces outside) passing indirectly through the glass into the room.

Still greater is the transmission of radiant shortwave solar energy—consisting of visible sunlight plus near-infrared radiation—which is largely transmitted directly through clear glass.
Secondly, heat is lost through conduction—direct transfer of heat from the warm side of the window to the cool side. In aluminium frames with no thermal break, heat is conducted up to six times more readily through the frame than the glass.

In winter, conduction from inside to outside also drives a convection current on the inside of the window, accelerating the rate of heat loss. Warm indoor air cools when it comes in contact with cold glass and falls to the floor, drawing in more warm air above it.

If your heating system has outlets directly under or above the windows, this will increase heat loss by increasing the temperature differential at the glass and breaking up the air layer on the inside of the window. Deflecting the warm air away from the window can thus save on heating costs.

A final method of heat transfer is air infiltration. This occurs when air leaks through the gaps between the inner frame (that holds the glass) and the outer frame (head, jambs and sill). Poorly sealed windows result in a high air infiltration rate and poor thermal efficiency due to the transfer of warm air.

But the main problem is plain glass. Standard unshaded single-pane, untreated glass windows are an energy efficiency disaster, but there are lots of alternatives. These include double and triple glazing, factory-applied glass coatings, add-on (secondary) glazing systems, stick-on window films and a myriad of window coverings.

But how do you know which glazing system or treatment is the best solution for you? It’s a complex task for the average homeowner, but the Australian Window Association has sought to address this problem and make things easier with the Window Energy Rating Scheme (WERS). First though, it’s worth looking at the window performance measures used in rating windows.

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