Window and film buyers guide

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

Reducing heat flows through windows and doors is critical for maintaining a comfortable temperature during weather extremes. Heat flowing through an unprotected single-pane window can be considerable, affecting the thermal performance of an otherwise well-insulated house. 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% of a home’s heating energy can be lost through windows and up to 87% of its heat gained through them. Choosing high-performance windows, combined with sensible window placement, insulating blinds and other window improvement methods such as special films and coatings, can reduce energy costs and improve thermal comfort. Understanding how different windows interact with the design of your home can be key in window selection.

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 long-wavelength 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 short-wavelength 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, as aluminium is such a good heat conductor.

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 from above. This heat loss method can remove a great deal of heat from a room.

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. This is particularly an issue in areas that see higher winds.

How do you know which glazing system or treatment is the best solution for you? It’s a complex task for the average homeowner, so here we look at window performance measures and the types of glazing you can choose from.

Read the full buyers guide in ReNew 143.

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