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Seeing Through Your Windows

Any opening, such as windows, doors or skylights, in your home's outer shell can affect its energy efficiency.

Since we normally have more windows, we should understand more about their effects, apart from the basics of views, natural light and breezes.

Firstly, glass conducts heat. Heat conduction is the transfer of heat from the warm side to the cool side. Radiant heat directly warms people and objects, rather than warming the air.

Purchasing a reverse cycle air conditioner may make you feel more comfortable but there is a big chance your electricity bill will be working overtime as the 'air con' pumps out more of the 'right' air while the window continues transferring heat in the opposite direction.

The greater the temperature differences, the faster the rate of heat flow.

On a hot day, other appliances will also work harder. Your refrigerator will add more kilowatts and its motor more internal heat.

Putting the 'right' windows, with the 'right' glass in the 'right' place, with the 'right' frame' and minimising infiltration are important considerations.

'Infiltration' is unintentional leakage through window joints and can leak about 15 per cent of your 'good' air. Different window types leak more than others and obviously the better quality the window, the lower the infiltration.

'Ventilation' is airflow we encourage. In summer, we can maximise this by positioning windows correctly to naturally capture cross breezes and ventilate ceiling areas.

Frames also play an important role. Aluminium frames are high conductors. Timber, fibreglass, or vinyl-clad window frames are better performers.

Window manufacturers can have their products rated by the Window Energy Rating Scheme (WERS - www.wers.net). A rating of 0 to 5 stars is applied to a window's heating and cooling impact on a house. The more stars, the better.

A window's thermal performance is rated with a U-value. The frame and the glass (centre and edge) are measured to determine overall resistance to heat flow. The lower the U-value, the greater the resistance and the better its insulating value.

Windows are also given a Solar Heat Gain Coefficient (SGHC) rating to determine how well they can block the solar radiation. The lower a window's SHGC, the less solar heat it transmits.

Installing double glazed windows significantly reduces heat flow.

If you've tried a hot drink from a glass, your burnt fingers will have shown you how a single layer of glass reacts with heat. Using two layers of glass separated with an air space eg a Thermos® reduces heat flow – even in windows.

Adding tints and glazes can also control heat flow but it is important to apply the right product to the correct degree and direction so you get the required natural light and heat exchange, all year round.

In our climate, windows facing east and west require more cooling stars. South facing windows get little direct sun and do not contribute much heat any time of the year. North facing windows gain the most beneficial heat in winter.

Stopping the sun from hitting windows is the best strategy to reduce heat gain in summer by providing exterior shading through overhanging eaves, awnings, shade screens and trees.