The priority in a heating (cold) climate is to retain heat in the home and maximise the use of ‘free’ solar energy in winter. Heat transfer takes three forms: radiant heat transfer, conducted heat transfer and heat transfer by infiltration of outside air.

The priority in a cooling (hot) climate is to keep unwanted heat out of the home. In order of decreasing importance, unwanted heat gain takes three forms: radiant heat gain, conducted heat gain and infiltration of hot outside air.

The priority in a mixed climate is to keep heat out of the home, except during a relatively mild winter season, when ‘free’ solar heat gain and retention of warmth in the house become important. Heat gain or loss takes three forms: radiant heat transfer, conductive heat transfer and heat transfer via air infiltration. Ideally, mixed climates call for strong solar control on east and west windows, but deliberate use of free sunlight admitted by northerly windows. This means different glazing solutions.

Radiant heat transfer
Radiant heat gain occurs when solar energy is transmitted through the glass. In addition, some solar energy enters indirectly. This occurs when the glass (and frame) are exposed to sunlight and heat up. Some of this heat is transferred inside by conduction and convection (air currents).On the east and west, the best way to stop radiant solar heat gain is to shade the glass, but this is not always possible. In addition, shading systems sometimes interfere with the view, reduce natural light, inhibit ventilation or add cost to the window system. Every situation is unique. An easier alternative is to select a window with at least three cooling stars - that is, follow the guidelines for a cooling climate.To the north, the windows should be shaded in summer by correctly sized eaves. To maximise the benefit of free heat from the winter sun, look for windows with at least three heating stars. In other words, follow the guidelines for a heating climate. This will also ensure that beneficial winter sun is admitted.

Infiltration heat gain
The final way that unwanted heat enters the home is through gaps and cracks around sashes and frames. When a window is shut it should be shut.WERS-rated windows must satisfy Australian Standard AS 2047 for air infiltration performance. Look for windows with good weather seals. Traditionally, those with ‘compression’ seals, as fitted to awning and casement windows, tended to have superior long-term infiltration performance. However recent advances in some sliding window seals have reduced the gap.