Different climates demand windows with different characteristics – in hot climates it is more important to choose windows that help with cooling and in cold climates it is more important to improve heating.
The first step in choosing energy efficient windows based on climate, is to understand how the heating and cooling performance of different windows is measured.
The U-value of a window measures its thermal performance. The lower the U-value of a window the less heat and cold it will conduct, indicating better energy efficiency.
When identifying the U-value of a frame, check this is for the whole window U-value and not just the separate components. A whole window U-value is indicated by Uw whereas the frame on its own is Uf and the glass Ug.
Solar Heat Gain Coefficient (SHGC)
SHGC is a measure of how well a window blocks heat caused by sunlight. SHGC is expressed as a number between 0 and 1. The lower a window’s SHGC, the less solar heat it transmits.
The next step in choosing energy efficient windows is to consider your climate and how you can use this to your advantage. Australia can be roughly divided into three climate zones – hot, mixed and cold. Basing your window selection on your specific climate conditions will provide the best energy efficient performance for your home.
Hot climates – Zones 1, 2 & 3
(e.g. Brisbane, Darwin, Northern Australia)
Hot climates require more energy to cool the home. Windows should be designed to keep the heat outside. The best results are achieved by installing windows that limit heat from the sun on all orientations (lowSHGC). Although climate zones one and two are frequently humid while zone three is not, they can all subject the home to the risk of overheating at any time of the year. Good insulation (a low U-value) is also beneficial, especially if the home is air-conditioned.
Hot climate window tips
- Use light coloured frames
- Choose windows with low U-Values and low SHGC
- Choose window styles that provide maximum openable area such as louvres and casement windows
- Locate windows on opposite sides of the building to promote cross ventilation
- Consider high openable windows to maximise air movement
- Make use of shading to reduce solar heat entering the home through clear windows
Mixed climates – Zones 4 & 5
(e.g. Sydney, Adelaide, Perth)
Mixed climates use a relatively even amount of energy throughout the year for heating and cooling. With heat gain required in winter and to be avoided in summer, mixed climates present more design challenges and require some compromises.
West and east facing windows will receive more solar radiation in summer than in winter – the opposite to what is desirable. Choosing windows with a low SHGC and operable shading that can be drawn in summer and opened in winter, will improve performance and comfort year round.
North facing windows receive valuable solar radiation in winter that can heat the home naturally. Choosing windows with mid to high range SHCG will help to harness this free energy.
Mixed climate window tips
- Choose windows with low U-Values
- Choose mid to high range SHGC for north facing windows and low SHGC for east and west facing walls
- Make use of operable shading that can be extended in summer and retracted in winter
Cold climates – Zones 6, 7 & 8
(e.g. Melbourne, Hobart, Canberra)
Cold climates use more energy to heat the home. The objective here is to maximise solar exposure and minimise heat loss for most of the year. Solar heat can be harnessed by locating the majority of windows to the north. Windows with a high SHGC will generally work best as more solar heat is admitted.
U-values are most important in cold climates. There is a greater difference between the indoor and outdoor temperature and choosing windows with low U-values will minimise heat loss.
- Choose windows with low U-values
- Choose windows with high SHGC except for windows that allow unwanted heat gains
- Locate majority of windows to the north – especially in living areas
- Avoid shading windows or use adjustable shading to provide shading only in mid-summer
What U-value and SHGC should I use?
Use the table below as a guide to the preferred U-values and SHGC suitable for your climate. It’s important to remember that U-values and SHGC are just two of the elements to consider in improving your home’s energy efficiency. Other important elements to take into account include elevation, shading and orientation.
Preferred window U-value & SHGC by climate
|Climate||Preferred U-value||Preferred SHGC|
|Hot (e.g. Darwin, Townsville)||Low: 5 to 7.9||Low: 0.4 to 0.65|
|Mixed (e.g. Sydney, Perth||Lower: 3.5 to 7.0||Mid-range: 0.6 to 0.8 (or ideally adjusted per elevation)|
|Cold (Melbourne, Hobart)||Lowest: 3 to 5||High: 0.8|
|Source: Australian Windows Association. This information is a guide only. Only a WERS accredited manufacturer can advise you on the right windows to suit your home or building.|
Choosing the right window
The Window Energy Rating Scheme (WERS) provides comparative energy information on many residential windows available. Windows are rated for their energy performance and illustrated in terms ofcooling and heating stars. No stars show the window is a very poor performer while 10 stars means the best possible performance. WERS-rated windows carry a sticker and certificate specifying their energy performance.
The WERS website provides an online tool to search for windows by U-value, SHGC and additional criteria if desired. An extensive list of the window systems matching these criteria is provided along with the manufacturers who can supply them.
Snapshot of WERS window information
% – The cooling and heating percentages, indicated by the percentage symbols in blue and red boxes, allow you to compare the products performance compared to a clear, single-glazed, aluminium-frame window.
Uw = The U-value indicates how well the window prevents heat from escaping. The lower the U-value, the better its insulating value.
SHGCw = The Solar Heat Gain Coefficient (SHGC) is a measure of how well a window blocks heat caused by sunlight. SHGC is expressed as a number between 0 and 1. The lower a window’s SHGC, the less solar heat it transmits.
Tvw = Visible transmittance indicates how much visible light is transmitted. Tvw is expressed as a number between 0 and 1. The higher the number, the more light is transmitted.
Air Inf = Air infiltration measures how much air will leak through the window. The lower the value, the less air is likely to leak through.