Windows are often the weak spot in a building's “thermal envelope”. An unshaded, single-glazed window can allow 10 times as much heat to escape from your home in winter, and let in 100 times more heat in summer, than the same area of insulated wall. In fact, a square metre of window can let in as much heat as a single-bar heater produces.
Heat can pass through a window by conduction, convection or radiation. Radiation is heat energy transmitted as electromagnetic waves in sunlight. Convection refers to heat moved by air flow. Conduction is the propensity of heat to move from a hot area to a cold area until the two are at the same temperature. Heat can enter or escape by conduction even if there is no light hitting the window.
The rate a window lets heat through is known as its conductivity, expressed as a U-value. The lower the U-value, the less heat is transmitted. Just to be confusing, U-values can be reversed and expressed as R-values.
When designing a new home, windows should be positioned for the best passive solar design. For existing homes, heat gain/loss through windows depends on glazing, shading, and the material and insulation of the window frame.
The best solution depends whether cooling or heating is a more important issue where you live, and whether or not you are happy to shut out natural light and views from a window.
Thicker glass will reduce conductivity to an extent, but if you want to significantly reduce heat loss/gain it’s worth considering double glazing. Double-glazing consists of two panes of glass with a gap of air between them, and can cut a window's U-value in half. Argon gas is sometimes used to fill the space between the panes instead of air, because it has a lower conductivity than air.
Double glazing in itself does not reduce radiated heat loss/gain, but you can cut radiated heat by up to a half by using tinted glass. A tinted plastic film stuck onto the window can be a low-cost alternative. The problem with tinted glass is it also reduces heat entering your home in winter, when you want it, as well as cutting down natural light.
Shading is the best way to reduce (radiation) heat gain from sunlight in particular. Shading should be adjustable so you can block out the sun on hot days in summer but still allow it to enter in winter. Light or reflective material for shading devices will help them reflect more radiation heat away from the window.
The angle at which sunlight enters the window significantly affects how much radiated heat gets in. Protecting west-facing windows from low, direct afternoon sunshine in summer is thus particularly important.
Windows can be shaded internally with curtains or blinds, or externally with shutters or awnings. Deciduous vines, trees or bushes can also function as adjustable window shading because they will shed foliage in winter to allow sunlight through.
Eaves and verandahs can provide summer shading for north-facing windows when the sun is high in the sky, but will still let in sunlight in winter when the angle of the sun is lower in the sky.
It is always best to stop heat reaching the window in the first place, so internal shading is best for retaining heat in winter, while external shading is better at keeping your house cool in summer.
The thickness and conductivity of shading material will help determine how much heat gets through your window.
Aluminium window frames are durable and low-maintenance, but also have high conductivity. Timber frames provide better insulation. However, the insulation value of aluminium frames can be increased by inserting a “thermal break” consisting of a layer of low-conductivity material such as a polymer in the middle of the frame.
Heat loss/gain (watts) = temperature difference x surface area x U-value. For instance, if it is 15C hotter outside than inside, then a 4sq m window with a U-value of 6 will be losing 15 x 4 x 6 = 180 watts an hour.
The Window Energy Rating Scheme (WERS) rates the energy performance of residential windows. WERS-rated windows carry a sticker and a certificate specifying their performance.
Noise: double glazing, thicker glass and sealing cracks and gaps around windows will help keep out noise.
Fading: shading, window tinting and double glazing will reduce sunlight, which causes furniture and furnishings to fade.
Condensation: condensation occurs in winter when moist air hits the cooler glass surface of the window. By keeping the inside pane closer to room temperature, double glazing will reduce condensation.
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