Double glazing is a technology used to decrease the amount of heat transmitted through windows. Double-glazed windows have two window panes with a small pocket of air or another gas between them. The extra pane of glass reduces the amount of solar energy passing through somewhat, but the air pocket is the really important part. Air is a poor heat conductor, and the air pocket insulates the inside from the outside, keeping the house warm in winter and cool in summer.
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The cheapest style of double glazing is secondary glazing. In secondary glazing, a second layer of glass is fitted over an existing window, creating a pocket of air that insulates the home. Because secondary glazing is not sealed, it offers less energy savings than other forms of double glazing, but it is still more efficient than a single-pane glass window.
Most double-glazed windows are sealed units that are installed as one piece. Two panes of glass are embedded in a wooden, PVC, aluminium, steel or composite frame with a space between them. This space is generally either filled with air, which is a good insulator, or with a noble gas such as xenon, which is an even better insulator. The material affects the efficiency somewhat; metal window frames are strong and durable but conduct more heat than wood or PVC, reducing their efficiency somewhat.
Low-emissivity glass uses a special coating designed to reduce heat loss in winter. The glass lets sunlight into the room, where much of that sunlight is absorbed as heat. When that heat is reradiated as heat, the double-glazed windows block it from exiting the room. This allows sunlight to warm the room in winter, reducing heating bills.
Toned or tinted glass has a coloured or reflective coating applied to the outside face of it. This coating blocks some of the sun from entering the room. This helps the double-glazed glass reduce the amount of heating from sunlight, keeping the room cool even in the summer. The drawback is that tinted glass blocks light all year round, which reduces valuable natural lighting in the winter.
Some new technologies allow double-glazed windows to change the amount of light and heat that go through, depending on environmental conditions or user preference, according to the Sustainablility Victoria website. Photochromic glass darkens when it is exposed to sunlight; it stays clear on low light days, but blocks solar energy in bright conditions. Thermochromic glass changes based on heat, blocking more light as the temperature rises. Electrochromic glass is controlled by an electric current; the user can darken the glass manually by applying a charge to it.
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