Most energy efficient glazing installed in UK homes is double glazed. This means the windows have two sheets of glass with a gap between them of usually at least 16mm. The gap between the panes may be filled with an inactive gas like argon, krypton, or xenon. These creates an insulating barrier to reduce heat loss through the unit. Triple-glazed windows have three sheets of glass and two insulating gaps.
When gas such as argon, which has low conductivity is used within this space, the window is then even more efficient at keeping heat inside the property. It also at interferes with sound waves from inside or outside the house, thereby reducing noise pollution. The inside pane of the double or treble glazed window units has an invisible metal oxide coating which lets in light and the warmth of the sun.
Energy-efficient glazing is rated according to its ability to reduce the amount of heat that can pass through the window, the capacity for sunlight to travel through the glass unit, and the capacity for air to move through the unit.
When energy rating is given, a whole window u-value is also calculated. The u-value refers to how easily heat can pass through the unit. The u-value scale works in the opposite way to an energy rating, in that the higher the u-value, the more easily heat can pass through the window and the window is less efficient.
Certain window manufacturers label the energy efficiency of their windows with an energy rating ranging from A++ to C, A++ being the most efficient. This rating system has been developed by the British Fenestration Rating Council (BFRC). A set of A-rated windows for a semi-detached house will typically cost around £7,500.