All properties lose heat through their windows. But energy-efficient glazing keeps your home warmer and quieter as well as reducing your energy bills. That might mean double or triple glazing or secondary glazing. A window's heat loss behaviour is characterised by its U-value, which depends on heat transmission by conduction, convection and radiation; the lower the U-value the lower the heat loss. The U-value for a single glazed window is about 6 W m-2 °C-1, but triple glazing, for example can reduce this to less than 1 W m-2 °C-1.
The gap between the glass panes reduces losses due to thermal conduction. The glazing is typically filled with a heavy gas like argon instead of air, which reduces losses due to convection. Heat can pass between the panes of glass by radiation, but adding a thin low emissivity coating on the inner pane to reflect radiation back into the building significantly reduces this. It is important that the frame and spacers also have low thermal conductivities and these now tend to be made from U-PVC and glass fibre respectively. Modern glazing units have better performance than those on the market a number of years ago. The most efficient windows are rated B and above and usually carry the Energy Saving Trust Recommended logo.
In listed buildings, there may be restrictions on alterations to windows. Secondary glazing can be an alternative way of insulating historic buildings from the inside which may be granted planning permission. Here, a secondary pane of glass and frame is fitted inside the existing window reveal. This won’t be as well sealed as a double-glazing unit, but may be cheaper to fit, and will still save energy.