Double-glazed units with glass spacer bars

When specifying double glazed units you can check energy efficiency by looking at the Energy Saving Trust Recommended logo and BFRC energy label. The Energy Saving Trust endorses any windows rated C or above. The higher the energy rating, the more energy efficient it is. Double glazing has two layers of glass with a gap of around 16mm between them. There's also the option of triple glazing, which has three layers of glass. Both A rated double and tripled-glazed windows are available.

The most energy efficient glass for double glazing is low emissivity (Low-E) glass. This often has an unnoticeable coating of metal oxide, normally on one of the internal panes - next to the gap. It lets sunlight and heat in but cuts the amount of heat that can get out again. Very efficient double glazed sealed units might use gases like argon, xenon or krypton in the gap between the 2 sheets of glass.

All double glazed windows have pane spacers set around the inside edges to keep the two panes of glass apart. For a more efficient window, look for pane spacers containing little or no metal - often known as 'warm edge' spacers.