Double glazing

Double glazing is a way of creating an insulating layer - either normal air or special heavy gases - between two panes of glass. It lets in as much sunlight as single-glazed windows, but is better at holding in heat. Double glazing is standard in most new houses but it's also worth installing in existing homes if you’re doing substantial repairs or replacing the windows. If you need to prioritise, install double glazing to the main heated areas of the house as well as to large windows.

Benefits of double glazing

Good double glazed windows:

  • can halve heat loss through windows
  • significantly improve thermal comfort of your home
  • reduce external noise
  • reduce or remove condensation build up in cold weather.

Checklist for double glazing

For best performance, look for the following:

  • Frames that have a thermal break - a plastic or resin section in the centre of the aluminium joinery or frames made from an insulating material such as uPVC or wood. These are less likely to attract condensation and will lose less heat than windows with standard aluminium frames. Compared to standard aluminium frames, thermally broken aluminium frames reduce window heat loss by 20% and uPVC or wooden frames by 40%.
  • Low-emissivity (low-E) glass - allows light and heat in, but reflects escaping heat back inside the room. Double glazing with low-E glass cuts window heat loss by about 20% to 30%, compared to double glazing without low-E.
  • Multiple layers of good seals - to keep draughts, moisture and noise out. The joint between the glazing unit and the frame also needs to be well sealed.
  • Spacers made of plastic or stainless steel - to separate the glass panes (instead of aluminium) to reduce heat loss and condensation at the glass edge.
  • Inert gas filling - such as argon, between the glass layers. This is a better insulator than air, reducing window heat loss 3% to 9% more than double glazing with air filling.
  • ENERGY STAR qualified windows - these are a step up in thermal performance, and will make homes warmer, drier and more comfortable. They reduce heat loss through windows by more than 18% compared with generic aluminium framed, standard performance double glazing.

 ENERGY STAR qualified products


When you choose skylights, it’s particularly critical to choose ones with good thermal performance characteristics to avoid heat loss in winter and overheating in summer. Apart from the features listed in the checklist for double glazing, you should consider effective shading like built in blinds or shades as well as getting an openable skylight (for cooling in summer). If your skylight is installed through a roof space, make sure the light shaft between the roof and the ceiling is well insulated.