Double glazing

Double glazing is a way of creating an insulating gas 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 your existing windows require substantial repairs or replacing. 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 in 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 or are made of insulating materials - 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 about 20% and uPVC or wooden frames by about 40%.
  • Low-emissivity (low-E) glass - allows light and heat in, but reflects some of the 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.
  • Spacers made of plastic or stainless steel (instead of aluminium) - these spacers separate the glass panes 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 by 3% to 9% compared to double glazing with air filling.


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.