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Making Sash Windows Energy Efficient

There are a number of practical steps that you can take to make your sash windows more energy efficient, from having them repaired to installing secondary glazing. Some of this advice can also be applied to other types of window. 

Simple draught-proofing

Ill-fitting window frames can be extremely draughty, and over time wooden frames can become warped. But many companies now specialise in repairing sash window frames, which can significantly reduce the amount of heat lost. 

Adding draught-proofing strips can also cut draughts. Research carried out by Glasgow Caledonian University and English Heritage (now Historic England) showed that a combination of these two measures can reduce air infiltration by almost 90%.

Heat can also be lost through the glass itself but the research showed that heavy curtains or even close-fitting roller blinds can cut the amount of heat lost by more than a third. 

 Watch our short video about how to draught-proof your sash windows.

Secondary glazing

Secondary glazing adds a second sheet of glass to a window frame, allowing you to retain original windows without making damaging alterations. The Glasgow tests found that it cut heat loss by around 60% and if it’s designed well, it can even be reversible.

Double glazing, on the other hand, invariably means losing the original windows. This can be highly damaging to the character of your home, particularly if replacement windows no longer follow the original window pattern. 

If your home is listed Historic England is likely to oppose the removal or alteration of significant windows because it destroys its historic character.


Sash windows were often made with accompanying internal shutters to keep the heat in at night. But during the 20th century the increasing availability of cheap heating meant that these were no longer used. They were often painted over or removed altogether.

Not only do these original shutters look very attractive, but if they fit well they can perform as well as double glazing. So if they're still in place it's a good idea to have them restored, but if not it’s also possible to have new shutters made. See our video about secondary glazing and shutters for examples.

Why retain sash windows?

Replacing sash windows is incredibly expensive, and original windows contribute significantly to the historic character of a building.

Original timber windows were made of very high-quality wood seldom found nowadays and therefore it would be a waste to replace them unnecessarily.

What’s more, producing new plastic windows consumes a great deal of energy. And they often end up as landfill after a useful life of only 20 years or so.

Taking one or more of the above steps to improve your sash windows’ energy performance should not only reduce your energy bills but also retain the character that makes your building special.

If your building is listed you may require consent for some of these works. If in doubt consult your local planning authority.

For information on other ways to make your home more energy efficient please visit the other pages of the Saving Energy section.

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