Advice and Support

Sash windows: How to reduce heat loss

Improving the energy efficiency of sash windows to reduce heat loss.

1 How to reduce heat loss

Single glazed sash and case windows lead to less heat loss than you might think – less than 20% of the total heat loss in a traditional stone building. Roof and walls are largely responsible for the rest.  

You can make your windows more energy efficient in a number of ways. Draught proofing is the most effective solution to keeping warmth trapped inside and should be your first goal.  

Aim to keep windows in good repair in the first place – frames that have warped or no longer fit well will let in draughts. Close shutters and curtains at night to reduce heat loss. You can also consider adding secondary glazing, easily removable in warmer months.  

2 Draught proofing sash windows

This involves removing a small amount of the timber of the window frame and replacing this with tubing, brushes or small metal fins. Draught proofing creates a better seal when the window is shut – and often makes for easier opening too.  

Draught proofing: 

  • reduces heat loss  
  • improves noise insulation  
  • helps to stop dust getting in 
  • is cheap and highly cost-effective 
  • doesn’t affect the look of sash windows 

Windows with very slender frames may be weakened by adding draught proofing.  

Some ventilation is good – both to reduce condensation and ensure the safety of open fires, stoves and gas appliances.  

Find out more about how to draught proof your property.

3 Use shutters and curtains

Use shutters if you have them – this is one of the best ways to reduce heat loss at nightSingle glazing lets in more heat from the sun and shutters help to trap warmth inside overnight.   

Timber shutters reduce heat loss by 51%. 

Consider having ill-fitting shutters restored. If the original shutters have been removed, it’s possible to have new shutters made.  

Heavy, lined curtains are a good alternative. These can reduce heat loss by 14%. 

4 Add secondary glazing

Adding secondary glazing to sash and case windows can help to reduce heat lost through the glass and through draughts.  

Fitting secondary glazing involves attaching a second sheet of glass or acrylic to the inside of the window frame. Minimal changes are needed to the window fabric, and the glass panel can be easily removed. The character of the building from the outside is largely unaffected.  

Secondary glazing:

  • reduces heat loss  
  • can filter out some outside noise 
  • helps to stop dust getting in 
  • is a cost-effective solution, generally cheaper than double glazing 
  • doesn’t affect the look of sash windows 

Installing secondary glazing can reduce heat loss by 63%, making it one of the most effective ways to improve thermal performance. Heat loss can be reduced by more than 75% when secondary glazing is used with other methods like blinds and shutters. 

You can remove secondary glazing in the warmer months when it’s not needed.  

You may need permission to make changes to the windows of your property, including fitting secondary glazing.  

5 Why keep sash windows?

Replacing traditional sash and case windows with double glazed units is very costly. The other energy efficiency measures listed here are far more cost-effective.     

Replacing traditional windows with incompatible windows such as double glazed PVC units can harm the look and character of a building – especially if the style differs from the original. It can also cause long-term damage to window frames, creating additional costs.  

From a conservation and sustainability perspective, existing window fabric should be retained and improved wherever possible. Sash and case windows are extremely durable and will last many decades if well maintained.  

Many timber windows in Scotland are in their second century. Modern plastic units may well end up in landfill after just 20 years. 

You may need permission to make changes to the windows of your property.