Making Sash Windows Energy Efficient
If you plan to replace historic windows based on a desire to improve thermal performance, we'd recommend that you consider other ways of getting these improvements, as outlined here. These include draught-proofing, use of shutters and secondary glazing. The thermal upgrading of windows needs to be seen in the wider context of the whole building. The heat loss from windows can vary considerably depending on the size of the windows and their ratio to the external wall area. Adopting a 'whole building' approach can help in understanding where energy savings can be made using a number of options rather than replacing windows.
Over time wooden frames can warp and deflect which can result in gaps and therefore draughts. There are now many companies that specialise in repairing sash window frames and adding draught-proofing strips, which can significantly reduce the draughts and the amount of heat lost.
Research carried out for us by Glasgow Caledonian University showed that draught-stripping can reduce draughts 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 30%.
Secondary glazing adds a second sheet of glass to a window with an air gap in between it and the existing window. This allows you to retain the original windows without altering them. The Glasgow tests found that secondary glazing cut heat loss by around 60%.
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 often no longer used and were painted shut or removed altogether.
Not only do these original shutters look very attractive, they can perform as well as double glazing. So if they're still in place it's a good idea to have them restored. It’s also possible to have new shutters made.
Where historic windows or later replacement windows which follow the historic pattern survive without historic glass it may be possible to introduce slim-profile double glazing. Many traditional windows have to be renewed when double glazing is added because the window sashes are not able to accommodate the additional weight and thickness of the double glazed units.
Glass found in older windows will typically be around 3mm thick whereas many double glazing units can be around 12-14mm thick or more. This is a particular problem where windows have small panes divided by fine glazing bars which are not deep and wide enough to accommodate the double glazed units. To overcome this problem windows are often re-made with thicker glazing bars which change the proportions of the historic window pattern. Alternatively, glazing bars are sometimes simply applied onto one double glazed unit which gives a very flat lifeless feel to the window.
Why retain sash windows?
Historic windows, whether they are original or later replacements, usually make an important contribution to the character of older buildings. When they do we advise that they should be retained and repaired. If beyond repair they should be replaced with accurate copies.
Historic timber windows were usually made of very durable timber seldom found nowadays and therefore with careful maintenance can last indefinitely.
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.