In a period home with original sash windows, curtains, blinds and shutters do more than decorate a room. They also change how much heat escapes through the window.
That is not guesswork. Researchers have measured how much heat curtains, blinds and shutters stop from escaping through a traditional sash window. This guide sets out what they found, so you can choose a dressing for both looks and comfort.
What the Research Tested
The main study was carried out for English Heritage, now Historic England, by Glasgow Caledonian University. Researchers fitted different dressings to an 1880s timber sash window and measured the heat lost through it. The results were published in 2009.
Historic Environment Scotland publishes its own figures, and they do not always match. Where the two differ, both are given below, because results depend on the dressing, how it fits and the window it is tested on.
Curtains
In the English Heritage tests, heavy curtains cut heat loss through the whole window, including the frame, by 41%.
Historic Environment Scotland gives a lower figure. It says heavy, lined curtains can reduce heat loss by 14%, and describes them as a good alternative to shutters.
Both sources agree on timing. Curtains help most when they are closed at night.
Blinds
The type of blind made a large difference in the tests. Measured across the whole window, the researchers found:
- A plain roller blind cut heat loss by 38%
- A “thermal” honeycomb blind cut heat loss by 51%
- A roller blind with a reflective, low-emissivity film facing the window cut heat loss by 57%
Fit matters too. The report notes that when the blind was fitted tightly, the window’s heat loss fell further than with a loose fit.
The direction of the film also matters. A blind with the reflective film facing into the room performed less well than one with the film facing the window.
Shutters
Shutters gave the best whole-window result of the dressings tested on their own, just ahead of the reflective blind. In the English Heritage study, shutters cut heat loss through the whole window by 58%. The shutters tested were three-panel shutters made from plywood, fitted with draught strip around their edges for a tight fit.
Historic Environment Scotland puts the saving from timber shutters at 51%. It calls using shutters one of the best ways to reduce heat loss at night.
Its advice depends on what your home still has:
- if your shutters are ill-fitting, consider having them restored
- if the original shutters have been removed, it is possible to have new shutters made
If your windows have no shutters, fitted shutters for sash windows can be installed inside or outside the window reveal.
Combining Dressings
The best results came from using more than one layer. In the English Heritage tests, a low-emissivity blind used with shutters reduced heat loss through the glass by 83%.
Historic Environment Scotland reports a similar pattern. It says heat loss can be reduced by more than 75% when secondary glazing is used with blinds and shutters.
Start With Draughts
The dressing tests measured heat lost through the glass and frame. Draughts around the sashes are a separate problem, and Historic Environment Scotland describes draught-proofing as the most effective way of keeping warmth in, and says it should be your first goal.
It explains that draught-proofing a sash window involves removing a small amount of timber from the frame and replacing it with tubing, brushes or small metal fins. It lists lower heat loss, better noise insulation and less dust among the benefits, with no change to how the windows look. It also cautions that slender frames may be weakened by the work.
A specialist in timber sash windows can check the frames before any work starts.
Going Further: Secondary Glazing
If you want more than a dressing can offer, secondary glazing adds a second pane on the inside of the window. Historic Environment Scotland says it can reduce heat loss by 63%, and that it can be removed in warmer months when it is not needed.
In the English Heritage tests, low-emissivity secondary glazing cut heat loss through the whole window by 58%. Used with shutters, the saving rose to 62%. The report advises that secondary glazing should be closed tight in cold weather to limit condensation on the original window.
Before You Make Changes
- Keep some ventilation. Historic Environment Scotland says some ventilation is good, both to limit condensation and to keep open fires, stoves and gas appliances safe.
- Listed buildings. In England, the Planning Portal says listed building consent is needed for any significant works, internal or external. Check with your local planning authority before fitting shutters or secondary glazing.
- Leasehold. The Planning Portal also notes that a leaseholder may first need permission from the landlord or management company.
Choosing the Right Dressing
The research gives a clear order of results. Curtains and plain blinds help, insulating and reflective blinds help more, and shutters gave the best whole-window result of the dressings tested on their own. Both sources say dressings help most when they are closed at night.
0
Leave a Reply