Do thermal curtains and blinds actually work?

Windows lose heat two ways: air leaking around the frame, and heat radiating through the glass. Curtains do nothing about the first and a useful amount about the second, which is why they help but do not fix a draughty window.

Step by step

1. Understand what the lining does.Thermal lining adds a layer of trapped still air between room and glass, and reflects some radiant heat back. Still air is a good insulator; the fabric itself is doing less than the air it holds.
2. Recognise that fit beats fabric.A cheap curtain that seals well outperforms an expensive one with gaps. Air circulating behind the curtain — down the cold glass, out at the bottom, back in at the top — cancels most of the benefit.
3. Hang them wide and long.The pole should extend well beyond the window each side, and the curtain should reach the floor rather than the sill wherever possible. That closes the bottom of the circulation loop.
4. Deal with the top.A pelmet, or a track fitted close to the ceiling, stops warm air being drawn down behind the curtain. This is the single most effective and most commonly missed detail.
5. Close them at dusk, not bedtime.The loss starts as soon as it is dark outside and colder than in. Closing them at four in the afternoon rather than at ten captures several extra hours.
6. Open them on a sunny day.Sunlight through glass is free heat in winter. Curtains closed all day on a bright cold day cost you more than they save.
7. Do not cover the radiator.A floor-length curtain over a radiator under the window sends its heat straight up the glass. Either tuck the curtain onto the sill there, or fit a shelf above the radiator to deflect the heat into the room.
8. Compare with the alternatives honestly.Draught-proofing the frame is cheaper and often more effective. Secondary glazing does considerably more. Curtains are the least disruptive and cheapest of the three, and they combine well with draught-proofing.
9. Consider blinds as an addition, not a replacement.A blind inside the reveal plus a curtain over it gives two trapped air layers and works better than either alone. Honeycomb or cellular blinds trap air in their own structure and perform noticeably better than roller blinds.
10. Apply the same thinking in summer.The same curtains, closed during the day on a sunny side, keep heat out. External shading works better still, because it stops the heat before the glass.

Tips

• Thermal lining can be added to existing curtains, either sewn in or as a separate detachable layer that hooks onto the same rings.
• The gap between curtain and glass matters — too tight against the window and you lose the trapped air layer; a few centimetres is right.
• A door curtain on an external door works on exactly the same principle and is often more effective per pound than window curtains.

Common mistakes

• Hanging thermal curtains that stop at the sill — Air runs down the cold glass, out at the bottom and back in at the top, so the curtain drives a convection loop instead of blocking one.
• Closing curtains over a radiator — The radiator then heats the space between curtain and glass, and most of that heat goes straight out of the window.

Questions people ask