Blackout
Do Thermal and Blackout Curtains Actually Keep the Heat In? What the Evidence Says
An honest look at how much heat curtains really save, what the research shows, and how to hang them so they work, including what to do about radiators under windows.
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Updated October 2026 · By Sam K, Fabrica Kraft
Quick answer: Yes, lined curtains do help keep heat in, but less than many adverts suggest, and only if they're hung properly. In laboratory tests for Historic Scotland, well-lined traditional curtains cut heat loss through a sash window by about 14%. To get the most from them, choose heavy, lined, floor-length curtains, hang the pole high and wide so they overlap the window on every side, close them at dusk, and keep them clear of any radiator underneath.
When the heating goes on in October, curtains become one of the cheapest things you can change at home. Shops are full of "thermal" curtains promising lower bills. Some of that is true, and some is marketing. Here's what the evidence actually shows, and how to make the curtains you buy work as hard as possible.
How much heat is lost through windows?
According to figures from the Energy Saving Trust, around 18% of the heat in an uninsulated home is lost through the windows. That's less than through walls or the roof, but windows are the easiest part of the house to improve without building work. Single-glazed and older sash windows lose the most.
What the research says about curtains
The most useful UK evidence comes from laboratory tests by Glasgow Caledonian University for Historic Scotland, on traditional single-glazed sash windows. They measured how much each type of window covering reduced heat loss:
| Window covering | Reduction in heat loss |
|---|---|
| Traditional lined curtains | About 14% |
| Victorian blind | About 28% |
| Honeycomb (double-layer) blind | About 36% |
| Roller blind with a low-emissivity film | About 45% |
| Traditional timber shutters, closed | About 51% |
Two things stand out. First, curtains do make a real difference on their own. Second, the closer a covering fits against the window, the better it works. Shutters and close-fitting blinds trap a still layer of air against the glass. Curtains can do the same, but only if they're hung to cover the whole window and overlap the edges.
Be wary of headline claims of very large savings. Results depend heavily on your windows, how well the curtains fit, and whether you actually close them every evening.
Thermal curtains vs blackout curtains
| Thermal lining | Blackout lining | |
|---|---|---|
| Main job | Insulation | Blocking light |
| How it works | A thicker, often brushed or coated layer that traps air | A dense, tightly woven or coated layer light can't pass through |
| Helps with heat? | Yes | Yes, because it adds a dense extra layer |
| Best rooms | Living rooms, draughty windows | Bedrooms, nurseries, media rooms |
In practice, any heavy, lined curtain helps. Blackout lining adds a dense layer behind the fabric, so it gives you darkness for sleep and some extra insulation. Our curtains offer a 100% blackout lining as an add-on. Heavier face fabrics, like velvet, add more weight still.
How to hang curtains so they actually keep heat in
- Go floor length. Curtains that stop above the floor let cold air from the window spill out under the hem into the room.
- Hang the pole high. Fit it 10 to 15 cm above the frame, or close to the ceiling, so warm air can't escape over the top.
- Go wide. Extend the pole 15 to 20 cm past each side of the frame so the curtains overlap the wall.
- Order enough fabric. About twice the pole width, so the panels overlap in the middle when closed.
- Close them at dusk. The sooner they're closed after dark, the less heat escapes.
- Open them on sunny days. Even winter sun warms a room for free.
- Consider a pelmet or a pole close to the ceiling to reduce air moving over the top.
Our guide to measuring for curtains in cm covers every step, or use the curtain size calculator.
The radiator problem
Many British homes have a radiator directly under the window. Long curtains that hang in front of it cause a problem: the heat rises behind the curtain, straight up against the cold glass, and much of it is lost before it reaches the room.
You have three good options:
- Sill-length curtains that stop just above the radiator, so heat flows into the room.
- Floor-length curtains tucked behind the radiator at night, if there's room, so the radiator heats the room side.
- A radiator shelf above the radiator, which pushes warm air out into the room instead of up the window.
Never let fabric rest directly on a hot radiator for long periods. Our guide on whether curtains should touch the floor explains when sill length is the right choice.
Do curtains help in summer too?
Yes. Closing lined curtains on sunny south- and west-facing windows during hot spells helps stop rooms overheating. Bedrooms benefit most, since blackout lining also helps with the early summer sunrise.
Other cheap ways to cut heat loss at windows
- Draught-proof gaps around window frames with foam or brush strips.
- Close curtains in rooms you're not using, and close internal doors.
- Layer a voile behind a lined curtain for daytime privacy without losing light.
- Fit secondary glazing film on single-glazed windows.
Curtains aren't a substitute for double glazing or insulation, but they're one of the quickest, cheapest improvements you can make.
Choosing curtains for warmth
- Living rooms: heavy, lined made-to-measure curtains or pinch pleat curtains.
- Bedrooms: blackout curtains for warmth and darkness.
- Period homes with tall windows: extra-long curtains, up to 366 cm drop.
- Extra weight and luxury: velvet curtains.
Curtains are made in 2 to 3 weeks. Delivery is free across the UK, with import duty and VAT already paid, so there's nothing extra to pay when they arrive.
Frequently asked questions
Do thermal curtains really work?
Yes, but modestly. Tests for Historic Scotland found lined curtains cut heat loss through a window by about 14%. Fit matters: floor-length curtains that overlap the window work best.
Are blackout curtains also thermal?
Blackout lining is designed to block light, but its dense layer also adds some insulation. Any heavy, lined curtain helps keep heat in.
Do thermal curtains keep heat out in summer?
They help. Closing lined curtains on sunny south- and west-facing windows during hot spells reduces how much the room heats up.
Do door curtains keep heat in?
A heavy, lined curtain across a front or back door helps cut draughts, especially if it is wider than the door and reaches the floor.
Should curtains cover a radiator?
Ideally not. Curtains over a radiator trap heat against the window. Use sill-length curtains, tuck them behind the radiator, or fit a radiator shelf.
When should I close my curtains in winter?
At dusk. Open them during the day to let sunlight warm the room.
Do curtains help with double glazing?
Yes, though the difference is smaller than with single glazing, because double glazing already loses less heat.
