How much they cut heat loss
Historic Environment Scotland, which looks after Scotland's historic buildings, gives these figures for how much each change cuts the heat lost through a traditional sash window:
| What was added | Cut in heat loss |
|---|---|
| Heavy, lined curtains | 14% |
| Timber shutters | 51% |
| Secondary glazing | 63% |
| Secondary glazing with blinds or shutters | More than 75% |
Curtains help, but much less than a second layer of glazing or a shutter that closes tight. They work by trapping a layer of still air between the fabric and the glass. Air that can flow over the top of the curtain or round the sides carries the heat away again, which is why the way you hang them matters as much as the fabric.
Modern double glazing loses less heat than single glazing to begin with, so curtains save less on it. On single glazing, especially in a flat you rent where you can't change the windows, good curtains and window insulation film are the two cheap things you can do.
What makes a curtain thermal
Most ready-made curtains sold as thermal don't state a measured heat-loss figure, so judge them by how they're made:
- A dense lining. Foam-backed or triple-weave blackout fabric stops air moving through the curtain.
- Weight. A heavier curtain hangs closer to the wall and moves less in a draught.
- A separate lining hooked behind a curtain you already have does the same job. It's the cheaper fix if your curtains are already the right size.
Thermal and blackout linings are often the same product. That's useful in summer too: closed on a sunny window, they keep the heat out. See how to keep a bedroom cool in a heatwave.
How to hang them so they work
- Wider than the window. Fit the pole or track well past the frame on each side, so the curtains overlap the wall when they're closed and air can't get round the edges.
- Long enough. To the floor if nothing is in the way. If there's a radiator under the window, stop the curtains at the sill instead.
- Never over a radiator. A curtain hanging in front of a radiator sends its heat up the gap behind the curtain and onto the coldest glass in the room.
- Closed at the top. Warm room air can spill over the top of the curtain, cool against the glass and fall out at the bottom. A pelmet, or a track fixed to the ceiling, stops most of that.
- Closed at dusk and open in the morning. In the day the sun through the glass warms the room for free.
Curtains and condensation
Closed curtains keep the room warmer, but the glass behind them gets colder, because less room heat reaches it. Moist air that gets behind the curtain meets that colder glass, so you may see more water on the window in the morning. Keep closing them at night, and deal with the moisture:
- Open the curtains when you get up, and wipe the glass and sill dry.
- Keep trickle vents open, or open the window for a few minutes to let the damp air out.
- Don't let the curtains rest against wet glass. Damp fabric is where mould starts on linings.
More on this in why your windows are wet every morning.
Don't forget the front door
If your front door opens straight into the living room, a heavy curtain across it at night does the same job as one at a window. A rising door rod lifts the curtain as the door opens, so it doesn't drag. Seal the gaps in the door first: see how to draught-proof a door.
Our picks
Chosen by how they're made. None of these makers publishes a spec page, so the details come from their listings. We haven't tested them ourselves. Measure your window, including how far past the frame you want the curtains to reach, before you choose a size.
New curtains for a cold window
Enhanced Living Nightfall Supersoft Thermal Eyelet Curtains (natural)
- Type
- Thermal curtains
- Size
- Pair, 66 x 72in (168 x 183cm)
A heavy triple-weave fabric, dense enough to slow air through the curtain without a separate lining. In this colour it's sold as room darkening rather than full blackout, and the maker says to hand wash it.
To upgrade curtains you already have
Homescapes Blackout Curtain Lining, 3-pass coating (pair)
- Type
- Curtain lining
- Size
- Pair, to fit 66 x 54in curtains
A coated blackout lining that hooks onto the heading tape of your own curtains, cut slightly smaller so it doesn't show. Check the size above against your curtains: this pair is a short, sill-length drop, which suits a window with a radiator underneath. Eyelet curtains need lining hooks, so check those are included. Dry clean only.
For a front door
Enhanced Living Nightfall Thermal Eyelet Door Curtain (natural)
- Type
- Door curtain
- Size
- Single, 66 x 84in (168 x 214cm)
The same triple-weave fabric as the window curtains above, in a single panel long enough for most front doors, to hang on a rising rod.
Sources
Checked 24 October 2026.