Before spending anything on windows, look at what hangs in front of them. A good pair of lined curtains, drawn at the right time, traps a layer of still air against the cold glass and cuts the heat a window loses overnight. It is about the cheapest insulation there is, and most of the benefit is in how you use what you already have. This guide explains why curtains work, the timing routine that makes the difference, how to hang them so they actually seal, and the one common mistake that throws the heat away.
The short answer. Heavy, close-fitting lined curtains trap a pocket of still air against the glass and slow the heat a window loses at night. Draw them at dusk and open them in the morning to let in winter sun. Floor-length curtains that overlap in the middle and have a pelmet or deep rail on top seal far better than short ones with gaps. The one rule: never let long curtains hang over a radiator, or you funnel its heat straight out of the window.
Why curtains keep heat in
Glass is a poor insulator. At night, when there is no sun to offset it, a window radiates room heat straight out and the inner surface runs cold, which is why you feel a chill sitting near one. A heavy, close-fitting curtain works by creating a pocket of trapped, still air between the fabric and the glass. Still air is a good insulator; it is the same principle that makes double glazing and loft insulation work. The curtain slows the heat leaving the room, much as secondary glazing does, only softer and far cheaper.
Fabric matters, but less than people expect. Thermal-lined or interlined curtains insulate noticeably better than thin unlined ones, because the extra layers add more still air and block the fabric's own draught. But the single biggest factor is not the cloth at all, it is the fit. A luxurious curtain with gaps round the edges leaks heat past it; a modest one that seals all round holds the warmth in. Heavier and closer-fitting beats expensive and loose every time.
How much it actually saves
Honesty matters here, because curtains are cheap but not magic. They reduce heat loss through the window itself, and windows are a smaller share of total heat loss than walls or the roof. Studies of well-fitted lined curtains and blinds suggest they cut heat loss through that window by something like 15% to 25%, with a snug pelmet-topped arrangement at the upper end.
| Window covering | Rough cut in heat loss through the window | Cost |
|---|---|---|
| No curtains, bare glass at night | baseline | nil |
| Thin unlined curtains, loose fit | ~10% | low |
| Heavy lined curtains, floor length, overlapping | ~20% | low to moderate |
| Lined curtains plus a pelmet, sealed all round | ~25% | moderate |
Indicative figures; actual saving depends on the window, the glazing and how well the curtain seals. Heat loss reduction is through the window only, which is part of the home's total. Worked guide, not a guarantee.
In money terms it is a modest few pounds a year per window, not a transformation. But it costs almost nothing if you already have the curtains, it works the very first night, and it stacks with the comfort benefit of killing the cold downdraught. Treat it as one cheap layer among several, alongside draught-proofing and the wider winter energy checklist.
The timing trick
The habit matters as much as the curtain. The rule is simple: closed against the cold and dark, open to the sun.
- Draw them at dusk. Close the curtains before the evening chill sets in, while the room is still warm, to trap the day's heat inside before the window starts radiating it away. Leaving it until bedtime means you have already lost hours of heat.
- Open them fully in the morning. Let the room breathe and, on a bright winter day, let free solar warmth flood in through south-facing windows. That low winter sun delivers genuine heat for nothing, so do not block it.
- Read the weather. On a dull, cold day there is little solar gain to capture, so keeping curtains closed on a north-facing or shaded window during the day loses little and keeps some heat in. On a sunny day, open everything that faces the sun.
Done consistently through the heating season, this single routine steadies a room's temperature for no cost at all and takes the edge off the coldest part of the evening. It pairs naturally with sensible thermostat settings, since a warmer-feeling room lets you hold the thermostat a touch lower.
Hang them to seal
A curtain only traps air if it fits closely, so how you hang it decides how much it does. Gaps are where the saving leaks out.
- Go floor length. Curtains that reach the floor hold heat far better than short ones that stop at the sill, because warm air cannot spill out underneath. Where a radiator sits below the window, see the next section for the exception.
- Overlap in the middle. Curtains that meet and overlap, rather than just touching, close the gap where the two halves meet and cold air would otherwise pour through.
- Fit a pelmet or deep rail. The biggest leak is over the top. Warm air rises behind an open-topped curtain, escapes at the top and draws cold air in at the bottom, a small chimney effect. A pelmet, a deep curtain rail close to the wall, or a box across the top stops that loop and is the single most effective improvement you can make.
- Fit wide and snug to the wall. Curtains that extend beyond the window frame at the sides and hang close to the wall seal better than narrow ones leaving gaps down each side.
Never trap the radiator
This is the one mistake that turns a saving into a loss. Most windows have a radiator beneath them, and long curtains drawn in front of that radiator funnel its heat straight up the back of the cold glass and out of the window, exactly where you do not want it. You end up heating the great outdoors while the room stays cold.
The fix depends on the window:
- Where a radiator sits under the window, use sill-length curtains that stop above it, so the heat rises into the room rather than being channelled behind them.
- If you want floor-length curtains for the look, tuck them behind the radiator onto the sill at night, or fit a curtain that finishes neatly above the radiator shelf.
- A shelf or a radiator reflector helps push the heat out into the room. See radiator reflectors for the cheap foil trick on outside walls.
Keep the heat coming into the room, not channelled out through the glass. Get the curtain length right relative to the radiator and you capture the warmth instead of exporting it.
Where curtains fit in the bigger picture
Curtains are a cheap first layer, not a substitute for fixing the fabric of the house. If your windows are cold and you want more than curtains can give, secondary glazing adds an inner pane for a fraction of replacement cost, and the honest case for new windows is laid out in is double glazing worth it.
The cheapest wins of all are usually upstairs and out of sight. Loft insulation and draught-proofing save far more than curtains for not much more money, and they are where to put your effort once the curtain habit is in place. For the room-by-room view, see heating one room or the whole house and the home heating hub. Curtains are the free first move; the others are the ones that move the bill.