Do curtains over a radiator waste heat, and what should I do about it?

Where the heat actually goes when a curtain hangs over a radiator, how much it costs, and the four fixes ranked by effort.

Difficulty: beginner
Time: 20 min
Read: 5 min

Short answer

Yes, meaningfully. A full-length curtain hanging in front of a radiator traps the rising warm air between the fabric and the cold glass, so a good share of that radiator's output goes up the window instead of into the room. The fixes, in order: tuck the curtains behind the radiator or onto the sill, shorten them to sill length, fit a shelf above the radiator to deflect the air outwards, or use a track that lets the curtains stack clear.

The radiator under the window is there for a good reason — it counters the cold downdraught from the glass. The curtain is there for a good reason too. Put them together badly and you get a warm chimney between the fabric and the window, which is exactly where you least want the heat to be.

What you'll need

Tape measure
Thermometer or the back of your hand, for checking (optional)
Shelf and brackets, for a radiator shelf (optional)
Curtains shortened or re-hemmed (optional)

Step by step

Check whether you actually have the problem.With the heating on and the curtains closed, put a hand between the curtain and the glass. If that gap is noticeably warm, heat is going up the window rather than into the room. If the curtain finishes above the radiator, there is nothing to fix.
Understand what is happening.A radiator works largely by convection. Warm air rises off it, and a curtain hanging in front turns that rise into a channel between fabric and glass, where the heat is delivered to the coldest surface in the room and lost through it.
Try the free fix first: tuck the curtains.Curtains that finish on the sill, or that are tucked behind the radiator, let the warm air into the room. It looks less elegant and it costs nothing, and at night it is the difference between heating the room and heating the glass.
Shorten to sill length where the radiator is under the window.Curtains finishing 100 to 150mm below the sill and clear of the radiator are the correct answer in that arrangement. It also stops the fabric from being cooked, which fades and weakens it.
Or fit a shelf above the radiator.A shallow shelf 50 to 100mm above the radiator, projecting 20 to 30mm past its front face, deflects the rising air out into the room rather than up behind the curtain. Modest but real, and useful when full-length curtains have to stay.
Or move the pole out and up.A pole fitted higher and with more projection lets the curtains hang clear of the radiator and the sill. Combined with a deeper stack-back at each side, the fabric spends the day off the glass altogether.
Fit a reflective panel behind the radiator while you are at it.Radiator foil behind a radiator on an external wall returns some of the heat that is otherwise conducted straight into the masonry. Cheap, quick and complementary to everything above.
Keep the curtains closed at night regardless.Closed curtains reduce heat loss through the glass substantially once the radiator air is not being funnelled up behind them. The point is not to open the curtains — it is to keep the radiator's output on the room side of the fabric.
Weigh it against comfort.Full-length curtains genuinely feel warmer and look better, and thermal linings reduce loss through the glass. If you want them, use the shelf and the projection rather than giving them up.

Tips

Thermal or interlined curtains help most at windows without a radiator beneath them. Under a radiator, the geometry matters more than the lining.
A radiator shelf is worth doing at the moment a shelf is going up anyway. It is not worth fitting purely as an energy measure on its own.
Check the thermostatic valve head is not sitting behind the curtain. A TRV in a pocket of trapped warm air closes the radiator down while the room is still cold.

Common mistakes

Boxing in or covering a radiator to hide the problem — It restricts the convection the radiator depends on and reduces its output to the room. The heat has to get out at the top.
Leaving full-length curtains hanging over a radiator all winter — It sends a share of that radiator's output up the coldest surface in the room, and the fabric fades and weakens in the heat.
Blaming the radiator for a cold room without checking the curtains — A room that is cold with the curtains closed and fine with them open has a geometry problem, not a heating fault.

If it doesn't work

The room is colder with the curtains closed

Cause: Curtain in front of the radiator, funnelling warm air up the glass — Fix: Tuck the curtains behind the radiator or onto the sill tonight. Then shorten them, or fit a deflector shelf.

The radiator turns itself down while the room is still cold

Cause: The TRV head is behind the curtain in a pocket of warm air — Fix: Keep the curtain clear of the valve, or fit a remote sensor head.

Curtain fabric has faded or gone brittle at the bottom

Cause: Sitting in the rising heat from the radiator — Fix: Shorten to sill length, or fit a shelf to deflect the air outwards.

Questions people ask

How much heat do curtains over a radiator actually waste?

Enough to notice. The warm air rises between the fabric and the glass, which is the coldest surface in the room, so a meaningful share of that radiator's output goes into the window instead of the room. The exact figure depends on the glazing, but the effect is easy to feel with a hand behind the curtain.

Should curtains be short if there is a radiator under the window?

It is the simplest correct answer: finish them 100 to 150mm below the sill and clear of the radiator. If you want full-length curtains for the look, fit a shelf above the radiator and give the pole more projection so the fabric hangs clear.

Does a radiator shelf really work?

It makes a modest, genuine difference by deflecting the rising air out into the room instead of up the glass. It is worth doing when a shelf is going up anyway or when full-length curtains have to stay. It is not a major energy measure on its own.