A curtain in front of a radiator: mistake or not?
A curtain that covers a radiator sends part of the heat towards the glass instead of the room. That is the only real mistake on this subject, and it is corrected without changing curtain: the panel just has to stop above the appliance, or pass in front of it without covering it.
Conversely, a correctly fitted curtain works in the same direction as the radiator: it slows the escape of heat through the window, which is precisely the coldest surface in the room.
What actually happens behind the curtain
A radiator heats the air in contact with it, and that air rises. If a curtain hangs in front of it down to the floor, it creates a chimney: warm air rises between fabric and glass, gives its heat to the cold pane, and comes back down cooled into the room. The radiator works, but part of its output heats the window.
That is not a marginal loss on a radiator placed under a window, which is the most common arrangement in France, precisely because that is where the cold comes in. According to ADEME, windows account for 10 to 15% of a home's heat loss.
The four configurations, from worst to best
| Configuration | Effect |
|---|---|
| Long curtain, pressed in front of the radiator, closed | The worst case. The heated air runs along the glass and a notable share of the heat leaves. |
| Long curtain, held a few centimetres off the radiator | Better, but the chimney effect remains as long as the fabric hangs in front of the appliance. |
| Curtain stopping at the sill, above the radiator | A good compromise. Heat rises freely into the room, the glass stays covered. |
| Long curtain, but radiator moved or room with no radiator under the window | Optimal. The curtain closes the air gap to the floor without obstructing the appliance. |
How to choose the length
The rule fits in one question: is there a radiator under the window? If so, the curtain stops at the sill, two to three centimetres above the top of the appliance. If not, it goes to the floor, which closes the air gap and also gives the best drape.
Both lengths are covered in detail in should curtains touch the floor, and the hanging height in how high to hang a rod.
What the curtain brings when it is well fitted
It acts where the radiator can do nothing: on the cold surface itself. The Union des Fabricants de Menuiseries notes that the ADEME estimate does not account for air leakage around windows already installed, and it is precisely that circulating air a dense fabric stops.
| Type of curtain | Weight | Effect in winter |
|---|---|---|
| Sheer or light-filtering | 265 gsm | No thermal effect. It does not obstruct the radiator, nor does it help it. |
| Plain blackout | 310 gsm | Blocks light, with no measurable thermal effect. |
| Thermal blackout, three layers | 620 gsm | Up to 7 °C difference at the glass, and 22 dB attenuated. |
Frequently asked questions
Do you need a shorter curtain when there is a radiator?
Yes, that is the simplest solution. A panel stopping at the sill, just above the appliance, lets heat rise into the room while still covering the glass. A floor-length curtain in front of a radiator sends part of the heat towards the pane.
Can you keep a long curtain despite the radiator?
Yes if the panel passes in front without covering it, for example on a rod wide enough that the open curtain clears the appliance entirely, and closed only at night when the heating is turned down. That is a compromise, not the optimum.
Does a reflector behind the radiator replace the curtain?
No, the two address different things. The reflector limits losses towards the wall behind the appliance; the curtain limits losses through the glass. They add up without replacing each other.
Does a curtain closed at night obstruct the heating?
No, if it does not cover the appliance. At night the heating is often reduced and the glass reaches its lowest temperature: that is when the curtain is most useful.
The rest of the season is gathered on our winter curtains page, and our made-to-measure thermal curtains are cut to the exact height your radiator imposes.
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