· By Diego Gómez
Turning off radiators in unused rooms: does it cause damp?
Switching off the heating in spare rooms saves money, but cold rooms attract condensation and mould. How to save without ending up with a damp box room.
With the Ofgem price cap at 26.32p/kWh for electricity from October to December 2026, plenty of households are closing the door on the spare room and turning its radiator off. It does save energy. The catch is what happens to that room by January.
Why cold rooms get damp
Moisture from cooking, showers and breathing spreads through the whole house, not just the rooms you heat. When that warm, moist air drifts into a cold room, it hits walls and windows that are several degrees cooler, and the water condenses there. That's why the box room or the back bedroom is usually where black mould shows up first.
How to save money without the mould
- Turn it down, not off. Setting the radiator valve to a low number (or "frost" plus a bit) keeps the walls from getting really cold.
- Keep the door shut. That stops moist air from the rest of the house drifting in.
- Air it briefly. Open the window for 10 minutes now and then, especially after drying clothes or if someone slept there.
- Don't use it as a drying room unless you run a dehumidifier in it.
- Pull furniture a few centimetres off outside walls so air can circulate behind it.
Keep an eye on it
A cheap hygrometer in the unheated room tells you if humidity is creeping above 60–65%. If it is, a desiccant dehumidifier is the right tool for a cold room: compressor models lose a lot of performance at low temperatures. A desiccant unit drawing 300–650 W costs roughly 7.9p–17.1p an hour at today's cap; with its humidistat it won't run all the time. For a wardrobe or a small box room, a non-electric absorber can be enough.
If mould has already appeared, follow our guide on how to get rid of mould, and if the windows stream every morning, see condensation on windows.
Recommended
ThermoPro TP49 hygrometer (4-pack)
- temperature and humidity
- 4
- AAA
Cheap digital hygrometers with a comfort-level face.
EcoAir DD2 Simple (desiccant)
- 9 L/day
- 1 °C
- 3 L
A light desiccant dehumidifier that keeps working in cold, unheated rooms down to 1 °C, where compressor models struggle.
Pro Breeze Osmo (refillable, no power)
- non-electric moisture absorber
- up to 1 L
- up to 10 m²
Reusable, non-electric moisture absorber for wardrobes, bathrooms and small bedrooms.
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