The short answer
Condensation forms when warm, damp indoor air meets glass that is colder than the dew point. You stop it by putting less moisture into the air, moving the damp air out, or making the glass warmer. A dehumidifier does the first job for about 18.2p per litre of water in a heated room and 42.1p per litre in a cold one at the October 2026 cap.
Every page on the first results page for this question tells you to buy a dehumidifier or run an extractor fan. We read five of them on 7 September 2026. Not one says what running either costs. That is the gap this page fills.
What condensation actually is
Air holds water as vapour, and warm air holds more of it than cold air. When that air touches a surface below its dew point, the vapour turns back into liquid on the surface. Windows are usually the coldest surface in a room, which is why they get wet first.
Everything you do about it works on one of three levers. Put less water into the air, take the damp air out of the room, or raise the temperature of the glass. Wiping the window removes the symptom and changes none of the three, though it does stop the water sitting on the frame.
What each fix costs to run
| Method | Which lever | Running cost at the October 2026 cap |
|---|---|---|
| Dehumidifier in a heated room, 20°C and 60% humidity | Less water in the air | 18.2p per litre removed |
| Same dehumidifier in a cold room, 10°C and 60% humidity | Less water in the air | 42.1p per litre removed |
| Larger 25 litre machine, heated room | Less water in the air | 15.7p per litre removed |
| Bathroom extractor fan, 15W, one hour a day | Damp air out | 0.4p an hour, about £1.44 a year |
| Passive moisture absorber tub | Less water in the air | No electricity, cost is the refills |
| Window vacuum | None, removes the symptom | Negligible, it is the purchase price that counts |
| Opening a window | Damp air out | Not priceable here, see below |
Dehumidifier figures are arithmetic on Meaco’s own published wattage and extraction rate for each stated condition, priced at the October 2026 cap electricity rate of 26.32p per kWh. The fan figure is arithmetic on a stated 15W, not a measurement of any particular fan. Full workings are on our dehumidifier comparison. We use the October rates because they hold for the whole heating season. Until 30 September 2026 the cap rate is 26.11p per kWh, which puts the same two cells at 18.1p and 41.8p.
The gap between the first two rows is the part worth remembering. The same machine, at the same humidity, costs 2.3 times as much per litre in a cold room as in a warm one, because a compressor works badly in cold air. If you are choosing where to plug it in, put it in the heated part of the house rather than an unheated garage or a cold spare room.
Size matters too, and not in the direction people expect. The bigger machine removes each litre for less. Our guide to what size dehumidifier you need works through that, and the passive absorber tubs are the cheaper answer for a wardrobe or one bad corner rather than a whole wet house.
The unpriced fix
We read the first page of Google results for this question on 7 September 2026 and recorded one thing about each. Does it put a number on what it tells you to do.
| Page | Fixes recommended | Any cost figure | Any running cost |
|---|---|---|---|
| Screwfix guide | 12, including dehumidifiers, extractor fans, insulation and new glazing | None | None |
| Endsleigh blog | 7, including buying a dehumidifier | Purchase price only, £38 to £140 | None |
| Stafford Borough Council | Behaviour changes plus extractor fans and fixed heating | None | None |
| Todi and Boys Joinery | Dehumidifier, thermal curtains, weather stripping, its own repair service | None | None |
| BBC | Video item, no written guide | None | None |
Five of the six results on page one. The sixth is a Reddit thread, which our tools could not read, so it is left out rather than guessed at. Read 7 September 2026. Pages change, and any of these may add figures later.
So four of the five give no cost of any kind, and the fifth gives what a dehumidifier costs to buy but not what it costs to run. Call it the unpriced fix. The advice is not wrong. A dehumidifier is usually the right machine. But telling somebody worried about a damp bedroom to run one, without saying whether that is pennies or pounds a week, leaves out the part they came to find out.
One more thing came out of reading them. The joinery page ranks on this question with an article about condensation on the inside of windows, and does not deal with the outside of the glass at all, which is what a large share of the current search traffic is asking about.
Condensation on the outside of the glass is not a fault
If the water is on the outer face of a double or triple glazed unit, nothing is broken. The outer pane has cooled below the dew point of the outside air, which happens on clear, still nights when the glass radiates heat to the sky. Dew forms on it for the same reason it forms on a car roof or a lawn.
It is more common on good windows than bad ones. A well insulated unit lets so little heat through that the outer pane never gets warmed from inside, which is exactly what you paid for. The glazing industry’s own guidance describes it as a healthy sign that the glass is doing its job, and says it clears as the day warms up.
Two things follow. There is nothing to fix and no running cost to pay, so do not let anybody sell you a repair for it. And if you are weighing up new units, our pages on energy efficient windows and what double glazing costs cover the choice.
Water between the panes is a different matter. That means the seal has failed and the unit needs replacing.
What to do first, in order of cost
- Stop adding water. Lids on pans, the bathroom door shut with the fan on, and clothes dried outside or in a vented tumble dryer rather than on a rack in the bedroom. This costs nothing and is the biggest single source in most homes.
- Run the extractor fan properly. During the shower and for a good while after, with the door closed. At 15W that is fractions of a penny an hour.
- Move furniture off cold outside walls. Free, and it stops the pockets of still, cold air where mould starts.
- Put a tub where the problem is small. A wardrobe, a caravan or one bad corner does not need a machine.
- Run a dehumidifier in the heated part of the house. This is where the pence per litre above applies, and where the cold room penalty bites if you put it somewhere unheated.
- Wipe the glass if the water is already there. A cloth works and a window vacuum is faster on a lot of windows. Neither changes why it formed.
The catch
Heating a cold room to warm the glass does stop condensation, and it is the most expensive way to do it. We have not put a figure on it, because the cost depends on how leaky the room is, what the weather is doing and how far you raise the temperature, and any single number we published would be wrong for most houses. The same goes for opening a window, where the cost is the heat that leaves with the damp air.
Persistent damp with black mould, or damp low down on walls rather than on glass, is usually not condensation. That is a building problem and it needs someone to look at it rather than a machine.
What this page cannot tell you
- How many litres your own home puts into the air. It varies with how many people live there, how you cook and whether you dry washing indoors.
- What any particular dehumidifier does in your room. The figures above are manufacturer conditions, and a real room is colder, damper and draughtier than a test chamber.
- What heating the room to warm the glass would cost you.
- Whether your damp is condensation at all.
- How the machines sound, feel or last. We have not run any of them.
Method and sources
- Electricity price. 26.32p per kWh, the Ofgem price cap rate for direct debit customers in England, Scotland and Wales, 1 October to 31 December 2026.
- Dehumidifier figures. Manufacturer published wattage and extraction rate at each stated temperature and humidity, divided out to a cost per litre. Sourced and set out in full on our dehumidifier comparison.
- Extractor fan figure. Arithmetic on a stated 15W at the cap rate. Check your own fan’s rating, they range roughly from 8W to 30W.
- External condensation. Clayton Glass guide to external condensation, read 7 September 2026.
- The page one audit. Google.co.uk, UK results, query “how to stop condensation on windows”, read 7 September 2026. We recorded only whether a cost figure appears, not whether the advice is good.
How we work is set out on our methodology page, and the same cap rates price everything else in our running cost index.
Questions people ask
Why is there condensation on my windows?
Warm indoor air holding moisture has touched glass that is colder than its dew point, so the vapour has turned back into water. Cooking, showering, drying washing indoors and breathing all add the moisture.
Why is there condensation on the outside of my windows?
The outer pane has cooled below the dew point of the outside air overnight. It is normal, it is more common on well insulated glazing, and it is not a fault. It clears as the day warms.
How much does a dehumidifier cost to run?
Measured per litre of water removed, about 18.2p in a room at 20°C and 60% humidity, and about 42.1p in a room at 10°C, at the October 2026 cap rate of 26.32p per kWh.
Does opening a window stop condensation?
It helps, because it moves damp air out of the room. It also lets heat out, which is why a fan run for a short period with the door shut is usually the cheaper way to do the same job.
The other half of a cold, damp room is the air coming in around the doors. Our draught proofing audit priced a gap-rated basket at £28.76 and found that 17 of the 23 excluders sold on keeping heat in never say what size gap they seal.