Last Updated on October 24, 2026 by Umar Farooq
Yes, you can leave a window open overnight in winter, and a gap of a few centimetres will usually cut morning condensation, because it lets the moisture you breathe out leave the room. The catch is that it works best on cold, dry nights, and a trickle vent or a vent-position latch does most of the same job with less fuss.
I know how this sounds. You pay to heat the bedroom, then open a hole in it. But the wet window at 7am is a sign the moisture has nowhere to go, and a small, deliberate gap is one of the cheapest ways to give it somewhere.
Should you leave a window open overnight in winter to stop condensation?
Mostly, yes, with one condition: the air coming in has to be drier than the air going out. In winter it nearly always is. Cold air can hold very little water, so even when it feels damp outside, the outdoor air carries far less moisture than the warm air in a closed bedroom. Swap a little of one for a little of the other and the room dries.
I am not going to redo the arithmetic here, because the dew-point comparison is in the guide on whether opening windows reduces humidity. The short version is that the rule runs on water content, not on how damp the weather feels.
Condensation itself needs two things: moist air and a cold surface to meet. A gap does nothing about the cold glass. It works on the other half, the moisture. (If you want the full mechanism for the glass, see why condensation forms on windows.)
What a night of sleeping puts into the room
People add water to a bedroom without trying. You breathe it out, and a shut door and shut window hold it in for seven or eight hours. By morning the room has been quietly collecting it, and the coldest surface, usually the window, is where it lands.
I have not measured the overnight figure for a sleeper, and I have not found a source I would trust for one, so I will not give you a number. For scale, UK housing factsheets put a whole household at roughly 12 litres a day, and a bedroom is one small slice of that. It is enough to matter in a sealed room, which is why bedroom humidity is its own problem.
The Adur & Worthing councils’ damp page describes the same pattern, suggesting what it calls “flush ventilation” to deal with moisture that has built up overnight, with windows open for 20 to 30 minutes in the morning. Note what that is: a daytime burst. It is a different tool from an all-night gap, and councils generally recommend it as well, not instead.
What it costs you in warmth
It costs some. I have no honest figure for it: it depends on the gap, the wind, the outside temperature and the house, so any pound-per-night number would be invented. What I can say is where the cost shows up.
Fresh air has to be warmed, and a draught on a cold night is felt most near the bed. The upside is real too. West Lindsey District Council points out that “dry cool air is actually cheaper to heat than warm moist air”. Trade a little heat for a drier room and the bedroom may not feel any worse.
If heat loss is the worry, shrink the gap rather than abandon the idea. A narrow opening on the latch, or a trickle vent, loses far less than a window pushed wide.
Five rules for a window left ajar
- Keep the gap small. A few centimetres, on a stay or latch, not a wide-open sash.
- Choose the dry, cold nights. Frosty and clear is ideal; see the next section for the nights to skip.
- Shut the bedroom door. Otherwise the draught pulls warm air out of the rest of the house.
- Do not point the bed at the gap. Move the bed, or open a window on the far side of the room.
- Keep the radiator on low. West Lindsey advises background heat rather than short bursts of high heat, and a cold room with a window open is where that matters most.
Pair it with a quick wipe of the glass in the morning and a short daytime airing, and most mild cases are done.
Try these before you prop the window open
A real gap in a real window is the crudest option. Several neater ones exist.
- A trickle vent. A slot in the frame that stays open while the window is shut, built for exactly this job. What trickle vents are and how to use them covers it. If you have one and it is taped over, open it first, before anything else in this list.
- The vent position. GOV.UK’s guidance on damp and mould mentions encouraging occupants to use windows in “vent position”, partly open but locked. That answers the security question too.
- Ventilating before bed. Ten to fifteen minutes with the window wide, then close it. This clears the day’s moisture but does nothing for what you breathe out during the night.
- A morning flush. The 20 to 30 minute burst described above, as soon as you are up.
My honest opinion: a trickle vent beats a propped window for every night in winter, and a propped window beats nothing for a bedroom that streams. If you have neither, the cracked window is a perfectly respectable stopgap.
Nights when the window should stay shut
A gap only helps if the outside air is the drier of the two. These are the exceptions.
- A very humid outdoor night. Mild, drizzly, foggy: the air outside may be carrying as much water as the air inside, and you are importing damp, not exporting it.
- Security. An open window on an accessible floor is an open window. GOV.UK lists security, noise, outdoor air pollution and losing heat as the usual reasons tenants hold back. A latch-locked vent position or a first-floor window helps, a ground-floor sash at the back does not.
- Noise or pollution. Traffic, a nearby road or smoke from neighbours’ chimneys are fair reasons to use a trickle vent instead.
- A draught on a cold wall. If the gap makes a cold wall or corner colder, the surface can sit below the room’s dew point and collect more moisture than before. Look at where the cold air is landing, and if it is on a wall, close it up and try a vent elsewhere.
- A room that is badly underheated. A gap cannot rescue a bedroom that is never warmed. If the walls are cold, condensation on the windows and walls needs heat and insulation first.
Crack it on dry cold nights, vent it on the rest, and dry the source
So here is where I land. On a cold, dry night, leaving a window open overnight in winter is cheap, effective and nothing to feel guilty about, as long as the gap is small, the door is shut and the heating is not off. On damp, mild or noisy nights, a trickle vent or a morning flush does the same job. Underneath all of it, find out where the moisture comes from, whether that is drying washing in the bedroom or an unvented tumble dryer, because ventilation lowers the damage and does not remove the cause.
FAQ
Is it good to leave a window open at night in winter?
It can be. A small gap lets the moisture you breathe out escape, which cuts condensation on the glass by morning. It works best on cold, dry nights and is less useful when the outdoor air is damp and mild.
Will leaving a window open all night stop condensation?
It often reduces it, but it will not fix it on its own. Condensation forms where moist air meets a cold surface, and a gap only tackles the moisture. If walls and glass are very cold, or washing is drying in the room, you will still see some.
Does leaving a window open overnight cost a lot of heating?
It costs some, and I cannot give you an honest figure because it depends on the gap, the wind and the house. A narrow gap, a closed bedroom door and the radiator kept on low keep the loss small.
Is it better to use a trickle vent than an open window?
Usually, yes. A trickle vent stays open while the window is locked, so it solves the security and draught worries, and it is built for continuous background ventilation. See the trickle vent guide for how to use it.
How long should I leave a window open in winter?
For a morning flush, councils suggest 20 to 30 minutes with windows open on opposite sides of the house, or up to an hour for a bedroom. For overnight, use a small gap or vent only, and close the window to heat the house properly during the day.
