The first cold nights of autumn always bring the same complaint: you open the curtains and the glass is streaming, the sill is soaked and there's a grey bloom creeping along the seal. Here's how to stop condensation on windows overnight properly — by getting the moisture out of the air before bed, rather than wiping the glass every morning.
Why the water appears while you sleep
Condensation is warm, damp air meeting a cold surface. Your window is the coldest thing in the bedroom, so that's where the air gives up its water first. Overnight the heating goes off, the glass drops several degrees, and two sleeping adults breathe out roughly half a litre of water each into a closed room. Add an Atlantic climate where outdoor humidity rarely falls below 75 %, and you have the classic Irish and British bedroom window in October.
Which means the fix is almost never the window itself. It's how much water is in the air, and whether it has anywhere to go.
How to stop condensation on windows overnight: the evening routine
- Open the windows wide for 10 minutes before bed. Not on the latch all evening — wide open, on two sides of the house if you can, for ten minutes. Rapid cross-ventilation dumps the humid air without chilling the walls, which is what actually costs you on the heating bill.
- Open the trickle vents and leave them open. The small slotted vents at the top of modern frames exist for exactly this job, and most people shut them or paint over them in September. A sealed room is the most common cause of condensation, not the type of glazing.
- Leave the bedroom door open, and don't push a wardrobe flat against the coldest external wall. Air that can't circulate takes its moisture straight to the nearest cold pane.
- Don't dry clothes in the bedroom. One load on a clothes horse releases around two litres of water into the house. If you've no alternative, keep it to one room with the door shut and the window cracked, or run a dehumidifier beside it.
- Keep a low, steady heat instead of big swings. A room blasted for two hours and then left to fall to 12 °C will condense far more than one held gently at 16-17 °C. Steady is both drier and cheaper.
- Run a dehumidifier for two hours before bed if one room is persistently bad. Aim for 45-55 % relative humidity; a cheap hygrometer means you stop guessing.
The three rooms that soak the rest of the house
Bedroom condensation is usually made somewhere else and walks down the hall to find you.
- The bathroom: run the extractor fan during the shower and for 15 minutes afterwards, with the door shut. Switching it off and opening the door just shares the steam with the bedrooms.
- The kitchen: lids on pots and the cooker hood on. Check whether the hood actually vents outside or simply recirculates through a charcoal filter — if it recirculates, the water stays in the house.
- The utility room or hall: wet coats hung up to dry and tumble dryers that aren't vented or condensing keep pumping moisture out all evening.
What to do with the water that's already on the glass
Wipe it, don't let it sit. Water that runs down onto the sill and stays there is what grows black mould along the seals and eventually rots the beading. A window vacuum, or a squeegee and a cloth, takes about thirty seconds per window; wring it out in the sink rather than leaving a damp cloth on the radiator, which puts the water straight back into the room.
If mould has already appeared, clean it with a proper mould remover or diluted bleach, let it dry fully, and ventilate daily afterwards — otherwise it's back within weeks. Rubbing it with a dry cloth just spreads the spores: use a damp cloth and bin it.
When it isn't condensation
Two problems look similar and won't be solved by airing. If the misting is between the two panes of a double-glazed unit, the seal has failed and the unit needs replacing — no amount of ventilation will clear it. And if damp shows low on the wall with a tidemark, or always in the same patch whatever the weather, that's rising or penetrating damp and needs looking at properly.
Everything else is moisture management, and most of it is free: ventilate hard and briefly, heat gently and steadily, and keep wet washing out of the bedroom. You'll see the difference on the glass the very first morning.
For more ideas on keeping a home comfortable through the colder months, have a look at the full Velia Select collection.