Ventilate
Condensation on windows: when to open them to stop it
Why condensation forms on glass, how to air your home in winter without losing heat, and how often to ventilate bedrooms, bathrooms and kitchens to prevent mould.
Water on cold glass is not about how much moisture is in the air — it is about the temperature of the surface. When warm, humid indoor air meets a cold window, the air at that surface drops below its dew point and moisture condenses. In winter, that happens more often because windows are colder.
The goal is not to dry out the home completely. It is to swap indoor air often enough that the moisture from breathing, cooking, showering, and drying laundry does not build up on the coldest surfaces.
Why condensation forms
Indoor air always carries some moisture. That moisture condenses when the air touches a surface cold enough to push it past the dew point. The dew point depends on how much moisture the air is carrying.
In a well-heated room, walls and furniture stay warm. Windows stay cold because glass conducts heat poorly and the outside is cold. The coldest surface in the room is usually the glass, so that is where moisture appears first.
Single-glazed windows and thermal bridges — spots where insulation is weak — are the coldest surfaces and the most likely to run with water.
Short burst vs tilted window
A tilted window cools the wall around it hour after hour without exchanging much air. A wide opening for a few minutes swaps the air quickly, then you close and the room heats back up.
French guidelines recommend at least 10 minutes of wide airing daily, summer and winter, and more while cooking, showering, or cleaning. Short and wide beats narrow and long when the goal is keeping warmth.
Cross-ventilation — windows open on opposite sides — swaps air faster than a single opening. If you only have windows on one wall, leave an interior door open to let air move through the flat.
Bedroom condensation in the morning
A sleeping person breathes out moisture all night. By morning, a closed bedroom can be noticeably more humid than the rest of the home, and the coldest glass will fog or drip.
Open the bedroom window wide for a few minutes after waking. Let the moist air out while the room is still warm from the night. Then close. Wiping the glass removes standing water that can feed mould.
Drying laundry indoors
Wet laundry releases litres of water as it dries. If that water stays in a closed room, it has nowhere to go but onto windows and cold walls.
Air the room while the laundry is drying, even if only for a few minutes at a time. A dehumidifier can help in a home without good ventilation, but it does not replace fresh air.
Bathrooms and kitchens
Steam from cooking and showering loads the air fast. An extractor fan pulls the moisture out before it spreads. Without one, open a window wide during and just after the activity, and close the door to the rest of the flat so the steam does not drift into bedrooms.
When the Ventilate app helps
Ventilate checks outdoor temperature, rain, and the conditions at each of your windows to tell you when a short opening is useful. It does not measure indoor humidity, but it can flag the best moment to swap the air without dumping all your heat outside.
In winter, the advice shifts from long flush to short burst. The app tells you when outdoor air is dry enough and not too cold for a useful airing, and when to stay closed because rain or a hard freeze makes the opening pointless.
Where to go next
- On dew point as a comfort signal: dew point, not humidity
- When true cold shortens the opening: how to air in winter
- On Atlantic coasts where the air feels clammy: damp home in winter
This is not medical advice or a mould diagnosis. Persistent damp may be a building issue. If condensation or mould is making you unwell, follow your local health agency’s guidance.