Boligfritid.dk is a Danish magazine for homeowners. This English version is written for international residents in Denmark. Read the original in Danish.
Updated 24 October 2026. When it turns cold outside, many people wake up to condensation on the inside of their windows. It is a sign that the indoor air is damp, and damp is exactly what mould thrives on. Here are the causes and what you can do about them.
Why does condensation form?
Warm air can hold more water vapour than cold air. When damp indoor air meets a cold surface, such as a window pane, a cold corner or an outer wall behind a cupboard, the water vapour turns into water. A family releases a lot of moisture every day from cooking, showering, drying clothes and breathing. Surfaces are coldest in winter, which is why the problem shows up most now.
How to avoid damp and condensation
- Air the rooms with a through-draught a couple of times a day for 5–10 minutes. Bolius (the Danish homeowners’ knowledge centre) recommends 3 times a day. Short, effective airing replaces the damp air without cooling down the walls.
- Remove moisture at the source: use the cooker hood when you cook, and an extractor fan or an open window when you shower. Sundhedsstyrelsen (the Danish Health Authority) singles out the kitchen and bathroom in particular.
- Keep all rooms heated: mould often grows on cold surfaces and in rooms that are not heated enough. Sundhedsstyrelsen recommends that bedrooms are also heated during the day and aired well.
- Dry clothes outside or in a tumble dryer, not on a rack in the living room.
- Give the air room to move: keep furniture a few centimetres away from outer walls, and keep radiators free of curtains and furniture.
- Wipe the windows in the morning so the water does not soak into the frames and joints.
When should you take action?
- If black spots or mould appear in corners, by windows or behind furniture.
- If there is a musty smell.
- If there is moisture between the panes of a double-glazed window. That means the unit is no longer sealed and should be replaced. See prices and grants for new windows (in Danish).
- If the problem persists even though you air the rooms. There may be a building defect, such as a leaking roof, rising damp or poor insulation, which needs to be found and repaired.
Insulation helps
The better a house is insulated, the warmer its inside surfaces are, and the lower the risk of condensation. New energy-efficient windows and extra insulation make a big difference. But the more airtight the house becomes, the more important it is to air it or to have mechanical ventilation.
Frequently asked questions
Is condensation on the windows dangerous?
A little condensation in the morning is not dangerous in itself, but it shows that there is a lot of moisture. If the damp persists, mould can develop, which can cause problems such as coughing and irritated eyes.
Does a dehumidifier help?
A dehumidifier can help in a room with a lot of moisture, such as a basement. But it does not replace airing and ventilation.
Should I keep the window ajar all the time?
No. A window left ajar cools the wall around it and uses a lot of heat without replacing the air effectively. It is better to air the room with a through-draught a couple of times a day.
See the full winter checklist: Winter-proof your home.
Sources
- Sundhedsstyrelsen: Indoor climate (in Danish)
- Sundhedsstyrelsen: Spending time in buildings with damp and mould (in Danish)
- Bolius: Mould (in Danish)
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