You probably know the feeling. You wake on a cold morning and the bedroom windows are completely misted up. Water is running down the glass and pooling on the window sill. You may also have noticed black specks in the silicone joint between frame and wall.
This is not just annoying — it can be a sign that something in your home is not working as it should.
Why do windows mist up?
Condensation on windows is a matter of physics: warm air can hold more moisture than cold air. When the warm, moist air inside the home meets a cold surface — like the glass in your window — the air releases its moisture as water droplets. It is exactly the same thing that happens on a cold beer bottle on a summer’s day.
In an average family of four, you collectively release 10–15 litres of water per day — through breathing, cooking, bathing and drying clothes. That moisture needs to get out of the home. If it does not, it ends up on the coldest surfaces. And that is usually the windows.
Three things determine whether your windows mist up:
- Indoor humidity. Above 50–55% relative humidity in the heating season is too high. This causes condensation even on good windows.
- Glass temperature. Old single-glazed windows or older sealed units with a broken seal are significantly colder on the inside than new energy glass. The colder the glass, the faster condensation forms.
- Ventilation. Many older homes have natural ventilation designed for draughty windows. If you fit new, airtight windows without thinking about ventilation, you trap the moisture inside.
When is it harmless — and when should you act?
Normal and harmless:
- Light misting on the inside of the glass in the morning, which disappears during the morning
- Condensation on bathroom windows right after a shower
- Condensation on the outside of new energy-efficient windows (this is actually a sign that the windows are insulating well)
Act now:
- Condensation between the panes of a sealed unit — the seal has broken and the unit must be replaced
- Daily condensation that runs down into the window frame — the moisture is destroying the timber and creating conditions for mould
- Black specks or discolouration in the frame, the joint or the wall around the window
- Persistent smell of dampness in the room
According to SBi (the Danish Building Research Institute), persistent condensation on windows and in window frames can lead to rot in timber structures and mould, which can cause respiratory problems — especially in children and allergy sufferers.
Five things you can do
1. Ventilate — briefly and effectively
Open windows for 5–10 minutes morning and evening. Cross-ventilation is best: open windows in two opposite rooms. This exchanges the moist air without cooling the home down.
2. Monitor humidity
A hygrometer costs under 100 DKK and gives you a precise figure. Aim for 40–50% relative humidity during the heating season. Above 55% is too high.
3. Stop drying clothes indoors
A tumble dryer or a ventilated drying cabinet removes a major moisture source. If you dry clothes on a rack in the living room, it releases up to 3–5 litres of water into the air.
4. Check the ventilation
Does your home have vents in the exterior wall or trickle vents in the window frames? Are they open? Many people close them in winter to save heat — but this traps moisture inside. If your home has mechanical ventilation, filters should be checked at least once a year.
5. Consider new windows or glazing units
If your windows are from before 1990, they probably only have two panes of glass without an energy coating. New energy-rated glazing units keep the glass surface warmer, so condensation forms far less frequently. You can often just replace the glazing units rather than the whole window — this is both cheaper and better for the environment.
The price for new windows varies considerably depending on type and size. Always get at least two quotes, and pay attention to the U-value: the lower the figure, the better the unit insulates. The Danish Energy Agency recommends a U-value of no more than 1.0 W/m²K for new glazing units.
Which homes are most at risk?
- Detached houses from the 1960s–80s — often with original sealed units, natural ventilation and retrofitted insulation in exterior walls that has made the house tighter without ventilation keeping pace
- Flats with a windowless bathroom — moisture from bathing has to go somewhere, and it often ends up on the bedroom windows
- Newer, well-sealed homes without mechanical ventilation — built tight to save energy, but without sufficient ventilation to handle the moisture produced by occupants
- Homes where clothes are dried indoors — one of the most overlooked moisture sources
Condensation on the outside — actually a good sign
If you see misting on the outside of your windows in the morning, this is actually positive. It means the glass is insulating so well that the outside remains cold — and the cool night air condenses on the surface. This disappears when the sun warms the glass.
This happens especially with new low-energy triple-glazed units. You do not need to do anything about it.
Do something — before it gets expensive
A little misting in the morning is normal. But if your windows mist up daily, or you can see signs of moisture in the frames, you should act. Mould often starts in the window frame and spreads from there to the wall.
Start with the simple steps: ventilate, measure humidity, and check your vents. If the problem persists, it may be caused by something deeper — missing vapour barrier, insufficient insulation, or windows that have simply reached the end of their service life.
If the problem is widespread, it may be connected to damp in the walls or generally poor ventilation. And if mould appears in the window frames, you should act before it spreads.
Sources: SBi “Moisture and windows”, Byggefejlregistret (Byggeskadefonden), Danish Energy Agency window guide.