Condensation on Windows: Causes and Lasting Solutions
In the morning the pane is misted up, there is a small puddle on the windowsill and dark spots appear in the corners of the frame: condensation on windows is a common nuisance in the cold season. A few drops on very cold days are harmless. If water forms day after day, however, this points to excessive indoor humidity or cold building components, and in the long run mould and damage to frames and reveals are likely.
In this guide, we explain how condensation forms, which three cases you should distinguish and which causes lie behind them. We also show immediate measures for everyday life and lasting solutions, from the right ventilation strategy to a ventilation system with heat recovery.
How condensation forms on windows: the dew point
Depending on its temperature, air can hold different amounts of water vapour, warm air more, cold air less. If humid indoor air cools down on a cold surface, the relative humidity there rises. If it reaches 100 per cent, the air has fallen below the dew point and the water vapour is deposited as condensation. In winter, the glass pane is usually the coldest surface in the room, especially at the edge where the spacer between the panes sits. This is why misting often starts at the bottom edge of the glass.
How quickly this happens depends on the indoor humidity. The following table shows the dew point at a room temperature of 20 degrees, calculated using the common Magnus formula:
| Relative humidity at 20 °C | Dew point |
|---|---|
| 40 % | about 6.0 °C |
| 50 % | about 9.3 °C |
| 60 % | about 12.0 °C |
| 70 % | about 14.4 °C |
At 50 per cent humidity, a pane therefore stays dry as long as its surface is warmer than about 9 degrees. At 70 per cent, it mists up below around 14 degrees. A few percentage points less humidity therefore make a noticeable difference. Incidentally, mould does not need dripping water: things become critical as soon as the relative humidity at a surface stays at around 80 per cent for several days.
Inside, outside or between the panes: three cases
Pay attention to exactly where the water forms, because the causes differ.
Condensation on the inside
If the room-side pane mists up, the indoor air is usually too humid or the glass surface too cold. Bedrooms, bathrooms and kitchens are typical. This case is the focus of the rest of this guide.
Condensation on the outside
If the pane mists up on the outside on cool mornings, this is normal with modern low-emissivity glazing. Because the glass lets hardly any heat escape to the outside, the outer pane cools down considerably on clear nights and the humid outdoor air condenses on it. As soon as the sun comes out, the misting disappears. This is not a defect but a sign of well-insulating glazing.
Condensation between the panes
If moisture forms in the cavity between the panes, the edge seal of the insulating glass unit is leaking. Wiping does not help here; the glass unit has to be replaced. A glazier or window fitter can assess whether only the glass or the entire window should be replaced.
Typical causes of condensation on the inside of windows
Every household produces moisture daily through breathing, cooking, showering, plants and drying laundry. With several people, several litres of water per day quickly add up and have to be removed via the air. If ventilation is too infrequent, indoor humidity rises and the water is deposited at the coldest point. The German Environment Agency (Umweltbundesamt) gives 40 to 60 per cent relative humidity as a guideline. You can read which values make sense in which room in the guide Optimal Humidity in Your Home.
A common trigger is replacing windows. Old windows were leaky and unnoticeably provided a certain amount of air exchange. New windows close tightly, the moisture stays in the room and condenses on panes, frames or reveals. In rented flats, it is then often disputed whether a structural defect or the occupants' behaviour is the cause: tenants must heat and ventilate to a reasonable extent, and landlords should point out the changed indoor climate after replacing windows. Cold building components add to this: old double glazing without a low-emissivity coating, aluminium spacers in the edge seal or poorly insulated window connections. Cool bedrooms, wide windowsills that keep warm air from the radiator away from the glass, and tightly drawn curtains also encourage condensation.
Immediate measures against condensation on windows
The most effective immediate measure is shock ventilation: open the windows fully for a few minutes several times a day; according to the German Environment Agency, five to ten minutes is usually enough in winter. Cross-ventilation through opposite windows exchanges the air even faster. Tilted windows, on the other hand, provide little air exchange and cool down the reveal. Ventilate specifically after showering, after cooking and in the morning after getting up.
Heat all rooms sufficiently and do not let rarely used rooms cool down too much. Keep the area in front of the radiator clear so that warm air can rise along the pane. Wipe off condensation in the morning with an absorbent cloth so that it does not soak into seals and wooden frames. A hygrometer shows you whether your measures are working.
Preventing condensation on windows for good: ventilation, windows and insulation
If you have wet panes every morning even though you ventilate regularly, you need a lasting solution. The most reliable is controlled air exchange that works independently of the occupants' behaviour. According to DIN 1946-6, a ventilation concept must be drawn up if more than a third of the windows are replaced during a renovation or, in a detached house, more than a third of the roof area is resealed. Our guide Ventilation Concept According to DIN 1946-6 shows what this means in practice.
Ventilation for moisture protection
DIN 1946-6 distinguishes between several ventilation levels. The lowest, ventilation for moisture protection, is intended to prevent structural damage caused by moisture even if the occupants do not actively ventilate. This is possible with window rebate vents or external wall air inlets, with exhaust air systems or with ventilation units with heat recovery. Decentralised units can be retrofitted room by room in the external wall, even in occupied flats, as our guide Retrofitting Decentralised Ventilation shows.
Warmer surfaces on glass and frames
The warmer the inside of the glass and frame, the less often the dew point is reached. Modern low-emissivity glazing with a thermally improved edge seal, often referred to as a warm edge, stays considerably warmer than old insulating glass units. Professional installation with an insulated connection joint is just as important. When windows are replaced, wall and window should match: if the new window is much better insulated than the old external wall, condensation can shift from the pane to the reveal. An energy consultation helps to find the right order of measures. In existing buildings, new windows and ventilation systems with heat recovery can receive grants from the German Federal Office for Economic Affairs and Export Control (BAFA) under the Federal Funding for Efficient Buildings (BEG individual measures), whose conditions were adjusted on 21 July 2026. Status: October 2026; BAFA publishes the current funding rates and requirements.
Frequently asked questions
Is condensation on windows dangerous?
Occasional misting on very cold days is harmless if the water dries quickly. If it regularly remains on frames and seals for hours, mould and damage to wood can occur. You should then tackle the causes.
Why do windows mist up mainly in the bedroom?
Sleepers release moisture overnight, and bedrooms are often kept cooler. As a result, the relative humidity rises and the pane reaches the dew point more quickly. Ventilate thoroughly right after getting up and keep the door to humid rooms such as the bathroom closed.
Does a dehumidifier help against condensation?
It lowers the humidity in the room and can reduce misting. In the long term, however, it does not replace sufficient air exchange, because CO2 and odours have to be removed as well as moisture.
Is condensation on new windows a defect?
On the outside, no, it is a sign of good insulation. On the inside, the cause is usually the greater airtightness and therefore too little air exchange, not the window itself. Moisture in the cavity between the panes, on the other hand, indicates a defective edge seal, which is a job for the window fitter.
Conclusion
Condensation on windows forms when humid indoor air cools below the dew point on a cold pane. Misting on the outside is normal with modern glazing, moisture between the panes indicates a defective edge seal. Against condensation on the inside, the first steps are shock ventilation, sufficient heating and indoor humidity in the range of 40 to 60 per cent. In the long term, the problem is solved by reliable air exchange according to DIN 1946-6, combined with warm surfaces on glass and frames. You will find ventilation units with heat recovery in our Ventilation Systems category, and fans and other accessories under Ventilation.
