What causes “sweaty” windows?


If you find condensation on your new windows, the reason is simple: the new windows are more airtight and are hindering ventilation in the room. How can you avoid sweaty windows?

The problem lies in the indoor climate

If you have had your windows replaced and the new windows are sweating, the problem is generally not the window, but the changed indoor climate. The likely source of the issue is that natural ventilation through the windows is obstructed, which causes excessive moisture in the air to condense on colder surfaces – in residential buildings these are generally the windows. Thus, the formation of condensation should not be taken as a definite sign that there is something wrong with the window, but rather as an indication of problems with the indoor climate.

This can be resolved through heating and regular ventilation of the rooms via the windows or forced ventilation. In other words, sweaty windows can be avoided by maintaining an appropriate level of humidity in the indoor air and keeping your rooms warm, including the surface of the glass. Instead of blocking air movement with thick curtains, allow air to circulate near the windows.

Air humidity is affected by the people in the room, in particular their breathing and activities, such as cooking, as well as the presence of indoor plants, etc.

Heat and ventilate!

The solution, therefore, is simple: heat and ventilate! If the building is not equipped with an appropriate and functional supply and exhaust ventilation system to maintain an acceptable indoor air quality, you will inevitably need to open the windows once or twice a day to air the rooms. Alternatively, you can try fitting the window sash or frame with ventilation flaps.

As condensation is a physical phenomenon that occurs when the temperature drops below the dew point, sweaty windows are particularly common in the autumn and spring, especially with modern windows that have excellent thermal insulation. This is also one of the possible causes of fog on the outer pane of the window.

Condensation forms on the outer pane when air humidity is high and the air is cool. Since this is in part also caused by the warm indoor air not being able to reach the outer pane, condensation on the outside of the window is simply a sign of proper thermal insulation. Hence, fog on your windows means that the windows are truly of high quality and energy-efficient.

Frost is not a threat

However, if it is cold outside, the room should still be heated, because colder temperatures will cause the barriers to cool, which means more energy will need to be expended to keep the room warm.

Sweating is less of an issue with triple-glazed units, as the surface of the inner pane is relatively warm in these, which lowers the risk of condensation.