Roof windows fog constantly - why can this be?

If condensation always forms on skylights, that is not only annoying. In the long term, it can also cause severe damage to the building fabric and the window, and often also contribute to the formation of mold. What is the misting of skylights and what you do against it, you can read here.

Explanation of condensation formation

Cold air can absorb less water than warm air. This is known, for example, from your own bathroom: when the warm, moist air from the shower hits the mirror, it can no longer hold the moisture, and it condenses on the cold mirror surface. It is similar with skylights. In insulated attic rooms, the wall surface is considerably warmer than the window glass when the outside temperature is cool. If the warm air (which has room temperature) hits the cold surface of the window, it cools down very quickly, and the moisture contained in it condenses on the cold glass.

Problem glass edge

A problematic point in all windows is the edge of the glass. At this point, the glass sheets are flush with each other. The glass edge is connected by a so-called spacer with the frame. At this very spot a very cold gap is created. If not specially insulated, the edge of the glass cools much more strongly than the glass, which is heated on the inside by the room air. This creates at this point a thermal bridge and it is particularly fast to condensation.

Roof windows fog constantly - why can this be?: this

window insulation

Roof windows usually have slightly worse U-values ​​than vertical windows. While vertical windows can have U-values ​​of up to 0.6 W / (m²K) and even less, there are few skylights with low U-values. In general, the roof window is thus a much colder area than a well-insulated wall surface (U-value wall normally 0.20 W / (m²K) or even less). Each skylight automatically creates a cold spot in the room.


By replacing the windows with better insulated skylights you can face the problem. If the U-value difference to the wall is low enough, the formation of condensation does not occur.

Skylight Heating

A cheaper alternative may be the installation of a roof window heater. This heats the glass at the edge and prevents condensation.

Tips & Tricks

Be sure to check if your skylights may be leaking. Leaky points are a very effective thermal bridge and cause a high degree of condensation, at the same time a huge loss of heat in the room.

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