The humidity increases after the rain

Is it true that you shouldn't ventilate in fog and rain?

There are many myths and legends about correct ventilation. One of them is: You shouldn't ventilate in fog or rain, as this increases the humidity in the room. But is that even true?

Regular ventilation is important in order to exchange the air in the room. This applies to too dry air as well as to too high humidity. Because humid air in particular can lead to many problems such as mold growth. But what if it rains outside or there is thick fog? Doesn't that mean that more moisture comes in when you ventilate the room? Is it true that I shouldn't ventilate in fog and rain?

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No, it is not true that one should not ventilate when it is raining or foggy outside.

Even on days when there is 100 percent humidity in the outside air due to heavy rain or fog, ventilation should be provided. Because the warmer indoor air can store more moisture than the cooler outdoor air. This is because more water vapor can be dissolved in warmer air. So warm air can store more water.

The humidity decreases as the room temperature rises

Conversely, this means: The cooler air of the mist is heated in the room, but retains its absolute water content. This decreases as a percentage. The warm air can thus absorb more water vapor that we produce by breathing, cooking or showering. Sounds complicated?

A calculation example: If you ventilate your bedroom at 10 degrees and fog with a relative humidity of 100 percent and then warm the air to 20 degrees room temperature, the humidity drops by about 50 percent.

You can also find a physical explanation on

If you now want to know how you can ventilate properly and effectively, read our article: