Dew point

Why we regulate according to the dew point and not the rel. Humidity.
Gefülltes Bierglas mit kondensierten Tautropfen
You know it: condensation on the windows. The clearest and simplest comparison is a beer glass. A cold beer is poured into a room temperature glass. The warm and humid air now hits the cool glass. The consequence of this – moisture from the air condenses on the glass as condensation.

This effect also happens in your home when moist air flows into the basement with the window open and is reflected on the cooler walls.