High humidity in the bathroom. What is the reason?

Hello everyone!

Since moving into my own house, I have had a constant problem with humidity in the bathroom. It is simply permanently too humid, not only after washing or showering, but just always. The towels don’t dry and mold forms on the walls from time to time.
The bathroom adjoins the garage, this wall is made of Ytong blocks (20 cm). The other walls are made of 24 cm bricks.
Ceiling construction: roofing felt, 30 cm insulation, foil, wooden panels
The bathroom has a window and is naturally ventilated regularly, yet I simply cannot reduce the average humidity.
I am really at a loss, what could the cause be?

Since you air the room regularly, and the bathroom still does not dry, there is a risk that moisture is entering the bathroom from outside. Where from, I cannot say, as I do not know your house. Possible causes could be leaks (roof, water pipes, bathtub drain, joint between bathroom wall and garage...). You should consult a professional/expert who will examine the situation with a moisture meter etc.

Unfortunately, it is really difficult to determine where the moisture is coming from! Is the moisture present permanently when the windows remain closed? If yes, then something must certainly have gone wrong during construction!