Sharp increase in indoor air humidity in summer

Currently always between 65-75%. That's normal.
Window permanently open.

The only really problematic time is actually in winter.

Here currently always between 65-75%. That's normal. Windows permanently open. Really problematic is that actually only in winter.

Not only in winter. Especially on cooler basement/souterrain walls, the humidity from the warm air can condense on the walls. Mold can also form there.

Here also 65%. One door or another is already sticking …

Windows open permanently.

Why is that?
I don't need a humid 28 degrees in the room.

Why is that? I don't need a humid 28 degrees in the room.

I am just like that. As long as it is not EXTREMELY hot, I keep the windows open. I like fresh air/natural sounds, even if it gets warmer. But I am also located in Northern Germany. It is rarely really intense here.

I also think that is normal. I don't have a meter, but in our 1921 house it is especially more humid in the basement in summer than in winter when you ventilate a lot (or simply the door at the back is often left open because we are in the garden and usually enter through the basement). The reasons have already been explained. I read up on it and everywhere it says you should only ventilate briefly in summer and preferably not in the middle of the day. The humid air from outside simply brings in even more moisture. I tried the test this summer and ventilated accordingly and indeed it is now significantly drier in the basement than in previous years and that is good for the house. And the basement door now stays closed as much as possible.