Insulating the attic, avoiding mistakes?

Yes, moisture is an issue. Especially when insulating, there can be problems with the dew point. You have to be careful here. In the old house, we used the attic as storage space, and it wasn’t great. You’re reluctant to go up the stupid stairs. In the new house with two full floors, that would be better possible. But we consciously decided not to make it usable. I would either make the attic heated and fully usable with a fixed staircase, or not use it at all.

Moisture should not be an issue at all, especially in an uninsulated cold roof.

Moisture should not be an issue at all, especially with an uninsulated cold roof.

I see it exactly the same way. We have a sensor from the hygrometer in every room, and in the attic it's often only around 35%...

Theoretically no, that's correct. But practically, it might be the case with faulty work. And it really has to be planned properly. In an uninsulated, ventilated cold roof, there are virtually never any problems. If warm air reaches the insulated attic, it will condense there. Therefore, work must be done meticulously regarding the airtightness between the upper floor (OG) and the attic (DG).

The temperature is not the problem, but the humidity. Ventilating through roof windows during hot summer temperatures brings warm and humid air into the attic. Therefore, keep windows closed as long as it is warmer outside than inside. In winter, however, open windows, then cool but dry air enters the attic.

Exactly, because an attic is sealed and usually also has roof windows... 100 points for the candidate in purple.