Attic problem. High humidity - Controlled residential ventilation?

Hi Need your help. We built solidly in 2017 with Ytong. We don’t have a basement but do have a large attic upstairs with 70 sqm. We also had the inside of the attic plastered. The entire attic peak is insulated with a vapor barrier. Clad with drywall panels but not yet (money is gone). The ceiling between level 2 and 3 is also insulated. Upstairs there are also two triple-glazed windows installed, lying parallel. Now we have the problem that we have a daily humidity of over 80%. The temperature is always between 5-10 degrees. On the windows, condensation forms every day on the inside on the frame and in the middle of the window between the frame and the window. Of course, it is not heated up there since no one lives upstairs. It serves as a substitute basement. Can you help me how to get the problem with the too high humidity under control? I’d rather not have to climb up the attic stairs every day to ventilate and then close everything again. I had both windows tilted open with half blinds for 2 days. Of course, the humidity was high but the temperature was below zero. I was also not sure if that is good in the long run. A controlled residential ventilation system costs 2000-3000 euros. I already got an offer. However, no one can tell me why it is like this in the attic. On the one hand, a stubborn mold spot has already formed on the plaster. I currently run a dehumidifier all day every two days. But that can’t be the solution either. We really had many problems during the construction phase. And now this :-( Can you give us tips or help on how to solve the problem? Thanks

You have to ventilate. There's no other way. You have sealed everything else. Draft, that's the only thing that helps. It would have been better not to isolate it and not to wrap it with foil. Our real cold attic is cold, now even frost. But it is dry. Karsten

Heating or ventilating, nothing else works. Don't you have a heater upstairs?

The moisture has to come from somewhere. Is there a vapor barrier installed in the top floor ceiling? Or is the attic hatch leaking, allowing moist air to rise into the attic? If the top floor ceiling is airtight, not so much moist air should come up.

So the roof hatch should be sealed. A vapor barrier is installed. Can I leave both windows permanently tilted open with a half blind (because of the weather side) without mold forming on the windows and thus save myself the residential ventilation system?

Hi,

it's not entirely clear to me:

The entire attic peak is insulated with a vapor barrier.

Where exactly is the vapor barrier? Is it on the floor of the attic or is it under the roof tiles?

Regards,

Andreas