ᐅ Attic Issue: High Humidity – Mechanical Ventilation with Heat Recovery?

Hi
I need your help.
We built a solid structure with Ytong in 2017.
We don’t have a basement but we do have a large attic of 70m² (750 sq ft).
Inside the attic, the walls are plastered.
The entire attic roof space is insulated with a vapor barrier and covered with drywall sheets, but not finished yet (ran out of money).
The ceiling between the second and third floor is also insulated.
Upstairs, there are two triple-glazed windows installed, lying parallel.

Now we have the problem that the humidity inside the attic is above 80% daily. The temperature is always between 5-10°C (41-50°F).
Condensation forms every day on the inside of the window frames and also in the middle of the windows between the frame and the glass.
There is no heating up there, of course, since no one lives there. It serves as a substitute basement.

Can you help me figure out how to control the high humidity problem?
I would prefer not to climb up and down the attic stairs every day to ventilate and then close everything again.
I also left both windows tilted open with half the blinds down for two days. The humidity was high, but the temperature dropped below freezing.
I wasn’t sure if that’s a good long-term solution.

A controlled mechanical ventilation system costs €2000-3000 (about $2200-3300). I already received a quote.
However, no one can explain why this is happening in the attic.
On one side, a stubborn mold stain has already formed on the plaster.
Currently, I run a dehumidifier all day every two days.

But that can’t be a permanent solution.

We really had many problems during the construction phase. And now this too :-(

Can you give us tips or help on how to solve this problem?
Thank you

You have to ventilate. There’s no other way. You’ve sealed everything else tightly. A draft is the only thing that helps.
It would have been better not to insulate it and not to wrap it in foil. Our real cold attic is cold, now even below freezing. But it is dry. Karsten

Heating or ventilating, nothing else works. Don’t you have any heating upstairs?

Moisture has to come from somewhere. Is there a vapor barrier installed in the ceiling of the top floor? Or is the attic hatch not sealed properly, allowing moist air to rise into the roof space? If the ceiling of the top floor is sealed well, there shouldn’t be that much moist air moving upward.

So, the attic hatch should be airtight.
A vapor barrier has been installed.
Can I keep both windows permanently tilted open with half-closed blinds (due to the weather-exposed side) without causing mold to form on the windows, and thus avoid using the mechanical ventilation system?

Hello,

I'm not entirely sure about this:

slygon schrieb:
The entire attic peak is insulated with a vapor barrier.

Where exactly is the vapor barrier located? Is it on the attic floor or beneath the roof tiles?

Regards,

Andreas