Insulated roof with a vapor barrier – what are the standards of humidity?
In our house the roof and suspended ceiling are insulated, the quality of the vapor barrier between the upper floor and the roof is mediocre.
The KWL is inside the roof. Our planner thought it was a good idea, and apparently it's not really unusual. In hindsight, though, I would have done things differently.
How do you recognize damage and how do you prevent it? Are there humidity limits or something similar that I have to observe?
I could easily vent the attic through the KWL, but the KWL has a moisture recovery feature. Does that make sense?
I think an insulated attic should be able to ventilate, so build a window. On our roof we have Velux, which I always open for half a day. It dries out quickly up there.
In your case, it's complicated by the fact that you're using OSB as your flooring. This completely envelopes your top floor (also acts as a vapor barrier). Especially if the vapor barrier underneath – as you wrote – was not done correctly, there is a significant risk to the structure. Moisture penetrates from below, but can no longer escape upstairs.
Because of the complexity (OSB, KWL) I can not give specific advice here, but imho, at least the flooring should be replaced with Rauspund (diffusion-open). Cut "strips" out of the boards or drill large holes in them, if necessary. But please consult an independent expert – who will also be responsible for his recommendations in this case.
But just to be clear: I see a dire need for action here. It can last for decades, but it can also cause the ceiling joists to rot within a very short time. Personally, the risk would be too great for me.