Attic expansion with a pitched roof using OSB, is this possible?

Good evening.

Following matter:

Structure from bottom:

1. Plasterboard 12mm
2. Battens + counter battens approx. 60mm
3. Vapor barrier installed by a professional
4. Rafters 22cm, so 22cm insulated
5. Can I then lay OSB boards if the rafters above have an added roof insulation with Steico boards 35mm. So it is an insulated attic.

Or is there still a risk of mold? Please share your opinions on this. Or choose thinner insulation between the rafters to allow some air?

Regards

No hints or tips?

You need to explain a bit better what you actually mean:

OSB is supposed to go on the ceiling joists of the top floor ceiling, right?

The construction you explained now also refers to the top floor ceiling? Or to the roof construction above? So is it a warm roof or a cold roof?

The structure from above looks like:

Roof tiles
Battening
Steico 35mm insulation boards
Rafters uninsulated
Then basically the attic space with 1.8m in the middle
OSB on the cross beams 22cm these are also insulated with 22cm rigid insulation wool
Vapor barrier
Battening + counter battens
Drywall

So above with Steico on roof insulation and below with rigid insulation wool. No cold roof.

The 35mm exterior insulation is insufficient, so the attic will still become very cold and the situation arises: OSB boards are very cold from above. All perforations of the living space vapor barrier, e.g. cable passage holes / chimney / wastewater ventilation / ... screw connections counter battens, and drywall studs as well as nail holes of cable holders, etc., allow moisture to pass through, which can condense from below on the cold but breathable OSB boards. --> Mold in the intermediate ceiling.

It would help to also insulate the rafters with flexible insulation to reduce the temperature difference above and below the floor ceiling enough to avoid dew point undershoot. Otherwise use breathable flooring aka tongue-and-groove boards.

Additionally, the room, which is unheated, should be well ventilated. If there is a boiler that cannot breathe on all sides, it is basically difficult. The construction therefore does not fit with Rauspund either, or am I mistaken?