Upper floor ceiling - insulate additionally? What makes sense?

OSB boards are vapor-permeable because their SD value is greater than 20, approximately 80 to 200. From SD=20 onwards, it is a vapor retarder. Since no moisture is to be expected from below with proper execution, or only minimal moisture, it will "pass through" the OSB boards, so to speak, albeit somewhat crudely.

Cannot edit with Tapatalk should be SD less than 20.

Hello everyone,

now I have some answers here but none really addressed my questions :-(
What would be the correct setup? Is it enough to replace the board? Or is it enough to install diffusion-open boards?
Is it worthwhile to install additional insulation? After all, I can no longer change anything on the pitched roofs (add insulation). And if the ceiling of the upper floor is better insulated than the pitched roofs, I don’t know if that actually makes sense. The pitched roof area is about 20% larger than the upper floor ceiling.

My suggestion: leave the old insulation and double it crosswise on top of the rafters. Then also put insulation wool in there and lay simple boards or something similar on top. You currently have no vapor barrier or vapor retarder due to faulty aluminum foil. That doesn't matter in this respect because the moisture can escape upwards through the joints. It also makes sense, because I think it can be done with relatively little work and investment.

You should install the vapor barrier new and correctly, preferably from below, so that you can also make the connections correctly. And definitely do not use an OSB board on top! Does doubling the insulation pay off? Probably not, financially speaking.

The possibility from below does not exist. Everything there is covered with [GK], high-quality wallpapered, furnished and inhabited (bedroom and 2 children's rooms, bathroom and gallery).