Rough-sawn boards or OSB panels for the attic (possible later expansion)

I filled and sanded it.

The OSB boards in the roof structure could tear along some joints, including the wallpaper. So either screw on another layer of gypsum boards directly and fill (with mesh), or sand, fill, and for example wallpaper the OSB directly with a sturdy painter’s fleece. Painting the OSB board directly can also look good and is found in places like new hotels. However, in this case, a more expensive sealing coat might be necessary because the glue from the boards can dissolve. I had forgotten that OSB may not necessarily be the first choice for living spaces. I don’t want to know what fumes off the glue over the years. Since it only serves as a storage room for us, it wasn’t that important to me; it looks "raw" and is okay so far.

The image in the initial post was only exemplary. We have about 2.20m from the beam layer to the ridge. Is it possibly already advisable to insulate the intermediate areas of the rafters with clamp fleece? Then the intermediate ceiling and the inside of the roof would be insulated. At which points would a vapor barrier foil be needed and which wood covering (OSB or tongue and groove) would be best in this situation?

Thank you very much for your experiences

Honestly: the compressed insulation (20cm thick WLG40) held up extremely poorly for us, (rafter spacing ~63cm) it slipped out here and there again. That means it was necessary anyway to cover the rafters from below right away so that the compressed insulation rests on something. If you want to prepare it as living space, the "proper" construction method would be: vapor barrier from below onto the rafters, then counter battens and then, for example, Fermacell boards. I don’t really see OSB there (as already mentioned). But for the floor, it’s quite good due to minimal height loss and good stability.

The "OSB = vapor barrier" idea of course has its pitfalls, depending on the manufacturer and thickness of the boards it’s either more or less airtight. Since you have the correct vapor barrier under the ceiling, not much moisture happens up there anyway, but in living spaces (office) it’s kind of different. I’m not an expert, I just read up on it before the renovation: what for what, and considered the OSB variant optimal for a purely storage floor.

So OSB with underlying counter battens or tongue and groove boards with underlying counter battens is ultimately irrelevant, as in both methods the air can circulate between the insulation board and the floor covering and with proper sealing of the vapor barrier, no mold can form?