Cladding on Styrofoam at the roof apex

Hello,

we have a cold roof in our house (built 3 years ago). On the attic ceiling, there is insulation with 2 layers of polystyrene. There is no vapor barrier. We have now waited 3 summers because of the construction moisture in the reinforced concrete ceiling so that it could dry before the boarding is done. I lifted the polystyrene once. There was no trapped moisture visible or noticeable there.

It's not about boarding the entire area for me. It would be enough for me to lay a wooden walkway in the middle on the polystyrene as a path (2 meters wide). We don't want to use the attic much. The chimney sweep should be able to walk on the path and maybe some items will be placed there occasionally.

I thought such boarding is done with OSB boards. But now I have read that OSB boards should not be used because of moisture or condensation formation, as OSB is not vapor permeable.

Rough-sawn boards are mentioned as a better alternative. But I have no beams on which I could screw/attach the boards. I have seen that rough-sawn boards are rather narrow and there is no such thing as panels.

Have others had this problem too? How did you do it? Lay rough-sawn boards directly on the polystyrene without fastening? Or are there alternative boards to OSB that are more vapor permeable?

How would you do it or which wood would you use? I would be very happy to get a small tip. Or am I just making it too complicated?

Thank you and best regards

Are they really polystyrene boards? They are actually not pressure-resistant and should not be loaded. Normal chipboards are more permeable to diffusion than OSB. Wood fiber boards even more so. Gypsum fiber boards as well. Tongue-and-groove boards should definitely be glued to the substrate. However, they are usually so warped that it is difficult to achieve a flat surface without screwing. With partial coverage and a well-ventilated roof, however, I would have no concerns with any material.

I would lay battens of the appropriate length on the Styrofoam and screw the rough planks onto them. This ventilates the Styrofoam and distributes the load.

Are they really Styrofoam boards? They are actually not pressure-resistant and should not be loaded.

That is not generally correct. Of course, there are floor insulation boards made of EPS (Styrofoam). The question is what was used here. It is quite possible that non-pressure-stable boards were used for the intermediate ceiling insulation. In any case, I would not glue tongue-and-groove boards on there.
I think 's suggestion is pragmatic and okay for really just maintenance work and a few light objects.

Thank you very much for all the help and tips