ᐅ Cladding on Styrofoam insulation in the attic space

Hello,

We have a cold roof in our house (built 3 years ago). There is insulation on the attic ceiling consisting of two layers of polystyrene. There is no vapor barrier. We waited through 3 summers for the construction moisture of the reinforced concrete ceiling to dry out before installing the decking. I lifted the polystyrene once, and there was no visible or noticeable trapped moisture underneath.

I am not concerned with covering the entire area. It would be sufficient for me to lay a wooden walkway in the middle on top of the polystyrene, about 2 meters (6.5 feet) wide. We don’t plan to use the attic much. The chimney sweep should be able to walk on the path, and maybe one or two items could be placed there at times.

I thought such decking is usually done with OSB boards. But I have read that OSB is not recommended because of moisture or condensation issues, as OSB is not vapor-permeable.

Rough-sawn boards are mentioned as a better alternative. However, I don’t have joists or battens where I could screw or fix the boards. I have noticed that rough-sawn boards are rather narrow and don’t come in sheet form.

Have others experienced this issue? How did you approach it?
Would you just lay rough-sawn boards directly on the polystyrene without fixing them?
Or are there alternative boards to OSB that are more vapor-permeable?

How would you do it, or which type of wood would you use? I would really appreciate a small tip.
Or am I just overcomplicating things?

Thank you and best regards

Are those really polystyrene boards? They usually are not compressive and should not be loaded. Standard chipboard panels are more vapor-permeable 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 often warped, making it difficult to achieve a flat surface without screwing them down. With a partial covering and a well-ventilated roof, though, I would not have concerns with any of these materials.

I would lay battens of the appropriate length on the insulation foam and screw the rough-sawn boards onto them. This way, the insulation foam is ventilated from behind and the load is distributed.

Grundaus schrieb:

Are these really polystyrene boards? They are generally not compressive strength and should not be loaded.

That is not entirely correct in general. Of course, there are floor insulation boards made of EPS (polystyrene). The question is what was actually used here. It is quite possible that non-rigid boards were used for the insulation between floor decks. Either way, I would not glue tongue-and-groove boards on top of them.
I think @Pitiglianio’s suggestion is pragmatic and acceptable for maintenance work only and for a few light objects.

Thank you very much for all the help and advice