ᐅ Installing flooring directly over insulation in the attic?

Hello everyone

I have a pitched cold roof with a concrete ceiling. A vapor barrier membrane will be installed fully across the ceiling, followed by 20cm (8 inches) of polystyrene insulation. What should I use on top of the polystyrene as the "floor surface"?

Tongue-and-groove boards would be good because they are vapor-permeable, but that would require a substructure.
I can install OSB boards over the whole area, but there might be condensation issues.
Cement screed is an option, but expensive?

What other alternatives do I have? The whole setup should be cost-effective and not increase the build height too much.

Thanks in advance for your help.

XPS + dry screed

I don’t think you will have any problems with condensation. At least not with a concrete ceiling and a vapor barrier.

@truce
Is OSB a "particle board" or where can I source it?

@Joedreck
I have read online about issues more often. When do problems with condensation usually occur?

As a general rule, I would advise against using OSB as the final layer on the cold side of wooden ceilings, since the vapor barriers are often improperly installed. However, in your case, you have concrete and are planning to put a vapor barrier on top of it. I can't think of anything less permeable to vapor diffusion than that.

Do you intend to use the entire area as storage space?

Yes, I want to use it as storage space.
Which OSB board thickness do I need, 18mm (0.7 inches) or 22mm (0.9 inches)?