ᐅ OSB boards directly on top of expanded polystyrene (EPS) insulation boards?

Hello everyone,
in our detached house, the floor of the attic (brick ceiling) is insulated using Styropor panels. Due to the low ceiling height, the attic is practically uninhabitable but useful as a storage room.
When stepping on the Styropor panels (I’m not sure if there are differences between them), they always give slightly underfoot. I would like to prevent this with a walkable surface. I am considering OSB boards for this. The Styropor panels are laid crosswise and are 10cm (4 inches) thick each. Is it possible to install OSB boards directly on the Styropor as a floating floor?

You can do that, but make sure not to install the OSB as a continuous layer. OSB acts like a vapor barrier, and if it is laid without gaps, any moisture that has entered may not be able to escape.

Oh no, I hadn’t thought of that at all! Thanks for the tip about the vapor barrier.
What other alternatives are there? I’ve read about plywood, Fermacell, and oriented strand board (OSB) panels. Are there more affordable options for a floor that will be used infrequently?

Standard expanded polystyrene (EPS) is not permanently walkable, and 10 cm (4 inches) of insulation is insufficient. If only light boxes are going to be placed on it, then cover it with 20/22 mm (0.8/0.9 inch) chipboard panels. These are cheaper than OSB and not vapor-tight.

OSB boards used as the upper layer of insulated upper floor ceilings beneath cold roofs act as a vapor barrier and can cause condensation from indoor air to form on the cold underside. This construction is therefore not permanently damage-free. A better solution would be walkable and load-bearing wood fiber panels specifically designed for this purpose (e.g., Gutex/Pavatex) or, above the insulation, a walkable floor laid with gaps, such as tongue-and-groove boards, on a separate wooden substructure that allows diffusion moisture to escape.