You're thinking correctly, it should not be a problem as polystyrene/foam is usually glued with polyurethane glue, and it is just polyurethane that is found in foam filler. I myself have done similar to seal joints before casting and it worked perfectly, gluing and sealing well against the boards. Make sure the boards are reasonably clean and spray them with a little water from, for instance, a spray bottle before you foam, and it will be even better.
I have also used foam spray against styrofoam without noticing any problems.
Any long-term issues have not surfaced yet though (1 year old). And since there is a concrete slab on top, I will probably never discover any possible problems either.
I think you can safely use foam spray.
Any long-term issues have not surfaced yet though (1 year old). And since there is a concrete slab on top, I will probably never discover any possible problems either.
I think you can safely use foam spray.
Foam does not withstand sunlight (well, some variants do), but otherwise it seems completely stable. I started using foam about ten years ago and I can still inspect some of these spots. They look exactly like when I applied the foam, albeit somewhat darker on the surface. In sunlight, however, it becomes brittle and falls apart into small pieces.