ᐅ Expanded polystyrene (EPS) granules for varying structural floor slab heights

Hello,
we are currently planning the ground floor floor construction in a single-family house that is being renovated. The house is only partially basemented, and the structural height in the basemented rooms is higher than in the non-basemented rooms. If we now install expanded polystyrene granulate (then underfloor heating and screed), there will be 12cm (5 inches) of granulate in the basemented rooms and 28cm (11 inches) in the non-basemented rooms. Can anyone tell me if this might cause problems because the granulate could settle unevenly over the years? Would it be an alternative to raise the non-basemented rooms with expanded polystyrene granulate to the same level as the other rooms and then install insulation boards everywhere?
Thank you! 🙂

Why use granulate instead of regular floor insulation boards? It’s used everywhere, just in different thicknesses.

One screeding company would use only granulate, while the other company would use granulate partly and then install boards on top. Honestly, we hadn’t considered using boards everywhere, but we don’t see any reason against it. Our main concern is that the floor does not settle unevenly due to different structural slab heights.

It depends on the level of unevenness. Using only insulation boards is only possible if the floor has been leveled, which is rather unlikely.
I would go with the approach of the second company: Use a screed or loose fill to even out the level, then insulation boards for the required thickness.

The floor is leveled, meaning there are no uneven spots, but it is not necessarily perfectly horizontal everywhere. Therefore, we thought that granulate might be better, also to ensure that the underfloor heating lies on a completely even surface.

Then you can also use floor insulation boards. They are leveled with the screed.