Insulation for the top floor ceiling - which material?

Can you just remove that so easily and install something else? After all, that's the thermal envelope that must be properly calculated in order to comply with the energy saving ordinance

That's right, you're correct. I'll talk to my energy consultant tomorrow, but I'm 99% sure that I can't just remove it like that. Another idea: I'll have the general contractor lay the polystyrene as agreed, but no OSB boards on top of it; instead, I'll put something on top myself afterwards. Or can I, for example, put rock wool over polystyrene? Would that even provide better thermal insulation, or would that be redundant?

I find the Styrofoam to be poor and of low quality. As long as the concrete ceiling still releases moisture, this cannot pass through the Styrofoam and condenses. Or is a foil installed in between? Furthermore, with Styrofoam as a panel material, there is the risk that it does not lie flat on an uneven surface and the insulation effect is not achieved. Wool on top is not better. Mineral wool generally has a better WLG and therefore provides better insulation at the same thickness. It is elastic, adapts to the substrate, and is vapour-permeable. In other words, it is open to the GU.

True, you’re right. I’ll talk to my energy consultant tomorrow, but I am 99% sure that I can’t just take it out like that.

That’s just because of the construction company. The energy consultant has no problem with it.

Or can I, for example, put stone wool over styrofoam? Would that even provide better thermal insulation, or is that redundant?

It’s redundant and a waste of money.

I also consider the idea to be a waste of money and see it as a mistake to completely dispense with walkability. As far as I'm concerned, the Styrofoam can/should be replaced with stone wool and OSB laid on top. With additional effort, one will not gain more thermal insulation, as other factors play a more important role in heat ingress. Additional costs can be saved and better invested in air conditioning or similar... One would probably regret giving up walkability at some point. There are always cases where it is important or practical to be able to move around on top. Whether it concerns repair measures, photovoltaic or SAT installations, or the like. With exposed 320mm stone wool, one does not do that just like that, especially since OSB also represents the "moisture barrier"...

Multi-layer lightweight panel could be worth a look. It's the best combination of everything. You don't need to make extra catwalks, it can be walked on directly...

If walkable, then please not with OSB but with tongue and groove boards