Insulation for the top floor ceiling - which material?

Yep, because with the reinforced concrete ceiling you already have so much summer heat protection, the stone wool insulation is completely sufficient. And with the savings, you can treat yourself to a nice garden.

I would glue styrofoam, then put ESB or Rauspund on top, as gu suggests. That way you can at least walk on the cold roof. That wood fiber has better summer heat protection is simply complete nonsense. Especially to pay 4k net for that, how big is the floor area? It has absolutely no effect. The vapor permeability is of course an advantage, but once the construction moisture is out, that should be irrelevant.

I would glue styrofoam, then ESB or OSB on top, as gu suggests. That way you can at least walk on the cold roof. The claim that wood fiber has better summer heat protection is simply complete nonsense. Especially to pay 4k net extra for that, how big is the base area? It has absolutely no effect. The vapor permeability is of course an advantage, but once the construction moisture is out, it should not matter.

I have to disagree a bit... "Wood is the building material with the highest heat storage capacity among construction materials. Therefore, wood fiber insulation boards provide significantly better heat protection in summer compared to conventional insulation materials. They ensure that the absorbed heat energy is stored longer (phase shift) and only passed on to the interior in small amounts (amplitude damping). This way, it remains pleasantly cool inside even during extreme heat outside." The text above is from Gutex, and that’s exactly how I understood it (and I think it works well for us too). Of course, 1-2 cm is not enough for the effect; for example, we have at least 8 cm (outer wall), more on the roof (I think 14 cm?).

I have to disagree a bit...

"Wood is, among building materials, the material with the highest heat storage capacity. That is why insulation boards made of wood offer significantly better heat protection in summer compared to conventional insulation materials. They ensure that the heat energy absorbed is stored longer (phase shift) and only transferred to the interior in small amounts (amplitude damping). This keeps the indoor area pleasantly cool even in extreme heat outside."

That is sales talk. Of course, the statement is true by itself. Wood has a high heat capacity. But: How high is the mass fraction x heat capacity of the insulation compared to the reinforced concrete ceiling? I estimate just under 1%, possibly 2% with wood fiber. 1% more mass. For €5000. To achieve the same effect, instead of 24 cm EPS, you would have to double it to: 24.24 cm..... The decisive factor is the thermal conductivity or the heat transfer coefficient. The heat that the insulation can absorb plays only a very subordinate role. The only advantage would be the sustainability aspect.