Insulation under the floor slab EPS or XPS?

We have just received an offer from the general contractor. Our house is supposed to achieve Kfw55. Therefore, among other things, he wants to insulate the base slab. The base slab consists of 28 cm waterproof concrete. The insulation is supposed to be a 12 cm EPS insulation below the base slab. Now my question: Is that okay? I tried to find some information on the Internet and keep reading that under the base slab XPS is used and on the base slab EPS. So is the offer wrong, or is EPS also possible under the base slab?

I would be surprised, because XPS is pressure-resistant, does not absorb water - this is not true for EPS.

Maybe he was just mistaken?

That would then confirm what I have read so far. It is definitely a point that I will raise. However, if he were to tell me that the EPS also goes under the slab, I would believe him then or insist on XPS?

Then it's time for you to get an independent expert for the construction. There is nothing to believe - he should show you the permits, then he will have a problem anyway.

We are not there yet. It's still the offer phase. But for the construction, we will definitely have a DEKRA expert with us. But the approval is a point that I will note down. I'll ask him about that.

As far as I know, EPS is not used under a load-bearing floor slab because it is not as pressure-stable as XPS and also lets more water through. EPS is nothing more than Styrofoam and consists of small beads. You always see XPS in new development areas when floor slabs are poured, it looks like fine foam.

Best regards
Sabine