ᐅ Insulation under the slab: EPS or XPS?

We have just received a quote from the general contractor. Our house is intended to meet the KfW55 standard. For this reason, they plan to insulate the foundation slab, among other things.

The foundation slab consists of 28 cm (11 inches) of waterproof concrete. The insulation is supposed to be 12 cm (5 inches) of EPS insulation beneath the foundation slab.

Now my question is: Is this okay? I tried to find some information online and keep seeing that XPS is usually installed under the foundation slab and EPS is used on top of the foundation slab. So, is this quote incorrect, or can EPS also be used beneath the foundation slab?

I would be surprised, because XPS is pressure-resistant and does not absorb water – this is not the case with EPS.

Perhaps he just made a mistake?

That would then confirm what I have read so far. Definitely a point I will bring up.
However, if he tells me that EPS is also used under the floor slab, I might believe him—or should I insist on XPS?

Then it’s time for you to bring in an independent building inspector.

There’s nothing to take on trust – they should show you the permits (building permit / planning permission), and then they will have a problem if they don’t.

We are not that far yet. We are still in the quotation phase. However, we will definitely have a DEKRA expert involved during construction. The issue of approval is something I will make a note of. I will ask him about it later.

As far as I know, EPS is not used under a load-bearing slab because it is not as pressure-resistant as XPS and also allows more water to pass through. EPS is basically expanded polystyrene, made up of small beads. XPS is commonly seen in new construction areas when slabs are poured; it looks like fine foam.

Best regards
Sabine