Insulation under the floor slab EPS or XPS?

Both are polystyrene insulation materials. XPS is predestined for perimeter insulation because it is pressure-resistant, water-repellent, and thermal insulating.

But there is also EPS for thermal insulation under the floor slab. This must then have the abbreviation "PB" at the end of the designation, e.g., EPS 035 PB. PB: Exterior thermal insulation under the floor slab against soil (outside the sealing)

Often, the two insulation materials can already be distinguished by their color. EPS is usually white. XPS is often colored pink, green, blue, purple, or yellow.

However, EPS sold as perimeter insulation boards also often has color. For example, EPS 035 PW (PW: Exterior thermal insulation of walls against soil outside the sealing)

Thank you, that is interesting information.
Regarding the basement exterior wall, the offer states " ... with external perimeter insulation WLZ 035, d = 10 cm."

So, I now have the information that the insulation will be made of XPS and I will get that in writing as well. So, all good.

Quick question regarding insulation of the floor slab: Our architect has now proposed glass foam gravel as a base under the floor slab. About 35-40 mm thick, compacted. Does anyone have experience with it compared to XPS? Both in terms of price and insulation performance?

The thermal conductivity of glass foam gravel is roughly three times higher than that of XPS. This means that for the same thermal insulation, the layer thickness must be about three times higher. For low requirements (according to your information, the floor slab is probably not intended to be suitable for passive houses ), this is generally more cost-effective. However, if you want to achieve U-values with the floor slab that are comparable to those of an external wall, XPS is significantly cheaper. An example: In our case, 20 cm of glass foam under the floor slab would have cost about €33/m² (glass foam gravel compacted in fleece) and would have provided a U-value of 0.55 W/m²K. Alternatively, 10 cm of XPS was offered, which would have cost about €36/m² (including 15 cm of filter gravel and 3-5 cm of sand for fine grading) and would have provided a U-value of 0.35 W/m²K. In the end, we had 14 cm of XPS (0.25 W/m²K) installed to meet all the requirements for KfW55. Are you sure it is only about a 35-40 mm thick layer?

Are you sure that it is only about a 35-40 mm thick layer?

Sorry, of course it is about 35-40 cm...