Insulation of the area in contact with the ground

Hello,

We have now noticed that styrofoam was used in the earth-contact area (external insulation of the base slab?).
At least, it looks like that. Is this correct?
It is the area below the base plaster ...

You have to ask the person who installed it. There are also EPS products approved for perimeter insulation.

Now we noticed that in the earth-contacting area (external insulation of the base slab?) Styrofoam was used.

For insulation in the ground, XPS is mostly used - this is a very densely foamed polystyrene material with closed cells and very high compressive strength. Those who are not familiar with the material differences might describe it as very dense Styrofoam. Insulating the concrete in the ground is energetically very sensible. We had about 160 sqm of this material installed for the wall insulation of the foundations under the base slab, using the product named Jakodur in an 80mm thickness.

You have to ask the one who installed it. There are also EPS with approval as perimeter insulation.

You are right, that looks like EPS (granular structure) and not XPS. It has a lower compressive strength than XPS. It is also approved for perimeter insulation at shallower depths. It looks fine to me.