Additional sealing of strip foundation with perimeter insulation

A dimpled membrane does not function as a plaster protection, but as "backfill protection" in the soil and ends where the plaster begins. Maybe there was a misunderstanding.

Yes, I expressed myself incorrectly here. Of course, it was about the "backfill protection"

What may be necessary is a sealing of the strip foundations (i.e. before installing insulation boards), if necessary according to the corresponding water exposure class (ground moisture, temporarily pressing water, etc.) and if they are not inherently resistant (i.e. waterproof concrete construction). Although, of course, this mainly concerns possible rising moisture strip foundation->slab, and you will certainly have a corresponding sealing on the slab anyway.

It could not be sealed before gluing the insulation boards, as the XPS boards were installed directly during the pouring of the foundations (like a kind of formwork, on both sides). The slab is, of course, protected by a foil. Inside the house, the Katjafolie will still be installed.

Then you don't need to think about it any further - seals on insulation boards do not provide long-term protection against water ingress, so you can just leave it. Make sure the perimeter insulation boards are installed without gaps, foam them if necessary (provided the gap is not too large), put on the dimpled membrane, and that's it.

How far should your floor slab "project" in the end? If necessary, you need to check in the area of floor-to-ceiling windows and doors that the sealing (this definitely directly on the floor slab, floor end profile & wall, before insulation) is executed sufficiently far downward.

I must also add at this point that I was mentally focused on the concrete slab, specifically the part of the slab that is later located in the ground and extends up to the surface level - that is, around the slab on the outside, not underneath. Up to that point, it must be sealed directly on the concrete/brick from the outside. As others have already said, sealing on the XPS boards is just a placebo. The strip foundation itself does not require any special sealing. On the inside of the slab - that is, the raw concrete - comes the barrier against rising moisture, as the forum colleagues have already mentioned.

Yes, I expressed myself incorrectly here. Of course, it was about the "filling protection."

It could not be sealed before bonding the insulation boards, as the XPS boards were installed directly during the pouring of the foundations (like a kind of formwork, on both sides). The slab is of course protected by foil. Inside the house, the Katjafolie is still being laid.

As you describe it, this corresponds to the typical structure of a slab. PE foil, XPS insulation, reinforcement, and then the concrete is poured. The formwork around it. For understanding: Were the strip foundations poured in one go with the slab?

Just looked at the structural plans. The concrete grade for the slab and foundations is C25/30. I assume this is standard, right?

Make sure that the perimeter insulation boards are installed without gaps, foam if necessary (as long as the gap is not too large), then the studded membrane on top and done.

Yes, we are checking carefully. One spot (at the multiple duct entry) is somewhat poorly insulated. The guys absolutely need to improve that.

you have to check in the area of floor-to-ceiling windows & doors that the sealing (this definitely directly on the slab, slab edge profile & wall, before insulation) is executed far enough down.

Floor-to-ceiling windows are sealed with Enkopur, the liquid plastic.