Foam glass gravel yes/no?

You don't have a low temperature there because you have insulation in place. You are making a logical error because you are considering the layers isolated from each other.

But if I go from inside to outside, then the temperature within the insulation drops from room temperature to the floor slab temperature (not exactly, but at least as a first approximation). And the floor slab has the ground temperature underneath, which acts on it unhindered because there is no insulation in between. The thermal resistance (i.e., the first possible temperature change) only occurs in the insulation layer.

I sketched this once in the U-value calculator (I hope inserting the graphic is allowed):

And where is the condensate there? By the way, it is always frost-free for me - at least with basement construction.

: The insulation is sealed against moisture at the top with a vapor barrier and at the bottom with "Knauf Katja Sprint" (sealing membrane). The amount of water that was present before (during installation) can be neglected; it should be close to zero and really does not matter.

No, he means that condensation forms due to the temperature differences - but it cannot, because no warm air can settle on the surface of the floor slab.

That is his misconception...

The condensate are the drops in the right image (by the way, I didn’t put those in, the U-value calculator did)

I don’t have a conceptual error in that sense, I am just questioning. The answer to my initial question about how the floor structure should look is that a vaporbarrier above the insulation is necessary to keep the indoor humidity away from the surface of the floor slab. Only then can no condensate actually form. This must therefore be executed very meticulously.

Considering the entire effort, in the end I find perimeter insulation under the floor slab simpler and less prone to errors.