ᐅ Concrete slab – basement insulation only on the interior for KfW70 standard

Hello, today a structural builder suggested insulating the floor slab of our basement only from the inside, meaning no XPS insulation under the slab. He said no one really knows how the material behaves after 20-30 years and that he would insulate only from the inside – the internal thermal envelope would also be easier to seal that way.

Is there any truth to this, or does no one build without external insulation nowadays?

Regards
Pit

We were given the same suggestion. Having insulation under the slab is simply more expensive, and insulating from the inside is sufficient.