Perimeter insulation under the floor slab and still XPS under the screed?

Is insulation under the slab sensible? Is it mandatory for KFW55 house construction?

KFW55 naturally places higher demands on insulation towards the ground. In this respect, it seems usual and purposeful to me. Also consider that the insulation under the slab is the only insulation on the house that you will never be able to access again. Moreover, it is not exactly the cost driver of the house. I would therefore focus on other things.

If you now omit the insulation under the screed but instead have better insulation beneath the slab, wouldn't you then perhaps have a larger storage capacity with the screed and the concrete floor? Of course, it becomes even more sluggish, but how often do you actually adjust anything there?

Regards
Judyyy

And what good is the storage capacity if you don’t adjust anything anyway?

Less cycle times of the heat pump.

If you now skip the insulation under the screed, but instead have better insulation below the slab, wouldn’t you then perhaps have a greater heat storage capacity with the screed and the concrete floor? Sure, it becomes even more sluggish as a result, but how often do you actually adjust anything there?

Regards
Judyyy

It's called a Swedish slab (then no screed is needed anymore).
Google "Swedish slab construction process"

Fewer cycle times of the heat pump.