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

Hello,

the house construction is getting closer for us, but today something caught my attention that I don't quite understand, maybe someone can clarify it for me.

Under the floor slab (according to the structural analysis 180mm, but probably 220mm) there should be an 80mm perimeter insulation. (125m²)
According to the architect, the floor construction looks like this.
...
5mm carrier board
120mm WD board WLG 035
20mm TSD board WLG 035
10mm welded membrane

Now my question is.
Isn't all that redundant?
Wouldn't the 120mm XPS insulation board, etc., above the floor slab be sufficient?

It will be a normal house according to the Energy Saving Ordinance 2014.

best regards
Rico

And where is the insulation layer between the strip foundation and the foundation slab? As I understand it, an 80mm base slab insulation is installed below the foundation slab, right? What is that supposed to achieve? Not much cold comes from below into the foundation slab, but instead cold enters the foundation slab through the uninsulated strip foundation and probably the laterally exposed strip foundation/foundation slab.

Certainly, the floor slab is also insulated on the sides. And no – that is not redundant; remember that you have warm pipes in the screed and you want to transport as close to 100% of the heat upwards and not into the ground. Whether it really needs to be 14 cm can certainly be questioned – but a good 10 cm is definitely reasonable (14 cm won’t do any harm either – it probably costs more to question it than to just do it).

And where is the insulation layer between the strip foundation and the foundation slab?

He didn't say anything about that at all, did he? Whether there is a strip foundation underneath or not is just a structural engineering question. For a single-family house, the insulation under the slab is also laid over the strip foundation and then the slab is placed on it.

This way, less cold gets into the masonry itself. The insulation under the screed, as BeHaElJa already said, is there so that the underfloor heating doesn’t heat your slab but the room. Otherwise, it gets even more sluggish .

We have to build the following minimum structure (KFW55 Energy Saving Ordinance 2016):

23mm floor covering (= total construction height 16cm)
60mm screed
2mm PE foil
20mm TSD board EPS WLG 045
50mm WD board EPS WLG 040
5mm waterproofing membrane
250mm waterproof concrete
80mm perimeter pressure-resistant WLG 040 (10cm perimeter insulation WLG 037 was installed)

Hello, I would like to bring up the topic for discussion again, as we are also in the planning phase and have looked at a few construction descriptions. Two construction specifications mention 140mm edge insulation between the base slab and the facade cladding. The third one refers to perimeter insulation 60mm under the base slab and edge insulation without specifying the mm. All descriptions include 110-120 mm of 2-layer insulation under the screed. Is insulation under the base slab sensible? Is it mandatory for building a KFW55 house? P.S. The house will not have a basement.