Floor slab inside or outside the insulation

Hello everyone,

the topic of the floor slab came up recently.
A construction supplier said:
The insulation layer goes on the floor slab.
Why? Because then you don't have to "pointlessly" heat the floor slab all the time.

Another construction expert says:
Include the floor slab inside the insulation, then nothing can happen to the floor slab from the outside.

What do you think about that?

Hope my question is understandable : )

The first one. At least that is the case here.

Both are possible. The former is more common. The latter, in my opinion, has no disadvantages. I would probably follow how the entrepreneur always builds. Because he can do that.

We have both in the KFW40 house: 10cm underneath, 12cm above. Personally, I would prefer exterior insulation in the case of "either or." Of course, you then heat the floor slab as well. But I don't think that matters if it is insulated.

We have 12cm insulation under the floor slab and 6cm under the screed in our KfW40 house. However, this is in the basement. With insulation under the floor slab, you are definitely free of thermal bridges. However, it depends on what kind of floor slab you have. Whether it is a load-bearing floor slab or if it has strip foundations.

8cm screed
8cm fill
20cm base slab
30cm insulation
20cm gravel layer

certified passive house

load-bearing base slab