ᐅ Concrete slab inside or outside the insulation layer

Hello everyone,

Recently, the topic of the concrete slab came up.
One builder said:
The insulation layer goes on top of the concrete slab.
Why? Because then you don't have to effectively heat the slab unnecessarily.

Another builder argued:
Include the concrete slab within the insulation, so the slab is protected from the outside as well.

What are your thoughts on this?

I hope my question is clear 🙂

The first one. At least, that’s how it is here.

Both options are possible. The first one is more common.
In my opinion, the second one does not have any disadvantages.
I would probably go with whichever method the contractor usually uses, as that is what they are capable of.

In our KFW40 house, we have both 10cm (4 inches) below and 12cm (5 inches) above.
Personally, if I had to choose one or the other, I would prefer external insulation. Of course, that means heating the foundation slab as well. But I don’t think that matters if it is insulated.

For our KfW40 house, we have 12cm (5 inches) of insulation under the foundation slab and 6cm (2.5 inches) beneath the screed. This is located in the basement. Insulating under the foundation slab definitely helps to avoid thermal bridges. However, it depends on the type of foundation slab you have—whether it is a load-bearing slab or if it uses strip footings.

8cm (3 inches) screed
8cm (3 inches) loose fill
20cm (8 inches) concrete slab
30cm (12 inches) insulation
20cm (8 inches) gravel layer

certified passive house

load-bearing concrete slab