ᐅ Perimeter insulation beneath the foundation slab and additional XPS insulation under the screed?

Hello,

Our house construction is getting closer, but today I noticed something that I don’t quite understand. Maybe someone can clarify it for me.

Under the concrete slab (according to the structural engineer 180mm (7 inches), but probably going to be 220mm (9 inches)) there is supposed to be an 80mm (3 inches) perimeter insulation (125m² (1,345 ft²)) installed. The floor build-up according to the architect looks like this:
...
5mm (0.2 inches) carrier board
120mm (5 inches) wood fiberboard insulation, thermal conductivity class 035
20mm (0.8 inches) wood chipboard insulation, thermal conductivity class 035
10mm (0.4 inches) waterproof membrane

Now my question is:
Isn’t this a bit redundant? Wouldn’t just the 120mm (5 inches) XPS insulation board above the slab be sufficient?

It will be a standard house built according to the 2014 energy saving ordinance.

Best regards,
Rico

And where is the insulation layer between the strip footing and the foundation slab? As I understand it, an 80mm (3 inches) insulation layer is installed beneath the foundation slab, right? What is its purpose? Not much cold comes up from below into the foundation slab, but cold is drawn into the slab through the uninsulated strip footing and probably the exposed sides of the strip footing/foundation slab.

The floor slab is certainly insulated on the sides as well.
And no – this is not overkill; keep in mind that you have heated pipes in the screed, and you want to direct as close to 100% of the heat upwards rather than into the ground.
Whether 14 cm (5.5 inches) is necessary can certainly be questioned – but a good 10 cm (4 inches) is definitely reasonable (and 14 cm (5.5 inches) won’t hurt either – it probably costs more to question this than to just do it).

Illo77 schrieb:
And where is the insulation layer between the strip foundation and the foundation slab?

He didn’t mention that at all. Whether or not there is a strip foundation underneath is purely a structural issue. In a single-family house, however, the insulation is placed below the slab and also over the strip foundation, with the slab then built on top.

This way, less cold penetrates into the masonry itself. As BeHaElJa already pointed out, the insulation beneath the screed is meant to ensure that the underfloor heating warms the room—not your concrete slab. Otherwise, it would become even slower to respond.

We need to construct the following minimum assembly (KFW55 Energy Saving Ordinance 2016):

23mm (0.9 inches) floor covering (total assembly 16cm (6.3 inches))
60mm (2.4 inches) screed
2mm (0.08 inches) PE foil
20mm (0.8 inches) TSD board EPS, thermal conductivity 0.045 W/(m·K)
50mm (2.0 inches) WD board EPS, thermal conductivity 0.040 W/(m·K)
5mm (0.2 inches) waterproof membrane
250mm (9.8 inches) waterproof concrete
80mm (3.1 inches) pressure-resistant perimeter insulation, thermal conductivity 0.040 W/(m·K) (10cm (3.9 inches) perimeter insulation thermal conductivity 0.037 W/(m·K) was installed)

Hello,
I would like to revisit this topic as we are currently in the planning phase and have reviewed a few building specifications.
Two specifications mention a 140mm (5.5 inches) perimeter edge insulation between the slab and the base cladding. The third refers to 60mm (2.4 inches) perimeter insulation below the slab and perimeter edge insulation without specifying the thickness.

All specifications include 110-120mm (4.3-4.7 inches) of two-layer insulation under the screed.

Is insulation beneath the slab advisable? Is it mandatory for a KfW 55 energy-efficient house?

P.S. The house will have no basement.