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

Created on: 27 Sep 2016 20:44

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

Alex856 Dec 2016 07:53

Paulus16 schrieb:
Is insulation beneath the slab useful? Is it mandatory for building a KFW55 house?

KFW55 standards naturally require higher insulation performance towards the ground. In this respect, it seems common and reasonable to me.

Also keep in mind that the insulation under the slab is the only insulation element of the house that you will never be able to access again. Additionally, it is not a major cost driver in the overall construction. Therefore, I would focus on other aspects instead.

Judyyy6 Dec 2016 18:08

If the insulation under the screed is omitted but a better insulation is installed beneath the foundation slab, wouldn’t that result in a larger thermal mass with the screed and the concrete floor? Of course, this would make the floor more sluggish in responding to temperature changes, but how often do you actually adjust it?

Regards
Judyyy

Uwe827 Dec 2016 08:47

And what good is thermal mass if you don’t adjust anything anyway?

Judyyy7 Dec 2016 10:51

#10

Reduced cycle times of the heat pump.

AOLNCM7 Dec 2016 11:01

#11

Judyyy schrieb:
If you skip insulation under the screed but instead use better insulation beneath the concrete slab, wouldn’t the screed and concrete floor together provide a greater thermal mass? Of course, this would increase the floor’s thermal inertia, but how often do you adjust that anyway?

Regards,
Judyyy

This is called a “Swedish slab” (in which case no screed is needed).
Try searching for "Swedish slab construction process."

Judyyy7 Dec 2016 11:09

#12

Fewer operating cycles of the heat pump.

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

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