Double insulation below and above the floor slab?

Hello everyone,

we are in the final planning stage of our single-family house. Since it is (probably like for most) our first house, we are of course at best amateurs in this field.

Here are some key data about the house:
Type: timber frame construction without a basement
Standard: KfW 40plus
Base area: 106m²
Living area: 165m²

Regarding the insulation, I have the following question:
A 140mm thick perimeter insulation (WLG035) is laid under the floor slab (at the ends it is 100mm)
The construction company has now told me that the floor inside the house will also receive another 100mm insulation towards the floor slab, on top of which the underfloor heating will be installed. My question is whether it is actually necessary to insulate the floor again with this thickness against the floor slab. After all, the installation costs for the underfloor heating including the mentioned insulation and the subsequent screed amount to a considerable €19,000 for 155m² of heated area.

Thanks for your assessment.

Best regards
Torsten

Why kfw40?

Why kfw40?

It's no problem at all with a timber frame. Whether it has to be 40+, meaning with photovoltaics and storage – well. We're currently dealing with that topic in many threads, I would just be repeating myself here.

I wanted to hear it from the thread creator because I think kfw is really for individualists. Especially since he asks so puzzled, it sounds more like he wants to save money.

And the additional costs compared to 55 or 70 never get saved again.

I wanted to hear it from the thread creator

And the thread creator wanted to know something about insulation in the floor structure and not about the purpose of the KfW 40 standard. We just don't always get what we want.

And the additional costs compared to 55 or 70 are never saved again.

My post was a hint that even wood studs in "standard version" mostly achieve insulation values that meet the KFW 40 standard. Ergo no additional costs.

Well, if he is already questioning the purpose of the double insulation, I still assume that he does not want it.

PS even though our KfW 70 timber frame house would easily reach KfW 40 on paper, other cost factors still come into play.

Example blowdoor test with a small n50, to achieve that you do need a few extra things.

I believe that with KfW 40, solar is even mandatory, etc.

But maybe the thread creator will say something about it.