ᐅ Double insulation below and above the concrete slab?

Hello everyone,

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

Here are some key details about the house:
Type: Timber frame construction, no basement
Standard: KfW 40plus
Footprint: 106m² (1141 sq ft)
Living area: 165m² (1776 sq ft)

Regarding the insulation, I have the following question:
Under the ground slab, a 140mm (5.5 inches) thick perimeter insulation (thermal conductivity WLG035) is being installed (the ends have 100mm (4 inches)).
The construction company has now told me that the floor inside the house will also get an additional 100mm (4 inches) insulation toward the ground slab, on top of which the underfloor heating will be installed. My question is whether it is really necessary to insulate the floor again with this thickness against the ground slab. After all, the installation costs for the underfloor heating including the mentioned insulation and the subsequent screed add up to a considerable €19,000 for 155m² (1668 sq ft) of heated area.

Thanks for your assessment.

Best regards,
Torsten

Why KfW40?

tomtom79 schrieb:
Why KfW40?

With a wood frame, that's no problem.
Whether it needs to be 40+—meaning with photovoltaic panels and battery storage—well, that's a topic discussed in many threads right now, so I would just be repeating myself here.

I wanted to hear it from the thread starter because I think KfW is really aimed at individualists. Especially since he asks so surprisingly, it sounds more like he wants to save money.

And the additional costs compared to 55 or 70 standards are never fully recovered.

tomtom79 schrieb:
I wanted to hear it from the original poster

And the original poster wanted information about insulation in the floor structure, not to discuss the purpose of the KfW 40 standard. Sometimes we just don’t get what we want.

tomtom79 schrieb:
And the additional costs compared to 55 or 70 are never really saved.

My post was meant to point out that even timber frame walls in a "standard version" usually achieve insulation values that meet the KfW 40 standard. Therefore, no additional costs.

Well, if he’s already questioning the purpose of double insulation, I still assume that he doesn’t want it.

PS: Even though our KfW 70 timber frame house would easily reach KfW 40 standards on paper, there are other cost factors involved.

For example, a blower door test with an n50 value that low requires a few additional measures.

I believe solar panels are even mandatory for KfW 40 certification, among other things.

But maybe the original poster will provide more details.