ᐅ Floor Construction for a New Single-Family House KfW55 Standard

Hello everyone,

this is my first post in the forum. I hope you can help me a bit.
I have planned a prefabricated house, and with the recent changes in the energy saving regulations and since I will be installing an exhaust air heat pump, my insulation requirements have decreased.
To save some money, I wanted to skip the 16cm (6 inches) perimeter insulation under the concrete foundation.
Now, a new energy saving regulation calculation was done. The floor insulation consists of 20cm (8 inches) C20/25 concrete and 8.5cm (3.3 inches) WLG025 insulation under the screed. This results in a U-value of 0.27 W/m²K, which seems quite high for a KfW55 house, doesn’t it?

I’m not very familiar with this, which is also why I handed everything over, but all other values (roof, exterior walls) are around 0.14 W/m²K.

Do you see a simple way to improve the floor insulation, and above all, does it even make sense, or is this value sufficient?
The floors on both ground and upper levels will be fully equipped with underfloor heating.

I don’t have the exact floor structure on hand right now, but I can provide it if needed.

Many thanks and best regards from the sandwich.

You are cutting costs on insulation and using an exhaust air heat pump. Actually, it should be the other way around: improve insulation to make the heat pump effective. Save money upfront and then pay more for heating???

I was told that 16cm (6 inches) of perimeter insulation doesn’t make sense energetically because only 8cm (3 inches) of it is actually effective and the rest just adds extra cost.

However, after seeing the new calculation with 8.5cm (3.3 inches) of WLG025 insulation, I’m wondering if that really makes sense since the U-value is already quite high. But as I said, I’m not an expert. Is it really too high?

Would perimeter insulation be the better option, or should the WLG025 simply be made thicker? Are there any other options?

Thank you very much for your help.

What does your architect or energy consultant say? They know best what is suitable or not for your house. After all, YOU have removed the planned perimeter insulation, which the professionals recommended, for cost reasons.

As I said, they said that 160mm (6.3 inches) of perimeter insulation would not be necessary. However, when I input this into a calculator, I get a value of 0.201 W/m²K. I do not have a professional energy consultant. So, on advice, I omitted the perimeter insulation.

I just asked quickly, and they said that I’m calculating it too simply because there is an additional coefficient (Fx factor) of 0.66 for the foundation slab.
“If you multiply the U-value of 0.268 by 0.66, the value is 0.177.”
But even with perimeter insulation, I would still have this coefficient, right?