ᐅ KfW 55 – Insulation Below the Concrete Slab

Zaba12 schrieb:

If the insulation is insufficient and exceeds the value (average U-value or W/m²K) of 0.259, there is no funding.

Are you referring to the U-value of the foundation slab?

Here, we have 0.218 W/(m²K) as well.

@Hausbauenxx

Hello,

how did you finally insulate your slab on grade?

I just came back from my energy consultant, and he told me that due to the extremely high thermal bridge penalties of a floor slab insulated from above, I won’t be able to achieve KfW55. U-values are not a problem at all, but unfortunately, the thermal bridge penalties are much too high with insulation over the floor slab. Is that possible? Or how do you manage to meet KfW55 with insulation above the floor slab?

Well.. My insulation above the slab-on-grade narrowly meets KfW 40 standards. It really depends on the details and the rest of the house.

Lumpi_LE schrieb:

Well.. My insulation above the ground slab just meets KfW 40. It really depends on the details and the rest of the house.

Could it be that you have different construction methods?

My data

Heated volume 1055m³ (37,270 ft³)
Usable area 330m³ (11,648 ft³) fully basement
Living area 185m³ (6,535 ft³)

U-values:
Roof 0.15
Exterior wall 0.20 (42.5cm (17 inches) T9)
Basement wall 0.24
Floor slab 0.25
Thermal bridge allowance ???

Brine-water heat pump (horizontal ground loop) with controlled mechanical ventilation with heat recovery.

As mentioned, it seems that the insulation of the floor slab has a significant impact on the thermal bridges.
The energy consultant says that the thermal bridge allowances are especially important for the floor slab, to the extent that better external insulation does not help much.
Are thermal bridges this relevant for KfW55 standard?