KFW 55 - Insulation under the floor slab

Good morning,

we are currently planning a single-family house with a double garage.

We intended to apply for KFW 55.
Acquaintances of ours have KFW 55 without additional insulation under the floor slab, and our construction company also advised us against it.

Our energy consultant believes that without such insulation we will not meet the requirements.

Does anyone know how we can achieve KFW 55 without insulation under the floor slab.

Thank you

Ultimately, the component must achieve a certain U-value. The construction is ultimately irrelevant. Whether you have 6cm underneath and 12cm on top, or vice versa, makes no difference from an energy perspective. The advantages and disadvantages may lie in the costs and the impact on the planned heights.

Hello,

we didn’t apply for KFW (because the conditions were poor last year) but would have had no problems achieving KFW55. We also don’t have insulation under the basement floor slab, probably for the same reasons why your construction company advised against insulation under the floor slab. Here (energy consultant, construction company, and I) none of us think it’s a good idea to put a house weighing a few hundred tons on styrofoam. This stuff has been used for 20 years now but no one knows how it behaves in 50 or 70 years. From an energy perspective, insulation under the floor slab only really pays off for a house without a basement (cold bridges at the transitions between floor slab & wall). For a house with a basement, this effect is very small since you are no longer in the frost zone. (at 3-5m depth you have year-round temperatures between 10-15°C)

Is the basement heated? Our energy consultant said you heat the ground rather than the other way around. Has your energy consultant also calculated without the perimeter insulation? I just dug out the calculation and we have 29.78 kWh/m²a.

Whether the insulation is on or under the board does not matter at all for kfw55. You got a professional...

What do you think could happen?

The insulation under the slab isn't that thick ... even if it gets crushed, the house won't fall over, right (-:

Insulation can also be partially placed under and partially on the floor slab (as in our house, but I no longer know why). In the end, it doesn't really matter how the required insulation value is achieved.