KFW 55 - Insulation under the floor slab

It is not technically justifiable why XPS should not be installed under a floor slab. On the contrary, anyone can read the technical datasheet. It is purely a matter of mindset.

Approximately 20% more energy is lost to the ground, e.g. in the basement, than to above-ground wall surfaces. That is also logical, as there is rarely sun at the bottom. And 15 degrees at 3m depth is, to put it mildly, a myth. Perhaps at the annual maximum. The average is rather about half of that. Heating in the basement is disproportionately expensive. It is better to have 2cm more insulation there than less.

Whether an insulation layer is located below or above the floor slab is by no means irrelevant, as the thermal bridge may be less favorable with internal insulation. Furthermore, the floor slab cannot be activated as a heat reservoir.

The KfW assumes the currently valid Energy Saving Ordinance, so the minimum insulation values of the thermal protection certificate must be complied with.
An energy consultant must also adhere to this, even if they are not the one providing the thermal protection certificate.
Minimum thermal protection is regulated by Part 2 of DIN 4108.

yes, one avoids a thermal bridge. Practically relevant? Well.

Insulation under the slab (or completely around the strip foundation) saves the corner area.

For the base slab against the ground, it is always about the effective thickness of the base slab. Thermal bridge according to DIN EN ISO 13370. Therefore, the requirements for the U-value of the component against the ground are the lowest, as the thermal conductivity of the ground must be taken into account. The majority of practical problems are not in the surfaces but in the thermal bridges

The requirements for thermal bridges are low. If the U-value concerned the conductivity of the floor, the requirements would have to be the highest. The requirements are low because the floor is simply warmer than the air and virtually no radiation losses exist.

... and with the next Energy Saving Ordinance the issue will probably be settled, since today's KfW55 or 40 will become the new Energy Saving Ordinance standard, meaning that without under-floor insulation you will no longer get by. The former KfW70 has also become the currently valid Energy Saving Ordinance.

I mean, that doesn't improve the controversy - pro/con under-floor insulation, but then you accept it as a given.