ᐅ KfW 55 – Insulation Below the Concrete Slab

There is no technical reason why XPS should not be installed under a concrete slab. On the contrary, anyone can read the technical datasheet. It’s purely a matter of mindset.

Against the ground, for example in a basement, about 20% more energy is lost compared to above-ground wall surfaces. This makes sense, since there is rarely any sunlight below ground. The idea of 15°C (59°F) at 3 meters (10 feet) depth is, to put it mildly, a myth. That might be reached at the annual maximum, but the average is closer to half that temperature. Heating a basement is disproportionately expensive. It is better to add 2cm (1 inch) more insulation there than less.

Whether the insulation layer is placed below or above the concrete slab is by no means irrelevant, as thermal bridging can be more problematic with internal insulation. Additionally, the concrete slab cannot be used as a thermal mass in this case.

The KfW bank requires compliance with the current energy saving regulations, meaning the minimum insulation values specified in the thermal protection certificate must be met. Energy consultants must also adhere to these standards, even if they are not responsible for issuing the thermal protection certificate. The minimum thermal protection requirements are specified in Part 2 of DIN 4108.

Yes, it avoids a thermal bridge. Is it practically relevant? Well, not really.

Insulating beneath the slab (or completely around the strip foundation) eliminates the capillary break.

For slab-on-grade construction, the effective thickness of the slab is always the focus. Thermal bridging according to DIN EN ISO 13370. Therefore, the requirements for the U-value of the component in contact with the ground are the lowest, since the thermal conductivity of the soil must be taken into account. Most practical issues do not arise from the surfaces but from the thermal bridges.

The requirements for thermal bridges are low.
If the U-value referred to the conductivity of the floor, the requirements would probably be the highest. The requirements are low because the floor is simply warmer than the air and there are virtually no radiative losses.

... and with the next energy saving regulation, the issue will probably be settled, as today’s KfW55 or 40 standard will become the new norm in the energy saving regulation, meaning that without insulation under the floor slab it won’t be sufficient anymore. The former KfW70 standard has also become part of the currently valid energy saving regulation.

I mean, this doesn’t resolve the controversy—pro or con insulation under the floor slab—but it is then accepted as a given.