KFW 55 - Insulation under the floor slab

The thermal conductivity of the soil is assumed on average as λ= 2W/(mK) for the R-value.
A U-value of a slab-on-ground, which previously was above 4.5, thus falls to under 2.9 just through the soil. (Relation cf. 0.24 AW to 0.3 BP)

The ground has two energy inputs: that of the sun and that of the Earth's core. Both still depend on the groundwater.
The ground usually only reaches the average air temperature value from a depth of 10m.
All inputs are radiation losses.

The requirements for thermal bridges are also anything but low,
therefore, supposedly optimized flat rates, which are often applied in new construction, must also be proven nowadays.

The reasons why edge insulation is not additionally implemented in residential construction, but rather incorporated into the U-value via the R-value, are quite different.


Under-slab insulation is required by the minimum thermal protection Part 2 of DIN 4108, the Energy Saving Ordinance or KFW include this standard; the KfW assumes the Energy Saving Ordinance.

Currently still waiting for the thermal protection certificate regarding insulation under the floor slab yes or no. At the moment I only have U-values with 80 mm Styrodur under the BPL = 0.22 W/m2xK and without insulation = 0.44 W/m2xK even without insulation I come to an approximate heating load of just under 3.2 kW for 102m2 floor area. Although I do not know if the controlled residential ventilation has been properly included. Will calculate right away with the 0.22 floor slab.

Underside insulation is required by the minimum thermal protection Part 2 of DIN 4108, Energy Saving Ordinance or KfW include this standard; the KfW presupposes the Energy Saving Ordinance.

I was specifically referring to XPS under the floor slab, which will become standard in the future.

... and with the next Energy Saving Ordinance, the topic will probably be settled, since today’s KfW55 or 40 will become the new Energy Saving Ordinance standard, i.e. without under-slab insulation, it will no longer be possible. The former KfW70 has also become the currently valid Energy Saving Ordinance.

I mean, that does not improve the controversy - pro/con under-slab insulation, but you then just accept it as given.

we have no perimeter insulation under the floor slab and could even insulate on the floor slab with WLG040... -> primary energy demand 28 kwh/m² and thus easily KfW55

KfW40 will, I think, not become a standard, otherwise building will become unaffordable.

There is no standard, which also wouldn't make sense, as the requirements are different.

we do not have perimeter insulation under the slab and could even insulate the slab with WLG040... -> Primary energy demand 28 kwh/m² and therefore easily KFW55

KFW40, I think, will not become a standard, otherwise building would become unaffordable.

You cannot achieve KfW55 only with the primary energy value. No matter how much technology you incorporate—controlled residential ventilation, photovoltaics, air-water heat pumps, etc.—if the insulation is insufficient and exceeds the value (average U-value or W/m²K) of 0.259, there will be no funding.

Both values must always be undercut, and not just barely! KfW55 undercuts the values of an Energy Saving Ordinance 2016 house by 30%. What that actually means in terms of real energy savings and therefore money remains to be seen.