ᐅ KfW 55 – Insulation Below the Concrete Slab

@Lumpi_LE
The thermal conductivity of the ground is generally assumed to be λ = 2 W/(mK) for the R-value.
A U-value of a slab-on-grade that was previously above 4.5 drops to below 2.9 solely due to the ground. (Comparison ratio e.g. 0.24 for exterior wall to 0.3 for slab)

The ground has two sources of energy input: from the sun and from the Earth’s core. Both also depend on the groundwater.
Typically, the ground reaches the average air temperature only from about 10 m (33 ft) depth.
All inputs result in radiation losses.

The requirements for thermal bridges are also significant,
which is why nowadays the supposedly optimized general values, often applied for new buildings, must be verified.

The reasons why edge insulation in residential construction is not additionally implemented but included via the R-value within the U-value are completely different.

@Mottenhausen
Perimeter insulation is required according to the minimum thermal protection part 2 of DIN 4108; the Energy Saving Ordinance or KfW programs incorporate this standard; KfW requires compliance with the Energy Saving Ordinance.

I am currently still waiting for the thermal insulation certification regarding whether to include insulation under the slab or not. At the moment, I only have U-values with 80 mm (3 inches) of Styrodur insulation beneath the slab at 0.22 W/m²·K and without insulation at 0.44 W/m²·K. Even without insulation, I am calculating a heating load of about 3.2 kW for a floor area of 102 m² (1100 square feet). However, I am not sure if the mechanical ventilation system has been correctly accounted for. I will now calculate using the slab with the 0.22 U-value.

nichts zu schwör schrieb:

Under-slab insulation is required by the minimum thermal insulation standard Part 2 of DIN 4108; the Energy Saving Ordinance and KfW subsidies incorporate this standard; KfW presupposes compliance with the Energy Saving Ordinance.

I was specifically referring to XPS insulation under the concrete slab, which is expected to become standard in the future.

Mottenhausen schrieb:

... and with the next energy-saving regulation, this topic will probably be settled, since today’s KfW55 or 40 will become the new energy-saving regulation standard, meaning that insulation below the slab will then be mandatory. The former KfW70 has also become the currently valid energy-saving regulation standard.

I mean, this doesn’t really resolve the debate—pro or contra slab insulation—but at least it is accepted as given.

We don’t have any perimeter insulation under the slab and could even insulate with WLG040 on the slab... -> primary energy demand 28 kWh/m² and thus easily meet KfW55 requirements.

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

@Mottenhausen

There is no standard, which also wouldn’t make sense because the requirements vary.

Grantlhaua schrieb:

We have no perimeter insulation under the slab and could even insulate on top of the slab with WLG040 (thermal conductivity 0.040 W/m·K)... -> primary energy demand 28 kWh/m² and easily meet KfW55 standards.

I think KfW40 will not become a standard because building costs would otherwise become unaffordable.

You cannot achieve KfW55 by primary energy demand alone. You can install as much technology as you want—controlled mechanical ventilation, photovoltaics, heat pumps, etc.—but if the insulation is insufficient and the average U-value (thermal transmittance in W/m²K) exceeds 0.259, no funding will be granted.

Both criteria must always be met comfortably! KfW55 requires values that are 30% lower than those specified in the 2016 Energy Saving Ordinance. What this actually means in terms of energy savings and cost savings remains to be seen.