ᐅ KfW55 calculation with or without including the standard thermal bridge allowance?

Hello everyone,

we are building a detached house with a gable roof. The house will have brick cladding. Our energy consultant provided two options for a KfW55 house.

Option 1) Thermal bridge allowance 0.05, exterior wall insulation 18 cm (7 inches) with thermal conductivity 0.032 W/(m·K), roof insulation 28 cm (11 inches) with thermal conductivity 0.032 W/(m·K)

Option 2) Thermal bridge allowance calculated individually, exterior wall insulation 18 cm (7 inches) with thermal conductivity 0.035 W/(m·K), roof insulation 24 cm (9 inches) with thermal conductivity 0.035 W/(m·K)

Is it because of the method of calculating the thermal bridge allowance that in option 2 a lower insulation thickness and a higher thermal conductivity can be used, or is there a typo?

Thanks and best regards
fabianbre

fabianbre schrieb:

Is it due to the calculation of the thermal bridge allowance?

Allowances are generally "inaccurate." If everything is calculated individually (and then paid for accordingly), you can insulate differently (less) for the same result on paper. But in reality, you end up with a worse house.

nordanney schrieb:

You effectively end up with a worse house.

... which of course also has to be paid for at a higher price. However, I assume that even with 18 cm (7 inches) of thermal insulation with a WLG 035 rating, there is still some margin left to meet KfW 55 standards.

That means our energy consultant was not mistaken (option 2 with less and lower-quality insulation than option 1, both KfW 55), and you can skip the calculations for option 1 but will have to pay more for the insulation?
In return, option 1 provides better insulation.

Thank you.

That's true. However, the calculation process might actually cost more than the flat-rate surcharge, so be sure to calculate carefully, young Padawan. 😀

A third option could be to aim for only 16cm (6.3 inches) of insulation with a thermal conductivity of 0.032 W/(m·K), including thermal bridge calculation. When it comes to cost, this might be the best option, depending on how expensive the calculation is.