Single-family house with offset shed roof - KfW consideration

Hello everyone,

we simply handed our floor plan draft ( ) over to an energy consultant to have it calculated. I gave the following specifications: desired wall construction: aerated concrete 36.5 or 42.5, brine-water heat pump with surface collector and underfloor heating, photovoltaic approx. 15kwp without storage, controlled residential ventilation with heat recovery.

It turned out that a KfW40 house with aerated concrete is not feasible?! To me, that seems pretty "harsh." Can the calculation be correct? I mean: Are the state requirements really that extreme?

Here are a few key points: floor slab: U-value 0.127, walls: U-value 0.147 (with Ytong lambda 0.07 and 42.5 --> 0.16 not achieved), roof: U-value 0.132, windows: U-value 0.82 (we already have an offer with 0.83), door: U-value 0.82 (we have an offer with U-value 1.0; below 0.8 costs about 2-3k € more).

What are your opinions on these values? Thank you very much in advance.

Yes, the specifications are really "intense." Maybe you can still get something out of the individual verification of the thermal bridges. We achieve KfW55 monolithic with a deviation of 0.001 from the average reference U-value.

The outer brick should at least reach the required U-value, so you need to improve here. The door as well. The rest is done by the individual bridge verification. Then it will work out with KfW40 :-)

Ok, I almost feared it! I also received the values for kfw 55 and the minimum for the Energy Saving Ordinance. I think I'll build without kfw funding and stick roughly to kfw 55, then I won't need a thermal bridge certificate or a blower door test. And honestly, for the €5,000 funding, additional costs of about €3,000 are not proportional. The kfw interest rate was worse than the bank's...

Well, it depends. Over 10 years, it is only 1.70%. With the subsidy then at KfW55, effectively 1.08%. The benefit for us is the unlimited special repayment. Thus, if you regularly make special repayments, you can pay off the loan in 5-10 years and have a monthly burden lower by about €364.

No one here is saying that it could or should somehow be worthwhile. Everything below 55 is done out of conviction and because one can afford the additional costs. The government subsidy is only meant to be support or a small incentive, but not to cover all costs. However, one should not forego a Blower Door Test, as it can reveal leaks.

I believe I understood that you want KfW40 with single-layer aerated concrete. What is not clear to me is: what priority do these three key points have - that is, if you had to remove one of the three wishes, which one should be the first to be compromised?