24 cm Ytong + insulation or 36.5 cm Ytong

Now our construction company has offered us to build with 36.5 Ytong instead of the 24 Ytong + 12cm full thermal insulation.

Where does the original idea of 24 Ytong plus insulation actually come from? Who chose it and why?

I would tend to go for monolithic here (i.e. 35.5 Ytong) if that is already sufficient for KfW55. But don't just compare the result, actually look at the transmission heat losses as far as available. There should be calculations if KfW55 is to be achieved.

Regarding the old religious war, whether Ytong or not. My contractor said it like this: The north builds with Ytong, the south with Poroton. It is simply tradition. There are no real reasons for it.

Faith war - good keyword!

One knows it from nutritional "science" that the fashion of who is the devil changes. Today, Styrofoam is hyped, tomorrow Styrofoam is the "bad cholesterol." Then the houses stand there, freezing in their 175-weight summer dresses with torn-off Styrofoam, and the facade masons celebrate golden times. I'll start reshuffling my stock portfolio

Just used the U-value calculator again. Pure Ytong 365mm in PP2-0.35 results in U 0.235 W/m2K. Reference value method for KFW 55 is not possible with this, but still definitely feasible. The original wall construction with the same Ytong only 140mm thick plus EPS 035 in 120mm results in U 0.159 W/m2K and thus approaches KFW 40. As a result, the initially proposed construction will lead to a somewhat lower heating bill. The heating load is lower, possibly allowing a smaller heat generator to be used and thus saving money. The better wall construction also means that other components require less effort to fit into the KFW calculation, especially the windows, which could be cheaper. But it won't help you, because it is being built with GU (but this might explain why he wanted to build that way initially).

I also consider the monolithic design to be better, especially more durable.

Glaubenskrieg - gutes Stichwort !

That's it. For me, Ytong is the worst... poor sound insulation and thermal bridge issues, leading to significantly more frequent facade damage. In addition, unnecessarily thick walls with proper thermal insulation.