42.5 cm aerated concrete and 400 cm wide windows and venetian blinds

Regardless of the rest, but that is not necessarily correct – the U-value of a wall with insulation changes almost linearly with the corresponding thickness of the insulation layer.

As I wrote, I am not a professional in this, but I remember our various calculation models that we carried out with our engineering office. Since we had access to insulation in abundance, we could have practically increased this arbitrarily. In this context, I remember the statement from there and also the calculation models that doubling the amount of insulation would not linearly lead to a doubled insulating effect, otherwise we would have done it. As I said, I cannot qualitatively substantiate this, only quote it, but I do not want to convince anyone with it; for me, however, it sounded plausible and I stuck to it.

The OP is now only a guest and deactivated.

Certain things about interpersonal dealings cannot be bought; you either have them or you don’t. Unfortunately, the number of participants who do not want to hear any other opinion, but only expect applause, is increasing. I think the OP was rather looking here for stronger arguments against his older gentleman to better justify his already made decision; that is why the other options were also presented in a rather negative context. I would be happy if people always brought their entire project here instead of playing childish hide-and-seek. Apparently not everyone is aware that they are allowed to build any stuff, even if the forum itself regards it as nonsense. Why should I not want to hear criticism that might point out mistakes to me before such a big life project? For that, I have absolutely no understanding, but I do actually experience it in the closer environment in other areas as well; asking seems to be the new sign of weakness nowadays, people just know it more often (not universally!). It rather reassures me if I can ask someone who might know. As the nice saying goes: "You are grown-up when you do something even though your father (mother, other adults, experts) told you not to."