T9 30cm no standard?!

Hello,
what arguments do you think speak for/against building with a T9, 30cm, Uw=0.28? (T9 36.5cm Uw=0.23 || T10 36.5cm Uw=0.25)
That would be sufficient to comply with the Energy Saving Ordinance 2014.

I hear from all sides "That is not standard." Of course, a better (more expensive) brick is always better ... but then I would have to take the best brick on the market. Somewhere, "economic efficiency" has to stop.

Well, it's not like a stone only protects against the cold. No. It also protects against heat in the room during summer, and thicker is definitely better there. And I actually don't know anyone who has installed only a 30 cm stone without insulation. We ourselves have installed the T9 with 36.5 cm and even achieve Kfw 70 with an air-to-water heat pump. And then also think about the resale value: nowadays, people place a lot of value on a house with good insulation values. And yes, it's true: you can also take the most expensive option, but the question is what is still worthwhile. In my opinion, at least Kfw 70 is definitely sensible.