Bricks: Are differences in the thermal conductivity value of 0.03 noticeable?

I want to build a single-family house/ two-story air-water heat pump with filled 36.5 Poroton bricks. Since there is no longer any Kfw55 funding, I am generally wondering whether it is economically and energetically worthwhile to use bricks with insulation values of 0.07 W/mKp or if it is just nice on paper. Is the difference between 0.07 and 0.10 W/mKp really noticeably felt in the house or is it rather negligible? Do I have to heat significantly more if I choose the 0.1?

There is no more funding, but you still have to build KfW55 as a minimum requirement. That is the standard today.

Inside the house itself, you don’t notice any difference. You could also build with 0.30 stones.

You notice the difference in heating costs. Although in your example, user behavior probably makes a much bigger difference than the different stones.

Apart from that, I would personally only build so "poorly" for one reason. If I lack the money. Because you have to or should also think about the value of the house. And in this regard, KfW 55 is already outdated and "poor" today.

Apart from that, I would personally only build so "poorly" for one reason. If I lack the money. Because you have to or should also think about the value of the house. And KfW 55 is already outdated and "poor" for the future today.

If you build to the 55 standard, you are very well positioned in terms of price-performance. Even before 2020, the 40 standard was only conditionally economical. Yes. Energy will become more expensive. But construction costs and thus the additional costs for better standards as well. Economic efficiency ≠ energy efficiency

If you build 55 standard, you are very good in terms of price-performance.

You also can't build worse. But in fact, for many market participants, KfW 55 is already a no-buy argument today, and every provider builds KfW 40 (without all the NH stuff) for a small additional cost. Certifications etc. for subsidies are already elaborate, but that is not necessary.

There is no more funding, but you still have to build KfW55 as a minimum requirement. That is the standard today.

Is that really so? Who controls that? For example, do I have to do the blower door test?

You notice the difference in heating costs. Although in your example, user behavior probably makes a much bigger difference than the different bricks.

That interests me especially. If I have a photovoltaic system with storage and an air-water heat pump with underfloor heating. Is it relevant whether I use bricks with 0.1 or 0.07? For example, doesn't opening the windows immediately ruin everything?

Is that really the case? Who checks that? For example, do I have to do the blower door test?

You have to submit the necessary calculations with the building application that prove compliance with the Building Energy Act standard (= KfW 55). Without proof, no building permit.

That interests me especially. If I have a photovoltaic system with storage and an air-to-water heat pump with underfloor heating. Is it relevant whether I use bricks with 0.1 or 0.07? For example, doesn’t opening the windows immediately ruin everything?

Part one of your question: Yes, it is relevant. Whether there is a difference of €10 or €18.99 in heating costs that matters to you, you have to decide. The wall is only one part of the house. Then there are windows, front door, floor slab insulation, roof insulation, and a few small details.
Part two of the question: Opening the window once doesn’t do anything. But regular, continuous opening you will notice again. The ventilation system also makes an impact. How much all of this makes a difference you only know if you don’t open anything for a year and then make a comparison year.
Or if you always have 19.5 degrees or 22.5 degrees. Or if the winter was a bit colder or warmer.

All together, worse bricks, constantly open windows, having to sleep with windows open, etc. can add up, but no one gets poor because of it.

P.S. I expect heating costs this year for my apartment (in a two-family house, own heat pump) formally renovated to KfW 55 standard (actually better) of €350-380 (at 25 cents per kWh electricity) including hot water for 120 sqm. I don’t care at all if it sometimes reaches €400 because the kids were constantly going in and out in winter or for whatever reason a lot of windows/terrace doors were open. You will always have moderate costs in new buildings – 10% more or less doesn’t really matter in the end.