Roof insulation from KfW 55 to KfW 40 on the floor of the attic

If you go to, for example, the Schlagmann page and select the T8 49er brick there -> 0.16 W/(m2K) U-value (approximate price 8,- EUR per brick). The newer T7 49er brick has a 0.14 U-value (price about 8.5 EUR per brick). Is the 50 cents per brick worth the 0.02 U-value improvement?

Yes, I have to admit I went over it too quickly and mixed up Lambda and U-value. Nonetheless, I would consider whether it's worth going to 49 cm. We have a 36.5 cm wall and reach 0.18 filled with mineral wool.
And don't forget that you lose a lot of square meters with the thicker wall.
Forget the prices. You don’t know where your builder sources from and what he prefers to build with. That will affect the price. And if the wholesaler is currently "pimping" a certain brick, it will be cheaper. You can currently see that here in our area. Of those who build with Poroton, 90% build with Bellenberg MZ70.

I can’t put numbers on it for you. And it certainly depends on the individual circumstances. But I would make it depend on the carpenter’s offer whether I insulate myself or not. Looking at our case, that was a relatively small item in proportion. (We insulate the roof between and above the rafters). If you only insulate above the floor ceiling now, it’s even less effort and accordingly the carpenter should not charge astronomical prices.

Yes, that is of course true again. But when you then submit the request for quotes to the construction companies with the input plan, you do indicate that you want to install a xx-wide brick insulated with air/perlite/wool, right?

As I said, I don’t want to aim for KfW 40 because then I could say "yeah, my conscience is now clear, after all, I have a KfW 40 house," but rather because of the higher repayment subsidy from the KfW (specifically 6,000 EUR more from 55 to 40). And it is also clear that with more insulation (lower KfW value) you will save more heating costs in the coming decades (you never know how prices will increase). But as I said, I wanted to know if, with the things I presented, you are more likely approaching KfW 40 or if it is just around KfW 55.

Yep, that’s the principle of the tender, so you should discuss with your architect/energy consultant beforehand where the journey should go. But it’s no problem to include alternative positions. So you specify one brick and have the alternative price per sqm for two other variants offered. Then you can compare.

So, you only know exactly when you calculate it. KFW 40 with a cellar is very demanding and you’ll probably have to invest additional money in its insulation. Whether it’s worth it financially is questionable, I rather don’t think so.
The tendency of your wall construction is clearly towards KFW 40. So I wouldn’t put half a meter of wall if I didn’t want to go there.
As I said, think about it carefully, depending on your intention: economic, ecological, sustainable, self-sufficiency goal, etc., there are many motivations.
And you really lose sqm. Depending on the case, you’ve lost 5 sqm of floor area straight away. If you want to regain that through larger external dimensions, it costs more again.
Better consider investing more in cellar or roof insulation.

But that’s just my opinion.

You'd better consider investing more in basement or roof insulation.

Don't you think that 16 cm of perimeter insulation for the basement exterior walls is already quite a lot?

Why do you actually advise against the 49er walls? Purely for aesthetic reasons?

I have often read from people here who also have > 42 cm walls that they are glad about their thick wall when they saw how many pipes / cables / slots came into the exterior walls.

Personally, I find thick exterior walls more visually appealing than thin exterior walls (plus, with wider exterior walls you usually have deeper window sills). This also applies aesthetically to the roof overhang (rather a longer overhang than flush with the exterior wall).

how many pipes / lines / slots went into the exterior walls.

I know it this way, that the lines are laid on the floors and only the last few centimeters are channeled into the wall. So that can't be a problem.

I personally find thick exterior walls visually more appealing than thin exterior walls

Even if that then more likely results in smaller windows resembling loopholes? When I read about "thick" walls, I always think of the effect of small windows in castles.

Don’t you think that 16 cm perimeter insulation for the basement exterior walls is already quite a lot?

I didn’t read it that closely, but that’s good if you have the corresponding WLG.
Also under the floor slab?

Why do you actually advise against the 49er walls? Purely for aesthetic reasons?

I’ve read here several times from people who also have > 42cm walls that they are glad about their large wall thickness when they saw how many pipes / lines / slots went into the exterior walls.

Personally, I find thick exterior walls visually more appealing than thin exterior walls (plus with wider exterior walls you usually have deeper window sills). The same applies aesthetically to the roof overhang (rather longer overhang than flush with the exterior wall).

- Because I personally don’t find it attractive. 42 cm is already solid, but 49 cm is another 7 cm more.
- Because it costs floor space. With 12x12 you have 48 m of exterior wall. Times 0.125 m more wall thickness equals 6 sqm of floor area that you lose.
- Because I doubt the cost/benefit ratio. I don’t believe you save much if you go one or two steps smaller and instead get a better insulation value if that’s so important to you.
- Because, as already wrote, small window dimensions look like loopholes in my opinion. If you’re renovating an old farmhouse, that has charm. In a new building, I personally don’t like that. Pipes on the floor / in the ceiling and/or a pipe shaft if it goes across floors. And the few remaining outlets on exterior walls… I wouldn’t do it just for those.