Same price: Kfw55 with Poroton monol. OR Kfw40 with Poroton WDVS?

That was done for me that way. You have to make sure that this row has a higher compressive strength and that the overlap dimensions are really adhered to everywhere for the second row, as the masons easily get confused by the shorter bricks (at least mine did)... Also, you should make sure that the sealing is done behind the insulation. This was not done for me because the first layer of insulation was used as lost formwork for the base plate. However, this is not entirely correct from a technical point of view. It was heavily criticized by most people at the shopping center around the corner. My expert assessed it as unpleasant but not further concerning, which is why I let it rest.

There is the option to lay the first row of stones with narrower stones, then insulation is placed in front of these and the floor slab. From the second row onwards, the normal stones are used.

I mainly keep my distance from this variant – as it seems to be unfortunately popular. One "side effect" of this is that a botched overlap dimension often "originates" here:

That was done that way for me. You have to make sure that this row has greater compressive strength and that the overlap dimensions are really kept everywhere to the second row, because the masons are easily confused by the shorter stones (at least mine were)...

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Furthermore, you should ensure that the sealing is carried out behind the insulation. That was not done in my case, because the first layer of insulation was used as lost formwork for the floor slab. Nevertheless, this is not entirely correct from a technical point of view. It was strongly criticized by most people at the shopping center around the corner. My expert classified it as unpleasant but not further problematic, which is why I let it rest.

One motive for this construction method might be to lead the insulation over the front edge of the floor slab or even the basement ceiling. The bed joint of the second stone layer is also a popular upper edge for the base plaster. Especially in combination with 6 cm / 2x 6 cm / 12 cm insulation thickness, a popular routine has probably developed here. This will not cause a house to collapse, but I still consider it a makeshift solution.

My expert had no objections to the basic design. In your opinion, what would then be the better solution for the base insulation in monoliths?

My expert had no objections to the basic construction.

I basically just state this as a principle, that I do not see such solutions as the be-all and end-all. The pure doctrine must die about eight and a half times in every real-world project. I always consider it desirable – with Pareto in mind, of course – to aim to reduce it to quarter to four. But this phenomenon will probably never be eradicated completely. Nevertheless, I see no reason to capitulate and simply switch to laissez-faire, which is why I at least mention it (with the addendum that a house will not collapse on its own because of this).

What, in your opinion, would be the better solution for base insulation in monoliths?

That is a tricky matter, to be determined differently in each individual case, regularly recommending a detail drawing and not answering as a general solution. The problem mainly occurs with floor slabs (slightly less so with basement ceilings, as far as the basement belongs to the thermal envelope). Concrete is thermally problematic, which is why the floor slab is insulated. Not to forget the front edge is indeed correct. The most elegant solution would probably be to work with Isokörben (R) – but also probably the most expensive. Here I would probably choose the most efficient insulation material to minimize its thickness and thus the weakening of the load-bearing stone thickness. For this purpose, I would exceptionally saw a whole row of stones if necessary. For a wall thickness in the normal layers of caliber 365, I would probably take stones of caliber 300 here – then the base with 6 cm insulation thickness in front of the first stone layer has a half-centimeter setback. Turning the stones here and thus working with a wall thickness of caliber 248 seems borderline to me even with higher density.

Wienerberger simply lets the perimeter insulation protrude underground in their principle drawing. The entire base slab as well as the first row of bricks are then also underground. Interesting, because the sealing naturally becomes even more important then. But after all, it is no longer part of their trade... I also don't know whether this works with the hydrological load case. Even with my groundwater level of 7m and medium infiltration capacity of the subsoil, the foundation recommendation was a base slab 20cm above ground level.

Wienerberger simply lets the perimeter insulation protrude underground in their principle drawing.

Then please send it to me (or the link to it).

The entire floor slab as well as the first row of bricks are then also underground. [...] Even with my groundwater level of 7m and average infiltration capacity of the subsoil, the foundation recommendation was a floor slab 20 cm above ground level.

I could still follow with a red-hot furrowed brow for floor slab top = raw floor top. But GOK above the top edge of the first row of bricks, oh dear / yikes, then a moat is definitely needed ;-) Even with the modern thick floor constructions usually 16 to 18 cm, a brick layer with normally 3DF height is still relatively higher = above - or do they only mean a capping brick layer?