Is a vapor barrier necessary under insulation on a floor slab?

That is also the reason why I said you should rather heat there as well.

Of course, then the insulation in the ceiling becomes unnecessary again.
For me, the gable wall is still unknown. Do you want to insulate it soon or sometime later? In the second case, you have a greater temperature gradient and need to be more careful.

I already wrote that in my first reply. In the long term it is not completely unnecessary, since it is an expansion reserve and will need sound insulation at the latest then. But yes, it should have been planned better somehow.

Important note about the wall! Once it is undamped, you should definitely use a vapor barrier and execute it super airtight.

I already wrote that in my first reply post. In the long term, it is not completely unnecessary since it is an extension reserve and will need sound insulation at the latest then. But yes, it should have been planned better somehow.

Important note about the wall! Once it is uninsulated, you should definitely use a vapor barrier and make it super airtight.

Whereas then the insulation in the ceiling is of course unnecessary again.
For me, the gable wall is still unknown. Do you want to insulate it soon or sometime later? In the second case, you have more temperature gradient and have to be more careful.

Honestly? More like sometime later. Right now it’s really about getting everything inside properly, reasonably, and cleanly finished and maybe being able to move in in the fall. The baby is coming in summer, who will have priority at first, and then the basement, garage, and garden still need freshening up. And then I have to see how everything fits with the budget (at the moment it looks very good).

So the original idea was that I don’t want to heat the attic. In other words: the heat I add to the living floor should stay there as much as possible. Hence the insulation. I didn’t put any emphasis on sound at first because I probably can’t achieve it well with mineral wool (I insulated / built up the wooden beam ceiling UG/OG with heavy fill, wood fiber, and dry screed boards).

That would have all been resolved if the attic were ever to become living space. Since that is not yet certain, but rather planned as a reserve (e.g., for a third child or whatever), we found the solution with the insulation to be good.

So I take away: ventilation in the attic is important, preferably with a ventilation system, and since the attic will cool down relatively much despite insulation above the roof, the 100% implementation of the vapor barrier is all the more important so that excessive moisture does not draw upward.

Last question on the topic: I have the wall connections here, which are quite holey at the top (there used to be the lattice battens underneath, which were torn out). Would you fill them with Rotband plaster / surface filler, smooth everything out, and then proceed with the adhesion at that spot, or would you start directly below? There are about 3 cm grooves. The ceiling structure from the wooden beam with battens and plasterboard is about 5 cm.

I can't really see much there. For example, the mentioned wooden beams are not visible. In general, I would place the vapor barrier as close as possible to the insulation. But I really lack experience in that area. Maybe can say something about it...