Full rafter insulation roof or insulation on concrete slab

He is right. It is exactly what he says that prompted us to insulate the floor afterwards with clamp fleece and Climafolie. Now it is fine. A tempered room, frost-free in winter, reasonably cool in summer.

But in this case, it is not about the floor, but about the insulation of the roof.

We considered insulating on the concrete slab and thus saving the full rafter insulation, since we do not want a wooden structure, but rather the aforementioned concrete slab.

We also consider insulating on the floor to be sensible.

Karsten probably did not mean the floor of the floor, but the attic / loft or the stage / mansard (or to complete the confusion: the Swiss even say Estrich) itself; in his case even specifically the insulation between the rafters of the floor

He is right. It is exactly what he says that prompted us to insulate the floor afterward with clamp felt and Climafolie. Now it is okay. A tempered room, frost-free in winter, reasonably cool in summer.

And the ceiling (the floor of the attic) still kept its insulation, right? Can you tell me more about that, please? I might plan something like that later...

And the ceiling (the floor of the attic) still kept its insulation, right? Can you tell me more about that, please? I might also plan something like that later...

Exactly. It's about insulating the attic! This could eliminate the need for full rafter insulation on the roof.

Only my general contractor doesn't want to do it for the above reason. The question is whether this is justified or if he just doesn't want a concrete ceiling above him and would rather do a wooden construction.

So I have insulation on the top floor ceiling, which is also made of concrete. Therefore, the room cannot be used as storage space for me. That doesn't bother me. Yes, the room gets cold and hot. That doesn't bother me either. There is always some ventilation with me, so mold is not an issue. For Karsten, it is a bungalow where he has fixed and built a staircase into the attic. Thus, it is ideal for storage!

Very precise. Bungalow. The ceiling here is made of wood and drywall and a lot of glass wool. On top of that, tongue and groove boards are nailed. This creates a walkable floor. This is the standard series of the BU.
Additionally, now there is insulating fleece between the rafters. And foil. The result is that the walkable floor is now also tempered.