Insulating the concrete ceiling of the upper floor / What to watch out for?

Exactly, and the energy saving ordinance Pollisei, on behalf of the [KfW], constantly monitors the floors.

I would insulate the roof if I were you... You can't generate more storage space! And the work is (almost) the same, I would say... Costs maybe a fraction more, but not in proportion to the generated storage space.

With a concrete ceiling, I don't see it that way. Heat rises upwards, and here I want to catch the energy at the ceiling, not at the roof.

I wouldn't put the panels in the roof. If it burns, you don't need a fire department. Suggestion: Put 8cm of rock wool on the concrete ceiling and then the OSB panels. Additionally, insulation between the rafters.

The panels are hardly flammable by now, right?! Honestly, before the panels burn, the entire wood/roof structure burns.

I don't see it that way with a concrete ceiling.
Heat rises upwards, and here I want to capture the energy at the ceiling, not at the roof.

With what justification?

By the way, we did exactly that: upper floor ceiling made of concrete, insulation between the rafters, and created a pleasantly tempered, seemingly infinitely large storage room for minimal additional costs. Of course, you end up heating a few cubic meters more.

not if it is a KFW55 house. that is stipulated by the Energy Saving Ordinance

No, it is not. It is not mandatory. We are building KfW55 and will also insulate the concrete ceiling, not the roof.

1. Sweeping is sufficient.
2. Lay out the vapor barrier foil with excess for foil overlaps over the entire surface and fold up sideways at least 10 cm high for connection to surrounding components such as foot purlins and gable walls. Seal the overlaps with suitable tape. Apply an 8 mm thick bead of SEALANT to the surrounding components such as foot purlins and gable walls and press the vapor barrier foil down without tension with the flat hand.
3. Lay attic insulation boards fully and tightly butted; cross joints should be avoided. Slight unevennesses in the concrete ceiling are compensated by the attic insulation boards. The insulation effect is increased and thermal bridges are avoided by multi-layer installation with staggered joints.
4. Lay moisture-resistant chipboard or OSB boards (thickness ≥ 19 mm) floating on the attic insulation boards and glue at the board joints and seams.
5. Yes, that would be an advantage.
6. Phew, there is quite a lot....
7. It’s best to insulate the roof as well, as my predecessors have already said.