Screed necessary? Unfinished but insulated attic

Hello community,

we are currently planning a semi-detached house with a developer and I need some advice on a question.
It is about the attic, not living space, unheated, but intended to be used as storage.
The roof will be insulated with 240mm mineral wool and OSB boards, the floor consists of a 20mm concrete slab.

My question: Is it necessary to seal the floor with screed or can you skip that for a pure storage room? The costs seem relatively high to me, so I'm asking about the benefits.
According to the developer, the screed would, of course, contribute to thermal insulation; without screed, the upper floor would heat the attic. But since I have insulated the roof, no heat is lost.
Can you say by how many degrees the attic would heat up more compared to with screed?
Is it really necessary to lay screed or would carpet be enough? I only want to store cabinets/boxes there.

Many thanks for your help and support,
regards
Holleradhiho

Normally, the ceiling on the upper floor is insulated and the attic remains uninsulated....at least that's how I know it...and screed in the attic is new to me as well...

What kind of roof do you have or what height does the attic have then? Is it accessible by a normal staircase or only by a pull-down attic ladder?

Uninsulated attic was included in the standard offer, but since I want to use it, we agreed on insulating the roof. The attic is walkable with a pull-down attic ladder; in the gable area there is a clear height of 2.3m, 30° roof pitch.

Is screed necessary for this? As mentioned, the developer talks about thermal insulation effect, otherwise the attic would be heated via the upper floor. Should this be avoided or is it negligible?

No screed necessary. Why even a concrete ceiling if it's only used for storage? ESB/OSB boards of decent thickness are sufficient.

However, to avoid heating the attic completely, I would also insulate the attic ceiling. Not a big cost or time effort, and can easily be done yourself with friction-fit insulation (stone wool or glass wool).

The concrete ceiling is its construction method, I will not be able/willing to change that.

die Dachgeschossdecke ebenfalls dämmen

I don’t quite understand that now:
1) The ceiling of the upper floor is a reinforced concrete solid ceiling, wallpapered with fiberglass fleece.
2) Above that is the attic, the roof is insulated, the floor is bare.

Am I supposed to insulate the ceiling in the upper floor now? After it has been wallpapered?
But that means the heat transfer from the upper floor into the attic through the concrete ceiling is not negligible, and something should be done about it, do I understand that correctly?

Is the attic then also located within the airtight envelope?