Floor construction with dry screed and underfloor heating

Hello everyone!

We are currently renovating a house from the 70s and I have a question about the floor construction with loose fill, underfloor heating, and dry screed in the attic on a wooden beam ceiling.

Situation:

[*]On the wooden beams are chipboards that have been completely newly screwed to prevent squeaking. However, the boards sag in some places, so compensation is necessary.

Framework conditions:

[*]Both the ground floor and the attic are heated, so I assume that no vapor barrier is required.
[*]Flowing screed is not possible for structural reasons.

Planned floor construction:

[*]Floor leveling with approx. 20 mm of bound dry fill
[*]on top of that a layer of 12 mm OSB (floating and glued) to ensure fundamental stability, for...
[*]the shaped parts of the underfloor heating (30 mm thick)
[*]On top of the finished underfloor heating comes a 20 mm dry screed (glued and screwed)

Question: Can this be implemented like this? Concerns? Suggestions for improvement?

The drywall installer says you can also lay the underfloor heating directly on the fill, without OSB. The installer has his doubts about that (and so do I).

Thank you in advance!

You have to be careful with the screed that 20mm is allowed. Back then, I had some that required at least 40mm buildup... Depending on what the molded parts are made of, I also believe that you can lay them directly on the screed (maybe you have to, due to the minimum height of the screed)...