Attic expansion, floor construction for underfloor heating

Hello everyone,

I want to convert the attic in my former parental house, the house was built in 1962. The existing structure is a wooden beam ceiling with a thickness of 20cm, about 50-60% of the volume between the beams is filled with a lime-straw mixture. On top of that are tongue-and-groove boards of various widths that are very loose, meaning they are fastened with very few nails.
Now a complete conversion of the attic is planned, including a completely new roof.
Since a completely new heating system is naturally to be installed, we have decided on underfloor heating for the entire floor. The attic will include a complete living unit, i.e. kitchen, dining room, living room, bedroom, bathroom.

I now have the question of how I should design the floor structure. A friend who is a carpenter recommended leaving the lime-straw layer in place, re-fixing the boards above it with Spax screws. Then installing the underfloor heating (1cm insulation, plug-in panels for pipe guidance and pipes, 5-6cm screed, then the floor covering).

What do you think? Should I consider anything else? I have concerns about the lack of impact sound insulation, which is absolutely poor in the current state. Do new boards or other panels help? Should the lime layer maybe be removed and replaced with mineral wool or something similar?

I look forward to any constructive contributions and ideas.

Thanks in advance and have a nice Sunday :)

Best regards, Alex

Hello,

I installed 25mm OSB boards during the renovation of an old house (also wooden beam ceiling), then 5cm impact sound insulation, and then a thin-layer screed with underfloor heating because a normal screed was not possible statically. Impact sound is not a problem there.