ᐅ Insulation of the top-floor ceiling / roof in an older building

Hi everyone,

Since my planned new build won’t be happening and I’m buying an older house instead, I have a question about insulation.
I’m not very familiar with this topic.
An energy consultant (BAFA) has already visited, but it will take some time before I receive the report.
I would appreciate it if someone here could share their experience so I have a point of comparison for what the energy consultant will say in April.

Specifically, it concerns the insulation of the top floor ceiling, which I believe I am legally required to insulate within two years after purchase, right? It’s a solid brick house built in 1967.
The current structure of the top floor ceiling (wood beam ceiling) is as follows:
I’ll start from the bottom:
- 12mm (0.5 inch) drywall
- approx. 8mm (0.3 inch) particle board
- 160mm (6.3 inch) wooden beams, with about 60mm (2.4 inch) thick polystyrene boards placed between the beams
- approx. 22-24mm (0.9 inch) spruce boards
- 20mm (0.8 inch) bitumen board (Gutex)
- approx. 15mm (0.6 inch) particle board

What changes should I make to this setup? Should I just add insulation on top or remove everything, including the polystyrene, and insulate differently?

The roof area around the knee wall also needs insulation.
The structure from the inside is as follows:

- 12mm (0.5 inch) drywall
- approx. 8mm (0.3 inch) particle board
- wooden beams
Between the beams:
- 50mm (2 inch) quilted glass wool, attached to the beams with a thin cardboard overlap
- approx. 22mm (0.9 inch) bitumen board (Gutex)
- battens fixed to the beams, with a 4mm (0.16 inch) hardboard panel attached between them (to protect from snow)
- tiles

What would you recommend here?
The tiles are still in great condition, so exterior roof insulation is out of the question for me!
Since it’s difficult to access the existing construction, does that leave only insulation below the rafters?

Thank you in advance for your advice.

Regards
Blackforest