Renew roof insulation from 1986?

So I would do the following if the attic is not being used anyway:

Remove the beams including the top floor ceiling on the upper floor. Then put 140mm stone wool between them. Add 40mm battens crosswise to the beams. In between, another 40mm stone wool. This reduces thermal bridges. Then 18mm OSB3 boards on top. These with tongue and groove. Please glue carefully! Wall connection with [Quellband] from the window segment. Then seal with acrylic.

On top of that, gladly your gypsum plasterboards. You have 3cm more build-up but a very good vapor barrier. Also minimizes thermal bridges, windproof and much mass against heat and sound.

You are at a U-value of 0.24, which is not great but OK. Additionally, it is diffusion-open outward in case of errors.

Doubling up would be nicer but OK. On the top floor ceiling, more is possible if you don't want to use the attic.

Thank you for your response. I forgot to mention that the attic/ top floor is to be used as a room. On the upper floor there are bedrooms, children's rooms, 2 bathrooms. What is your suggestion here?

But 4cm is more than 20% of 14cm Why is doubling better?

Well, I would basically not recommend anything under 24cm. As a general rule. But compromises have to be made in the existing stock.

If the attic is used and heated completely, then insulation against heat/cold in the top floor ceiling is irrelevant. It's then about soundproofing. I am uneducated on that topic.

I would then pull the roof surfaces all the way up as described.

You could also save the OSB and work with foil. Then use a vapor-permeable one.
But OSB brings nice mass.
On the topic of insulation (especially in EL) my motto is: more is always better.
But 14+4 would be an acceptable compromise. For you too?

I was on site again yesterday and measured the rafter depth. You can forget the ruler of the Lidl spirit level. It's not 16 but at least 17 cm. Therefore, I tend towards the following construction, which Joedreck also suggested. 14 cm Rockwool stone wool Klemmrock 035 > vapor barrier foil > 4 cm under-rafter > 4 cm under-rafter insulation > gypsum boards

[*]I have no experience with aluminum foils, what do you say? Intuitively, I tend more towards a "normal" diffusion-open vapor barrier foil. I also have concerns that if I cover the roof with aluminum foil, the mobile phone reception might be disturbed and the phones will always operate at full antenna power, which is not exactly healthy.
[*]Should I stay within the system or can I buy good foil, adhesive tapes, glue and not spend so much money? Do you have any brand/model recommendations regarding these materials?
[*]Which gypsum boards and which thickness can you recommend?
[*]Should I close the hole in the roof membrane or leave it as a ventilation opening?
[*]With what should I insulate the ventilation pipes against condensate?

Mh then just take 16cm of rock wool. That makes a total of 20cm.

Use a vapor-permeable foil. It forgives mistakes.

You can insulate the pipes with 4cm wool. I would halve it to 2cm, wrap the pipe, and secure it with wire.

Over the hole, stick two strips of tape and that's it.