Renew roof insulation from 1986?

You don't seem to have practical knowledge. I have often come into contact with old buildings and almost always there are errors in the execution or damages in the old insulation. And in such cases, a replacement is always sensible, provided it is done properly.

Also, please read properly. I wrote that he can put the drywall on the OSB. The OSB3 board saves the vapor barrier there or acts as a vapor retarder. Again, provided it is done correctly.

And I repeat once again: existing structures mean compromises. Everyone who deals with them must be aware of that. The OP wants to lose as little space as possible. That only works by saving the additional layer or with a 40k investment for insulation above the rafters.

And with the 20% rule, the OP surely means his setup. He adds 40mm of mineral wool on the room side. You have to check exactly whether moisture condenses at the foil.

Regarding the Energy Saving Ordinance, no questions have been asked so far. And in renovation or refurbishment, usually no one cares about that anyway.

It is not about errors or damage to old mineral wool.
Aren't you pretending to have practical knowledge?

Old mineral wools often have a carcinogenic potential K1b or K2. However, this can possibly be clarified by the year of construction (before June 2000) and the manufacturer.

In practice, the removal is the problem, because with "old" mineral wool, during demolition, renovation, and maintenance work, TRGS 521 and the corresponding protection level concept must be observed and applied. Currently, there is no obligation to renovate "old" mineral wool; however, the reinstallation of dismantled "old" mineral wool is not permitted.

What kind of OSB do you want to use, 22mm?
What does the OSB cost compared to GK?
Foil makes the movement of the roof much easier and is flexible to connect to the existing structure.

You, on the other hand, are obviously the theorist. The expansion is not a problem, as long as you protect yourself. And it is not about an obligation, but about the sense. And old, damaged, thin, improperly installed insulation simply makes sense from many perspectives.

OSB3 has worked wonderfully. The wall connection can be perfectly made with permanently elastic sealing tape from window construction. The foil actually has to be plastered over. Otherwise, it is glued. But the foil, of course, fulfills its purpose. And naturally, it does so well.

Since you are so good at theory and reading, please read my posts carefully again. If you have already done so, the problem is probably with text comprehension. You can also answer the posed price question yourself. Or was that just a rhetorical device to make me look as stupid as possible? I would like to ask you to stop that. On a factual level, we can gladly discuss. I will refrain from the rest in the future.

@

I did not respond to your comment at all.
Apparently, it was the OSB in my comment that addressed you.
It was about the mass and summer thermal protection, not about airtightness.

There would also be a reason to actually use OSB, for example if there is no wind bracing band at all and a covering was done over battens. But handling possibly OSB4 overhead is very cumbersome.

Unless OSB would be pointless, as in fire protection for boundary buildings, but I read nothing about that.

No arguing

Thank you for the answers!
As I said, the old insulation was cracked, not present at all in the area of the roof hatch 1m around the roof, and I wanted to remove the old / hazardous wool.
I have bought protective suits (Cat III, Type 5 and 6) and masks (FFP3). Hope that was sufficient.
Here is what I realized:
14cm insulation between rafters (035 stone wool - Klemmrock), vapor barrier foil (RockTect Intello climate Plus), and then 4cm insulation under rafters (035, Formrock).
Thus, I have at least 3cm for ventilation.
For cladding, I am using 12.5mm drywall boards.
The problem I have now is the eaves in the area of the window where the beams run. I can barely reach there and will probably have to remove the wooden panels in the eaves.

Then at least you are retroactively compliant with the Energy Saving Ordinance 2002.