Insulation of the attic as living space

Well, you don't have to live there in the summer either. That makes more than a little difference. TE: enter u value calculator on Google and try it. Unfortunately, I don't have the time right now.

A few more centimeters of insulation don't make much difference here. The mass to achieve a corresponding phase shift and temperature amplitude damping is simply missing. This is a fundamental problem in the attic. For example, one could insert a double layer of OSB boards or a clay building board between the vapor barrier and the insulation. This provides significantly more heat protection than, for example, even 30 cm of insulation.

I agree with you. I generally recommend OSB boards as a replacement for the vapor barrier. They add a reasonable amount of mass. It's just that there isn't enough time to respond thoroughly right now. No one here has that either. I also prefer stone wool over glass wool. Probably even better would be wood fiber boards.

When in doubt, ask a carpenter. No, no drywall in my case. It's an attic that remains uninhabited. So why? Here again, floor before insulation.

Man, your roof looked just like ours in its raw state In our case, it was called a studio truss