Insulation of the attic as living space

That there can be problems with insulation in my case has become clear to me by now, even if it cannot be done otherwise than in a complicated way. One side of the roof is covered with underlay membrane, would it be correct to install the insulation completely without ventilation there? Possibly I would then have to have the second side freshly roofed and remove this old roofing felt layer.

Apart from that, 12cm is NOT enough and is standard from the 80s. Please keep in mind that it is about cold and heat protection. In this case, mass is power. So double up the beams crosswise. I would recommend AT LEAST 20cm. Although space is lost, under the roof it simply gets stiflingly hot in summer and in winter it is an energy guzzler.

Before I determine the insulation thickness, I first have to solve the actual problem ....

You have already noticed that. I just wanted to point out that the 12cm is so 90s...

Got it. But can you also help me with the actual problem?

If you install a vapor barrier in the room, I see that as less problematic. I also wouldn’t build thicker than necessary. Insulation wool doesn’t help optimally against heat anyway.