Vapor barrier for insulation on pitched roof

[britt]

Living in a house built in -42. The previous owner finished the attic, and the insulation was very poor. I have now taken down the ceiling in the middle to access the slanted roof for insulation. There are only 15 cm between the outer and inner roof. Since there should be an air gap of at least 5mm to the outer roof, I'm considering what insulation material to choose. The ceiling height in the room is already low, so I thought of adding 20 mm of foam insulation inside before installing renovation plasterboard. Do I need to install some form of vapor barrier (plastic film) on the slanted and middle roof?
Wondering lady who loves to renovate!

 

[Mats]

Spontaneously, I think 120 mm insulation sounds like a good idea. Theoretically, 145 should fit exactly, but I would probably go with 120 just to be safe.