ᐅ House built in 1966: Installing roof insulation between the rafters

Hello, I bought a house and now want to insulate the roof myself. The space between the rafters is 17 cm (7 inches) high for the insulation batt. With an insulation thickness of 14 cm (5.5 inches), that leaves 3 cm (1 inch) for ventilation. On the outside, behind the roof tiles, there is a tongue-and-groove boarding about 18 mm (0.7 inches) thick, nailed directly to the rafters. There is no underlay membrane. At the side of the eaves, the roof overhang is open to the ceiling, which currently only has 4 cm (1.5 inches) of mineral wool, and this opening should be sealed, but with what kind of material? Do I need a vapor barrier if I insulate with 14 cm (5.5 inches) of glass wool? I plan to convert the attic into a hobby room later.