What type of attic insulation is sensible?

Attention: Vapor barrier is crap, vapor retarder is something different... Vapor retarder would of course be good, but it must be on the warm side, so on the inside. Or directly behind the drywall, still in front of the beams. A ventilated attic is very good, then there should be no problems (even without a vapor retarder - as said: should... of course also depends on whether you heat up to e.g. 25 degrees under the roof and never ventilate, etc... but with normal use it should work - attention: I am not an expert - I have only dealt with the topic very intensively), but only if you use diffusion-open material! So e.g. stone wool or wood fiber. For example: lay Rockwool Varirock between the beams and make sure it fits tightly against the beams. Leave a few cm of air to the [Rauspundbretter] (so without contact, quasi ventilated underneath) and also leave air between the boards! For the slopes it gets quite difficult from the inside, of course... Maybe try a more stable clamp fleece here - but also leave air to the roof! After winter you can test by removing the insulation at some spots and check if everything is dry or measure the wood moisture. Please absolutely do not use the Styrofoam crap!!! And even if you get 1,000 EUR more, you should not take that crap. Pour a glass of water over a Styrofoam board. Where does the water go? Ergo: the moisture cannot diffuse and condenses on the underside... Just google "insulation top floor ceiling." You will find quite a bit...