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Definitely go with a vapour barrier - if you can spare the head height cover the whole ceiling in supafoil then cross-batten before plasterboarding. That will give you even more insulation and a vapour barrier if you tape the joints.
It’s good (vital!) you’ve got 50mm above the PIR - it’s a pain in the arse but i’d drill through the firrings to encourage cross-ventilation as much as possible too.
Dugtheslug-
thanks this is all useful in organising the process in my head.
the joist hangers on the previously sagging side are really poorly installed so they have a gap under the joist of a cm or so and hang down hindering placing the boards flat all the way up to the wall so cross battening will actually help me sidestep that issue nicely. we were gonna overboard the old ceiling so some loss of height is not anything i wasn't already prepared for either.
HatBeard
it's a cold flat roof. above the wood is the top of the extension
previously it was insulated with a few inches of pink fluffy insulation with I'm guessing 10cm polystyrene slabs below it.
i haven't measured yet but I think i need a min 50mm gap above the insulation so 100-150mm thick pir is what I'll probably need, I think I also should have a vapor barrier on the underside of the joists between the insulation and the plasterboard. the previous ceiling didnt have one and is probably what caused it to sag so badly we had to take it down.