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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
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.