-
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.
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.
Dugtheslug
You’ve done some good work there but proceed with caution.
If you’re insulating that you need a good gap between the top of the insulation and the osb, with good ventilation. Otherwise you will get moisture building up in there, more than you might imagine.
If you have the space above that roof, put the insulation on top of it rather than within it.