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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.
Dugtheslug-
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
am going to measure the depth of my joists to the top of the battens and order PIR board to fit.
to stop the rodents returning and making it home again I've ordered some 6mm wire mesh and will staple it up as a barrier at the end of each joist run to block access to the insulation cavity between joists. coupled with closing up the possible entry points I can see along the outside edges with steel wool and foam hopefully it'll be enough to discourage their presence again.