Thanks and apologies for the confusion.
So if I leave the existing buildup alone and put another ceiling in with plasterboard, joists with PIR it would need ventilation with a through draft?
I had thought ventilation might not be needed as there shouldnt be any moisture getting into the void.
Thanks for your reply!
The room has got two new windows so that should be ok. There are two external walls. One wall seeems to have two small cold spots so not sure if the wall insulation has moved about over the years. The floor is concrete so like the rest of the downstairs probably lacks...
We have a cold north facing office room (2.2m wide by 2.2m long) which has a pitched ceiling thats 3.6m at it's peak.
The ceiling has pine cladding and under that is some pastic sheeting then insulation roll.
Looking at the heat calcs if I made the ceiling flat (say 2.4m) I would need a lot...