I am insulating a detached garage to use as a gym. The roof is a flat felt roof. The ceiling joists are 300mm apart and the fascia board is nailed directly to the joists with no overhang. If I install Celotex between the joists I can only use 70mm to leave a 50mm ventilation gap and then I have the problem of venting through the fascia.
I have seen the mention of completely filling the void with 120mm Celotex so there is no air gap. The vapour barrier on the Celotex and tape would prevent air ingress. Is this still a valid design as I cannot see this anywhere on the web? I am fitting downlights as well, so would that breach the vapour barrier and make this design invalid?