cold flat roof

Looking for some advice on roofing if I may: I’m building a cabin with cold roof in SW UK and concerned about the air gap between insulation and OSB deck. The roof comprises (inside to outside): 12mm Q mark plywood Thick vapour barrier (black DPM) that is fitted best I could but without butyl...

I have a small cold flat roof. It has plenty of ventilation, followed by PIR insulation, followed by a vapour barrier. There is still a fair amount of space between the vapour barrier and the plasterboard below. Can I just pack this with extra insulation? I'm only asking because i read that...