by Mike G » 03 Feb 2022, 09:53
You don't have to comply with building regs on this (and you couldn't achieve compliance with an unventilated cold roof anyway). In the absence of re-building the roof or lowering the RSJ I think getting the best air-flow you can achieve is going to be your best bet. Regs call for the equivalent of a continuous 1" gap, which means if the joists are on 400 centres, for instance, a 100mm diameter hole provides approx the same open area. Obviously some of that would be obstructed by your insulation, but it does show the sort of ball park you might aim for..... maybe a bit less than you might think. I would include a vapour barrier in your ceiling, in addition to lining it with OSB or ply.