Hi there, im fixing 12.5mm PB over the top of 40mm celotex on sloping ceilings. The construction is 150mm celo between rafters with 40mm celo over the underside of rafters then 12.5mm PB. The rafters are at 600 centres, question is should i have a batten set behind the horizontal PB joins or just screw them through to the vertical rafters on 600 centres and leave the horizontal joins unsupported?
Have you got any metal 'C' channel from some of the wall lining systems? We used
some that was 50mm across by about 12mm deep, we put that across the rafters to catch the boards either side of the join. You have to notch the C channel for the rafters and cut into the celotex as well. But it's a LOT faster than cutting and nailing noggins.
600 centres is a lot to be un-supported. That is a 8x4 board with a single rafter down the centre, imho, that should certainly have a noggin. We are doing the exact same thing, boarding the inside of a roof, we have 100mm kingspan between rafters and 25mm over them to avoid the cold-bridge. We went with 400mm centres for the rafters.
I don't know about the membrane/vapour barrier the other poster advised. Best practice is probably yes.
