RobertL wrote:
On Jul 15, 8:47 am, Tim S wrote:
OK, stupid time.

Accidently bought 6 sheets of ordinary 9.5mm PB in 3x6' sheets. Need to use
vapour barrier PB for the ceilings on some bay window ceilings which will
be insulated with celotex above. I don't even think the use of vapour
barrier PB is that critical as there's a huge void over half the bay
ceiling as the pitched roof meets the lead flat roof of the bay, but vapour
barrier was suggested.

I'm interested in why a vapour barrier is needed. is it because there
are no ventilation points into the void?


Normally its to prevent condensation on the far side of e.g. porous
rockwool insulation.

Robert