RobertL coughed up some electrons that declared:

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?

Robert

I think in this case it is to reduce the risk of condensation on cold
bridges, the rafter faces. And to reduce the amount of warm vapour leaking
into the cold side. These will be nominally air tight but there's no way to
make it 100% so - too many bits of random wood to seal every interface.

I have no way to overlay insulation on these, only between them.

The cold side has the requisite 50mm air gap.


Cheers

Tim