On Oct 27, 7:20*pm, The Natural Philosopher wrote:

Ive got perfectly standard plasterboard above my showers. No problems.

Unless you deliberately spray the ceiling water generally comes
downwards out of showers....

It's not direct spray from the shower that's the issue, it's the water
vapour in the air, which is why we have vapour barriers: to prevent
water vapour penetrating into a cold space and condensing within the
roof structure. Two people sleeping in a bedroom can generate enough
of that, let alone a shower.

My roof space is ventilated, and there will be extraction in the
shower room, so I'm not anticipating a problem. Having said that, I've
just fitted a 25mm sheet of foil-backed Kingspan to the ceiling joists
before installing the PB, as part of my insulation regime, so I'd say
that was both belt and braces taken care of.

None of which answers my original question ;-) Just how moisure-
resistant is moisture-resistant PB?

Regards
Richard