I'm going to use moisture-resistant plasterboard for the ceiling of a
shower room in my renovation. This will be screwed under 3x2 ceiling
joists, with rock wool above. How moisture-resistant is it, and would
I be wise also to install a vapour barrier? The roof space above the
ceilings is ventilated (soffit vents and vented ridge tiles).

If relevant, this is the first floor of a dormer bungalow, so the roof
space left above the ceilings is pretty small.

Regards
Richard