"T i m" wrote in message
...

Understood, so anything non permeable 'above' the ceiling plasterboard
and between the joists could cause moisture to collect and via
capilary action wet the roof timbers? Even of it didn't get *through*
the plasterboard to 'cool to the dew point' where would it go?


If it doesn't "cool to the dew point", then it stays as vapour.

Either the "damp air" then gets circulated out of the house or, if the more
vapour is added, you reach the point where the air at the ambient
temperature can't 'hold' the all vapour and it condenses out.

In fact, if this didn't happen in the wider world, we'd not need umbrellas
;-)

Foil under new insulation isn't as good as a proper vapour barrier (new
houses should have foil backed plasterboard for ceilings) but it is better
than nothing.

--
73
Brian, G8OSN
www.g8osn.org.uk