Second, how does
the presence of a patch of fibreglass cause water to move across from
outer to inner leaf? Polystyrene would, as it soaks up water, but
surely not fibreglass, which afaik doesnt.

Polystyrene is impermeable.Even foam. Thats why they make buoys out of it.

Glass fibre is like any fibre, capillary action causes it to transfer
moisture.

However, I don't see how the cavity would get wet either unless the rain
is ****ing down the outside and its full of soft permeable mortar - or
made of soft permeable rock, like clunch.

Perhaps the issue is that some stone hoses have no DPC and need to let
internal moisture get out...


Puzzled of Tunbridge Wells.