T i m wrote:
snip
Where is the 'vapour' coming from Brian? I would have *thought* you
wouldn't want to trap vapour in the room but let it permeate out
through the walls / ceilings etc and if there was a vapour trap above
the ceiling it would keep the plasterboard damp?

Nope.
Plasterboard isn't an insulator.
It'll be at essentially the same temperature as the room.
However, it's essentially porous to water vapour, and what that means
is that you get warm damp air on the far side of the insulation, shortly
after you start breathing or cooking in a room.
It then permeates through insulation, cooling as it goes.
If you are unlucky, it cools to below the dew-point, and you get
condensation in the insulation, which can cause whatever's touching it
to rot rapidly.

The right way to insulate nearly all structures is to have the inside
a vapour-tight box, with air entry and exit done through ventilators,
not a porous structure.
Otherwise, if you try to insulate highly, you're likely to get condensation
where you don't want it.