T i m wrote:
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

They're quite right. The warm air absorbs water vapour from breathing,
plants, laudry, cooking, etc.. People generally prefer Rh around 50%,
so it's an advantage. It's not a problem whilst the warm air stays in
the heated space. If it can get into the loft it will condense in the
insulation & can waterlog it and reduce it's efficiency. It rarely
causes problems in the UK, for some reason. In colder climates you can
find the insulation saturated and frozen.

You can often get rot/damp problems above the airing cupboard, where
great holes are usually hacked for small pipes and where damp laundry
is dried.