In article , Pete wrote:
While floors still up.....
Our "undreground" bathroom has very little ventilation only a small
opening window on the outside wall next to the extractor fan.

When the fan is on and the door closed there is no through draught to
allow fan to draw out moist air.
I have been toying with the idea of running some "squashed"
ventilation/extractor vent pipe stuff from the outside wall to the
diagonally opposte corner to the extractor fan thus allowing fresh drier
air to enter the bathroom and creating a decent air flow.

Or would it be better just to put a vent in the bathroom door to allow
interior "warmer" air to flush through to the fan?

TIA
Pete

Even better would be a heat recovery ventilator which uses a
countercurrent heat exchanger to heat the incoming air with the
outgoing exhaust. That way you'll spend less energy on heating and
you'll reduce draughts. However, these devices are an order of
magnitude more expensive than simple extractor fans. They are
available as 'through the wall' units for single rooms. Here's an
example: http://www.kiltox.co.uk/products/hrv.htm I'm currently
thinking of fitting one in my (upstairs) bathroom. IIRC Baxi (now
part of Vent Axia) make a similar poduct.

--
Jan