My installation was guided by the fan requirement to ensure that ducting
sloped away from the fan! It therefore now falls away from the fan in two
directions: towards the external grill in the gable end and towards the
grill in the bathroom.
I was hoping that, if I lag the duct sufficiently, then the moisture will
remain air born until it reaches the outside world.
I have heard of condensate trapping, but wouldn't have expected a serious
issue in just 3m of 5" duct. In any case, the only place I could see to
drain condensate would be via hose or similar through the outside wall. I
reckon that, the time when this would be most needed ie winter, would see
this arrangement frozen up.
So my question revolves around: How effective would lagging the duct be?
How much difference would a switch to plastic duct make?