If windows and mirrors are very
cold, condensation can still form on
these surfaces. And if your bathroom
is sealed tightly, replacement air may
not be entering the room fast enough
to displace the moist air. Be sure to
undercut your door sufficiently to
draw fresh air into the room. Fan
placement can also be a factor.Your
fan should be located far from the
replacement air source to ensure the
moist air is drawn out first. Finally,
too many twists and turns in the
ducting will significantly reduce the
ability of the blower to remove moist
air. Make sure your ductwork is as
short and straight as it can be, with
gradual turns rather than tight
corners where necessary.