Possible source of dripping from bathroom exhaust fan?
If installing an insulated duct does not get rid of the leakage problem you might try changing the exhaust fan with one with a greater efficiency to push out the warm moist air out faster. The CFM rating should not be less than the square footage of your bathroom ceiling. To be sure your fan blower is adequate, I would use a fan with a rating 20% greater than the recommended CFM for your bathroom size. Also pay attention to the sone level. Get one with a rating of 2 or less if at all possible. Finally, if you don't want the fan to operate longer than necessary replace fan wall switch with a timer switch.