My heating contract was called into a house that was about 8 years old. The homeowner had a unique problem. In the master bedroom was located on the 2nd floor the toilet bowl was frozen and the bathroom temperature was 70°. The contractor though at first this was a joke. The owner insisted that he wanted the problem fixed. With a little bit of looking around the contractor saw the bath fan right above the toilet bowl. But the real problem was in the attic the exhaust vent from the exhaust fan was not insulated. With the out side temperature of –20F below. What happened the air inside the exhaust pipe being cold (and cold air is heaver than warm air) dropped from the ceiling fan and fell into the toilet because the husband left the toilet lid up the night before, Allowing the cold air to collect in the toilet bowl and actually freeze the water.
Another job site the homeowner had a new roof put on 6 months earlier and had the attic reinsulated. The first cold morning about –15 below 0 with a hot shower the ceiling fan started dripping water. So he called out the roofer to fix the problem, it was not the roofing but instead the exhaust fan piping again was not insulated. While the high humidity was being blown out and the metal piping was below 0 the humidity was condensing rapidly on the piping, then run back into the ceiling fan, through the vent louvers and end up dripping to the floor.
You have to insulate the entire exhaust vent to the exhaust point.