This bath fan was poorly vented to the gable vent where air comes in to ventilate the attic. So when you are trying to push your hot humid air out to the outside it is trapped and can cause mold and wood rot. This bath fan duct from the master bath was poorly hung as well and just got torn apart.
The proper venting of the bath fan through the gable side of the attic will prevent these issues. Also, we like to use a insulated flex duct so condensation can't occur in the winter from going through a cold attic. A butterfly valve is added as well to reduce air from coming in from outside back through the bath fan.