This is an example of the right kind of duct work to use for bathroom exhaust fan systems. The duct work is an insulated pipe with an aluminum exterior skin.
This allows for the hot air from the bathroom to go thru the pipe and to be exhausted to the exterior without putting any heat or humidity in the attic space…….that would cause a lot of mold. Notice that the piping is well sealed to the fan with tuck tape as well as sealed to the direct connect roof trap.
It’s also important to use the right roof exhaust trap (vent) on the exterior. Most common exhaust traps have a very low profile. We prefer to use the Ventilation Maximum trap that rises 8 inches above the roof. This keeps it out of the snow in the winter. Also, the Ventilation Maximum vents also has an anti-reverse damper that allows the hot moist air to be exhausted to the outside, without allowing cold winter winds to blow inside. So, a properly designed bathroom exhaust system gets rid of all the host moist air in the house directly to the outside, without allowing any hot, moist air into the attic.