My mother wants a ventilation fan installed in the bathroom. It uses a 4" outlet. To minimize the resistance, I want to use 4" round metal as it will have to run about 20' and that plus an elbow puts it around the max for the Panasonic fan I'm looking at, but I'd like to insulate it to minimize condensation since it will be running through the attic. I'd thought about buying some insulated flexible duct and running the solid stuff through it and I think I'd probably need to use 6" stuff, since I do not think I could slide the 4" solid into a 4" flexible duct (I haven't found 5", if it exists but haven't searched everywhere yet). The fan can also take 6" ductwork. Should I just forget about running solid material and use a 6" flexible duct? What's the best way to minimize chances of condensation and moisture dripping back into the room in the winter with this?