Ducting Advice for New Bath Fan
I have a 1964 Levitt house, and the one bathroom that had a ventilation fan... basically didn't. We had mold on the ceilings and when I took out all the ceiling drywall, come to find out that the fan (such as it was) had zero ductwork attached to it, and was venting into the joist space. Granted, that joist space was open to the outside at the end through a grill installed in the soffit, but still... it didn't work.
Not wanting to tear up the ceiling in the bedroom over which this joist bay runs, I plan to run the ductwork for the new fan the opposite way, which will allow me to get into an area of open attic. I will then have about a seven foot lateral run to get to the side of the house (gables) and plan to install a backdraft preventer along with a vent on the siding that will prevent birds/rodents from entering the home.
The question I have now is about avoiding condensation buildup in the duct. Running through the attic is good for convenience of install, but our attic gets really cold (we are working on getting insulation up to code, but its not quite there yet). I am assuming we should insulate around the duct and minimize the amount of vertical sections. So I'm thinking...
6 foot straight duct run to get into the open attic space
90 degree elbow to get above the level of the floor joist
90 degree elbow to run perpendicular to the original run (across joist bays)
6-8 foot run to get to the outside of the house.
I know the elbows will decrease efficiency but i'm not sure what other options I have. Any thoughts on that? Or other advice for running the ductwork to avoid problems with condensation or inefficiency? I can post pics later if that will help.