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Old 01-31-2013, 03:04 PM   #1
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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.



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Old 01-31-2013, 03:29 PM   #2
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You should post where you are located.

They sell flexible vent lines that are wrapped in insulation. I believe code here in MN in the vent must be insulated at least 36" from the edge of the house (could be wrong). I pulled the flex line out of the insulation and inserted a solid 4" ducting pipe and pitched it slightly to the inside so any potential condensation will drain.



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Old 01-31-2013, 03:32 PM   #3
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Thanks Beepster! I'm located in central Maryland, suburb between DC and Baltimore.

Your idea of replacing the flex duct with straight to get the insulation was a good one- I definitely want to reduce the static pressure by using smooth solid duct as possible.
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Old 01-31-2013, 04:14 PM   #4
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Welcome to the Forum!

check out the exhaust fan manufacturer's specifications for ducts. typically they are installed and vented to the exterior of the dwelling and not into an open attic. moisture will accumulate on the bottom of the roof sheathing and freeze in cold weather. you know what happens when it warms up then .....

Hope this helps!

"You get what you pay for, and sometimes free costs more!"
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Old 01-31-2013, 06:12 PM   #5
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Thanks for the welcome! I definitely will not be venting into the attic- just using the open attic as a place to run duct to vent outside because it will be much easier to work in there than tearing out ceiling!
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