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Community and Q&A

Routing bathroom ducting

kevinjm4 | Posted in General Questions on

This is a little hard to explain but hopefully not too confusing…

I will be routing my bathroom fan exhaust through the gable wall in my attic, which is about 8’ away from the fan, and will be traveling perpendicular to ceiling joists.

The fan sits inside the cavity as most fans do I suppose. But I cannot just run the duct in the cavity until I reach the wall, since that would mean doing a 15-20’ run going this direction, and would exhaust out of soffit. No thanks.

So here’s my concern: In order to get up and over the ceiling joists before sloping it toward gable wall I need to take a hard 90° vertical up with my ducting right out of the fan, since the outlet is on the side of the fan as opposed to on the top of the fan… Won’t moisture get trapped right at that elbow before it can even get anywhere?

(Wouldn’t a fan that can route moisture through the top of the fan make a whole lot more sense?)

I cant really think of anything else to do BUT to route the ducting up and over the joist, then run it toward wall, and exit…

Will I need a higher cfm fan than what is req’d for my size bathroom? Any other options? Or is my situation completely fine and nothing to worry about…

I see a lot of pictures where the ducting is just routed either straight up through the roof. Which would still mean a hard 90°, just straight up, and I do not want to do that. Or it is simply routed parallel to the joist bay the fan is already in, then out the wall, or even boared through joists which I think might be a bad idea for me since I have 2×4 joists in this part of the house.


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  1. Expert Member
    Peter Yost | | #1

    Hi Kevin -

    You are doing the right thing routing your exhaust fan ducting to the gable end. Going through the roof or turning down into a soffit is problematic.

    I always insulate and air seal up and over the duct even as it runs on top of the attic framing (and slope to drain any condensation down and out).

    And yes, your fan is rated for a certain amount of resistance (elbows and overall length) so possible your fan does not have the force needed for your run.

    I do have a soft spot for Tamarack Technologies fans: high efficiency, lots of configuration options, and excellent tech support (


  2. Expert Member
    BILL WICHERS | | #2

    You want to exit through the gable, and you want to slightly pitch the duct towards that vent for drainage.

    You’re correct that you’ll lose a little bit of airflow each time you increase the resistance (long runs, 90s, etc). Fans are rated to maintain their CFM spec with a certain amount of ducting and the install guide should have info about that. If you can manage two 45s instead of one 90 that will save a little since two 45s results in less back pressure than one 90. Don’t use flexible ducting, any kind of flex duct is much worse than any kind of rigid duct for airflow.


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