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Community and Q&A

Bathroom exhaust ducting plan

NewLaneProperties | Posted in General Questions on

I’m in the process of roughing in a new bathroom in my basement. I’m looking for feedback on my exhaust fan plan. Bathroom is 5×11, eight foot walls. The fan will sit in the joist bay (obviously) and the duct will run about one foot in the bay to the wall, where I then need to drop the duct below the joist to run it along the wall in a chase for about 18 feet. At that point I hit a plumbing obstruction (main stack). I plan to use flexible duct for the first two feet, PVC for the next 15ish and flexible again at the end to get around the stack. There’s an existing vent cap in the rim joist which means the duct will need to go back up at the very end to connect to it. To sum up, it will go down, over and back up for a total run of around 18-20′. No 90s if i use the flex stuff, but will use PVC in the middle. I plan on insulating the entire thing. Fan will be minimum 110CFM (probably Panasonic).

Am I missing anything? Are there any problems I’m not noticing?

Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    Even without elbows, those changes in direction increase the static pressure in the duct system. Flex duct adds more static pressure than smooth-wall duct, and curved duct adds more static pressure than straight duct. Your 20-foot run might have an "equivalent duct length" of 40 feet of straight duct. (See attached table.)

    What this means is that you will probably get 35 cfm or 45 cfm (just a guess) of air flow from your 110 cfm fan. One way to reduce the static pressure (and increase the air flow rate) is to increase the duct diameter from 4 inches to 6 inches.


  2. NewLaneProperties | | #2

    Thanks Martin. Any recommendations on a high cfm fan? I don't have any other options to shorten the run so I'll have to get a better fan. Another problem is a planned addition will require the wall cap to be moved further away. By the way, I have no gas mechanicals in the house so I'm not worried about air backdrafts with a high power fan.

  3. GBA Editor
    Martin Holladay | | #3

    Panasonic will sell you a fan in almost any size you want:

    Panasonic 190 cfm fan

    Panasonic 290 cfm fan

    Panasonic makes more powerful fans, too, if you want even more cfm.

    But I'll repeat: From an energy-efficiency standpoint, a larger diameter duct is a better solution than a more powerful fan.

  4. wayno_from_vt | | #4

    As always, great stuff.

    For the sake of simplicity, I was planning to use 4" pre-insulated flexible plastic duct for my basement bathroom fan exhaust. Will help with noise and heat transfer. It's a 110CFM Whisper Warm Panasonic. It's a straight shot run of ~8 feet, with a slight up/down jog of 1/2" or so over the PEX water lines and vented through the rim joist ~2 feet above grade on the south side of the house below the eaves.

    Understand that straight, smooth ducts are more efficient.

    Could I use this on the exhaust to keep snow out? Seems like the run is so short, the pressure drop this causes may be OK.


  5. Expert Member


    What is it about the exhaust that makes you think that snow will get into the vent terminations usually used for bathrooms?

    1. wayno_from_vt | | #6

      Hey Malcom;

      The vent'll only be ~1 ft above grade (I mistakenly said 2 ft before), so was more concerned with snow accumulation possibly blocking it.

      It's below the eaves where we have solar (south side). Usually, any sudden clearing of accumulated snow on the solar lands a solid 3-5' from the house.

      The wind can whip around the SW corner, which is about 6 ft from the vent hole. I have not noticed large drifts. The NW of the house is wide open to the prevailing winds = not ideal.

      Our house is oriented nearly aligned with the compass points, with the gable ends facing east and west.

      - Wayne

      1. Expert Member
        MALCOLM TAYLOR | | #7


        The only knock I've heard about those Lambro vents is that the plastic breaks down after a few years. If you locate the connection so replacement isn't difficult, I don't know why it wouldn't work.

        You might want to run the vertical pipe up a couple of feet outside before putting the termination on. It won't look great, but may help with the snow.

        1. Expert Member
          BILL WICHERS | | #8

          Sometimes a “U” shaped vertical guard, open on both ends, placed on the wall over the vent can help with snow too — especially drifts. The guard will also keep UV from the sun off of the vent which will probably stop any issues with breakdown of the plastic parts. It won’t be super attractive, but at least it’s somewhat streamlined in appearance due to the smooth surface and no formed edges.


  6. wayno_from_vt | | #9

    Yeah. It's right in the front of the house. Ugh. Good news is we have some greenery that helps in the summer. I might just use the vinyl J-box from the siding supplier and see what happens this winter.

    I'm expecting the fan to get little use this first winter, so can see how it really performs.

    Thanks all.

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