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I am looking to install an exhaust fan in a bathroom where one does not already exist. I've read that it is preferable to run exhaust ducts down instead of up, but does that apply here? We would be talking about 20-some feet to run an exhaust duct from the 2nd floor ceiling down to the first floor ground level. The alternative would be to run the exhaust duct up about 5 or 6 feet and out the roof fascia (seems easier to drill through wood fascia than brick wall, but that may be a separate question). I'm not even sure I can get a duct down through the wall yet, but thought I'd ask before I spend too much time figuring it out. It's an exterior wall, if that makes any difference.

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    I hadn't heard that bathroom exhaust ducts should go down; very often they go up and through the roof. Where did this information come from? Is it that there shouldn't be a low point for water to gather?
    – Daniel Griscom
    Jun 2, 2016 at 11:27
  • I guess it's really just one source: uexpress.com/first-aid-for-the-ailing-house/2014/6/4/… His logic seems to make sense, but I'm not sure if the extra ductwork involved would negate any efficiencies.
    – Jerrad
    Jun 2, 2016 at 11:53
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    I have always gone horizontal or up with the vent having a back draft damper
    – Ed Beal
    Jun 2, 2016 at 13:13
  • I think that's what most people would do (and what I was about to do). I'm not going to rip out a chunk of wall in order to run the vent down, but if it's a straight shot down to the first floor, I'm just wondering if that's a better way to go or not.
    – Jerrad
    Jun 2, 2016 at 13:30
  • Given that the exterior walls are brick, I agree that going up & out either the fascia or the roof proper is the way to go. That's how most 2nd-floor vents are set up.
    – Carl Witthoft
    Jun 2, 2016 at 14:32

2 Answers 2

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I believe what they mean by plumbing the vent downward is to make sure it has a slight slope so that moisture condensation from venting a shower will not drip water back into fan unit and cause problems.

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  • No, the article states: "The most efficient way to vent bathrooms is to do so downward through a basement or crawl space and outside through the rim joists."
    – Jerrad
    Jun 22, 2017 at 17:23
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    I may be wrong but punching a 4 inch hole through your rim joist sounds like a bad idea and as a contractor I have never seen a vent plumbed like that in either new and old commercial or residential.
    – David Moritz
    Jun 22, 2017 at 17:28
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One purpose for the vent is to remove lingering odors. Venting high has to do with peoples noses and, once upon a time, truly noxious gas; restrooms today are not as toxic as they once were. Still, if the gases are vented downward, they are less likely to be further evacuated from the area because there is less wind on the ground than in the sky, especially next to a wall that has at least some wind shielding.

Technically, moisture, heavier gases, and odors settle; so it's understandable that someone might think it's more efficient to move odors (etc.) downward, but the amount of power or efficiency is really negligible. A far more important factor of efficiency would be the length and size of the vent (fan and duct). In your case it's a no-brainer, 6ft vs 20ft.

But if the lengths were the same then it really wouldn't make a huge difference, besides the lingering smell.

edit- Historically, (before 1900) fires were lit/used to create a thermal draft (perhaps this is the true source of lighting a match when making a stink, or why candles are used in the bathroom... it's a cultural memory). Exhaust was pulled through vents via a stack effect. The stacks were then powered by fans when electricity became common. Source: http://aceee.org/files/proceedings/2004/data/papers/SS04_Panel7_Paper21.pdf

Impetus (handed down) is one reason that the exhaust vent goes up, but the fact is, it still usually works better to go up for the other reasons mentioned above.

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    Be careful with the term "vent" when it comes to bathrooms. It could be taken to mean plumbing venting, which must go up because the venting for plumbing must be above the water level of the highest fixture in the house as well as to ensure that sewer gasses which will come through the vent are released well above people. A bathroom exhaust fan (room air only) is not likely to carry ANY toxic gasses.
    – The Evil Greebo
    Jun 10, 2016 at 17:06
  • @TheEvilGreebo Once upon a time it was toxic... glad that's changed. Actually, the vent could still be considered a failsafe, (if things aren't working).
    – Ben Welborn
    Jun 10, 2016 at 17:14
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    Again, are you sure you're not confusing plumbing venting with room venting?
    – The Evil Greebo
    Jun 10, 2016 at 17:30
  • @TheEvilGreebo Ivent havent novent ideavent.
    – Ben Welborn
    Jun 10, 2016 at 18:02
  • Moisture rises. Thus I'd prefer to have vents in the roof, as moisture would be the main reason for ventilation...
    – vidarlo
    Dec 25, 2017 at 19:18

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