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During my latest foray into the attic, I discovered that my bathroom exhaust fans vent directly into the attic. I'm worried about moisture buildup and I'm considering routing the exhaust to an external vent. How major of an issue is this, and which type of vent is best?

From my perspective, a soffit vent is going to be easier to do and doesn't have the potential to allow rain to leak into my attic. Is there any strong argument for a roof vent?

I found this article explaining a basic how-to of each procedure, but it doesn't give any pros/cons for each one.

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  • There is a flap on most vent housing units that prevents air from back flowing in from a soffit.
    – user11182
    Commented Jan 20, 2013 at 18:43

2 Answers 2

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Soffit

Pros

  • Vent easier to install
  • In heavy snow areas, not likely to be blocked

Cons

  • Soffits are designed for intake, so it's possible the air will be sucked back into the attic
  • Moist air can collect under the soffit

Roof

Pros

  • Heat and moisture want to rise
  • Keeps moisture away from walls, windows, siding

Cons

  • Can be blocked with heavy snow
  • More difficult install

I don't think it's that difficult to install a roof vent - this is a pretty good video

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  • +1: I hadn't considered that soffit is for air intake, so the moist air might end up in the attic anyway. Also, great video resource!
    – Doresoom
    Commented Oct 26, 2010 at 17:14
  • Won't the exhaust heat melt the snow buildup?
    – Joe Phillips
    Commented Oct 26, 2010 at 17:33
  • Snow buildup isn't really a concern for me - I live in Alabama.
    – Doresoom
    Commented Oct 26, 2010 at 17:42
  • @Joe I wouldn't worry about the snow thing in most cases, but my brother tells me some states require an install above the roof snow line. May or may not affect the placement and make a soffit vent an alternative. Also can't really speak for places that live under feet of snow, I know they've got exchangers for stoves and whatnot, not sure how bathroom exhausts are handled.
    – Steve Jackson
    Commented Oct 26, 2010 at 18:09
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    Little late to the party here, but if you put a p-trap in the line, it won't allow heat to escape through the roof vent... (unless, of course, the bathroom fan is on.)
    – Michael
    Commented Jul 26, 2011 at 7:50
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The natural flow of air in an attic is going to be pulling air in through the soffit vents and out through the roof vents/ridge vent.

The problem I see with exausting the air through a soffit vent is that the natural flow is going to tend to pull that moist air right back in through the soffit vents and into the attic again.

Given that the air flow through an attic is mostly based on convection and probably pretty slow, this may not be an issue in reality, but it would certainly make me think twice about doing it.

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  • +1: Great answer! Steve just barely beat you to the answer, so I'm accepting his.
    – Doresoom
    Commented Oct 26, 2010 at 17:15
  • I always question how much moist air could be sucked back into the attic through a soffit vent. As you mention, the velocity of air into the soffit will be low. That with the fact that the vent fan is actively blowing the moist air away from the soffit would seem to combine to let very little moist air back into the attic.
    – hazzey
    Commented Apr 21, 2016 at 18:28

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