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I'm installing vent fans in both bathrooms on 2nd story of my two story house. Open attic access. Instead of venting to the roof or through the gable which requires risky ladder work, Wanted to vent through the soffit. The soffit is made of solid wood trim no existing soffit vents, So I think I avoid the issue of having any moisture seep back into the attic, right? My roofing system only has two gable vents and one ridge vent.

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IMO the easiest place to install a vent in an open (unfinished) attic is a gable. You can do almost all the work from inside without crawling around and reaching into invisible soffit space.

But ... if your soffit is much safer to access from outside than your gable, that's a great argument for using it. A winning argument. Do it!

Properly installed, the soffit vent moves moisture through a pipe into the outdoors. There is no special path from there back into the attic. I've heard the allegation before but I don't understand it. Just as with a roof or gable vent, a relatively small amount of moisture gets pushed forcefully into the vast outdoors where it mixes with outdoor air.

  1. What special path does the air from a soffit vent have, back into the attic, that the air from a gable or roof vent does not have? None, I say.
  2. Even if there was some path through alternate dimensions of space and time back into the attic .... how much of the moisture from the bathroom will remain in that air after it has been expelled at speed from the vent and mixed with outdoor air? Effectively none, I say.

Just go for it.

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    "There is no special path from there back into the attic." If your soffits are vented, that's the path. Built right, air is supposed to flow into the attic through the soffits (the lowest part of the attic) and exhaust through the ridge or gable vent.
    – SteveSh
    Aug 18, 2022 at 13:25
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    OP's soffits are not vented. But it's a good point. If your soffits are vented in places, my answer doesn't change much. If they are fully vented (perforated entire length) .... I've never had to deal with that but I think I'd still rely on the second half of my answer ... once mixed with outside air the relative amount of moisture re-introduced into the attic is trivial, and could find its way there almost as easily from a roof vent. Still it's a more interesting question.
    – jay613
    Aug 18, 2022 at 13:31
  • @jay613 moved comments into an answer
    – Armand
    Aug 18, 2022 at 20:28
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Two major considerations here:

  1. The air exhausted from a bathroom is likely to be warm and moist; the warmth will cause it to rise once outside the vent, where some of it will hit any structure above it, like the underside of the roof edge in the case of this soffit vent. Water will condense there; afaik soffits are not intended to have repeated direct water exposure, so some sealing or special paint may be needed. Perhaps a short 45 degree duct corner on the outside directing the vented air away from the house? Keep an eye on this area in cooler weather to see if any/much condensation forms.

  2. As @SteveSh notes, a properly built roof will have at least as much unobstructed air intake area down low (e.g. soffit vents) as exhaust area near the top (ridge and gable vents), so cooling air can flow up the underside of the roof, reducing roof temperature and extending its life.

  • If OP's attic indeed has zero air intake area, I would think a priority should be to add a sufficient unobstructed area of soffit intake vents to more than match the total unobstructed area of the current gable and ridge vents. Their locations can be coordinated with those of the bath vents so no soffit intake vents take in air from the soffit exhaust vents; perhaps a vertical panel between the two types of vents will be needed to minimize sideways flow under the roof edge.

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