How to properly vent a bathroom fan

oklahomagregJanuary 30, 2011

During my home inspection the inspector noticed that the builder (years ago) had vented the bathroom fans into the attic but not out the top of the roof. It looks like just regular dryer type venting, not insulated. So, I need to know how to properly vent it. I find conflicting advice on the net. I've seen things that say that if it's a short distance from ceiling to roof vent hole that I can use the uninsulated pipe but that if its a longer distance then I should use insulated pipe. But, aren't vents supposed to be on the upper 1/3 of the roof (why is this)? I've also seen one place that recommended it be vented out the soffit. There is no problem currently with any condensation dripping back into the house and I don't want to create one.

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It's better if the duct in the unconditioned attic space is insulated, it'll help the evacuated air stay warmer and help prevent held moisture from condensing on the walls of the pipe and running back down into the fan housing during cool seasons. Again, condensation may not be a problem in your locale, with cold attics condensation is more of an issue.

There really aren't any code restrictions on locating a bathroom vent termination hood. Except that they can't vent into an attic or into a crawlspace. Plumbing vents, yes, there are restrictions. Pot/attic vent outlets, there are guidelines for those too.

I'm in New England, with snowfall and roof penetrations in mind, personally my first choice is to vent out an attic gable wall.

If you're going out the soffit, do not let the duct be a blind termination behind perforated soffit material. You can go through the soffit, there are termination caps designed for this. One caveat with soffit venting is that it can sometimes create a mold/mildew build-up on the sidewall of the house and on the underside of the soffit, and the evacuated air can be pulled back in to the attic if you have soffit-to-ridge vents. That's not as big of a deal with bathroom vents as it is with clothes dryer venting, but I thought I'd toss it out based on the stuff I've seen over the years.

If you need to go out the roof, that's a common location, no worries there.

Your bath vent has a certain CFM air movement rating, it's based upon a certain amount of resistance as the air moves through the exhaust duct. If you use ribbed flex duct, if you have too long a length, or too many turns in the dict, the resistance increases and your CFM decreases. So short and straight works best.

    Bookmark   January 30, 2011 at 11:03AM
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