Can I vent a bathroom through the soffit if I use a product designed for it?

In the past I was a big fan of soffit vents. I figured if you used quality vents with a good back-draft damper, and sealed and insulated the duct well, it was a great way to vent bathrooms. There are several key benefits:

• First and foremost, it avoids penetrating either the roof or a gable wall, so it eliminates any risk of liquid water intrusion from wind-blown rain or unsheltered gable walls. However problematic humid air is for an attic, I think liquid water is an order of magnitude worse..

• It also allows for short efficient duct runs, keeps the duct low (in between the floor joists), and therefore well underneath blown in insulation (so you can use rigid duct and avoid the need for separate insulation).

• Finally using a soffit vent sort of creates a thermal trap- warm air from inside the duct wants to rise not sink, so there should be less heat loss when the exhaust fan isn't running. Even with dampers at the fan and the vent, those dampers are certainly not airtight. Often the end of the duct goes up a little bit before going down and out the soffit vent, so that only enhances this thermal trap.

However despite all these benefits, which I think are significant, I'm starting to reconsider whether soffit venting is generally a good idea. I've done it this way in the past and I'm a little embarrassed that I never even considered the fact that a substantial portion of the exhaust air could get sucked right back into the soffit vent or adjacent soffit vents.

I do think that closing off the soffit vents (into the attic) in the joist bay that you're venting from, and one on either side, would greatly mitigate this issue. The moisture should diffuse into the surrounding air very rapidly, especially when there is even the slightest breeze. As will making sure that the underside of these soffits is painted or otherwise moisture resistant. I also think it's important to realize that this is just bathroom air we're talking about, not a dryer vent, so while the air will be humid during showers it's not like we're blowing clouds of steam..

Also keep in mind that the whole point of soffit vents is to allow air to constantly wash along the underside of the roof decking, and remove any moisture from the attic, so any moist air that gets sucked in or condensation happening should be quickly removed- not saying that this can't possibly pose a problem (especially in winter), but I think there's a lot of built-in tolerance.

So I'm somewhat on the fence. I'm leaning towards thinking venting through a gable wall is better, especially if it is protected by an overhang, but I don't think soffit venting is as horrible as some make it out to be. And I would probably choose soffit venting over making another hole in the roof any day, for retrofit especially. I also think Dave makes a lot of good points.

A vent at the soffit can be done, but so could venting directly into the attic, and both are poor choices. This is because soffits are used for air intake into your attic (and out a ridge or gable vent). If you vent too close to the soffit, the warm moist air that you're trying to get out of your home will get sucked back up the soffit and into the attic, where it will cool and condense on the underside of your roof.

OK--the UCC code specifically says:

• Section 1501.1 Outdoor discharge
– Air removed by every mechanical exhaust system shall be discharged to the outdoors
• Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.
– Exception: Whole house ventilation-type attic fans that discharge into the attic space of dwelling units having private attics shall be permitted.

BUT. BUT. BUT. What this specifically means that you cannot run exhaust INTO (inside) the soffit itself. If you do, you're dumping your exhaust inside the soffit space, not outside the building.

You certainly CAN run the vent THROUGH a soffit. Then the vent terminates outside.

So, then the argument is that your humid bathroom exhaust comes out and is sucked back into the attic by the soffit vents built into most roof/attic systems.

BUT, if you have a vent several feet away (10 feet is required for noxious exhaust, but bathroom exhaust is not considered dangerous and not subject to this), you do not have a problem, code-wise or not.

So, the final argument is that your humid exhaust will linger in the eave of your soffit/roof and eventually rot it out. The easy answer to this is getting an end piece or vent that discharges sideways away from the house. Hardware stores have these or can order them--I'm not sure I'm allowed to put a link here.

So, if you consider these and plan for it, you're good.

I personally believe that even a regular round vent in my soffit does not rot my roof during the once a day or so use for a few minutes. I think outside evaporation takes care of that fine. But this last paragraph is just my opinion. I'm much more concerned about making sure that an insect screen or the equivalent is in my vent.

I think the bigger issue here, is that DIYers see these types of products and think "Hey, I can use this to make my job easier!". The trouble is, just because they make the product does not mean it's applicable in all situations.

There may be situations where soffits are not used as vents, in which case these exhaust vents are perfectly acceptable. However, if your soffit is being used to vent the attic, you do not want to use one of these. The warm moist air will be sucked back into the attic, and you'll eventually run into problems.

One of the major difference between an amateur and a professional, is that a professional knows what products exist and when to use them. Whereas an amateur has a limited knowledge of available products, and adapts those products to meet their needs.