Do your exhaust vents terminate at your soffits?

The building codes specify that exhaust fans shall not terminate in the attic or soffits. While there are those that argue “at the soffits” is not the same as “through the soffits,” I think it pretty much amounts to the same thing.

The reasons we do not want to vent warm moist air into attics is well known and documented. It can lead to mold growth and other four letter words.

So what exactly does the code say:

M1501.1 Outdoor discharge. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.

That seems straightforward enough, and since “aimed at the soffit” is still in the attic, my personal opinion is, when the code says “Shall not be exhausted into a …..soffit,” they are meaning “through the soffit” as well.

So let’s forget about the code, and let’s see if my opinion can be supported by building science.

Wow, do we really have to go to “science?”

What are the building conditions that would come into play to sort this all out? Typically, or at least most of the time, in a properly vented attic, the attic space is under negative pressure relative to the higher pressures at the soffits and at the ridge. Because of this, air is attempting to push its way into the negative air space to make balance–24/7. All air in the vicinity of the soffit vents is forcing its way into the vents.

Now lets place a bathroom exhaust fan vent right at the soffit vents.

We are exhausting warm, wet, buoyant air that it is already moving upward and increasing the pressure in the area of the soffits. This increase in pressure difference between the attic space and the soffits makes that warm wet air work even harder to get into the attic.

In this picture, you can see evidence of where corrections have been made of the four vents that terminated too close to the soffit. The opening have been covered over, and hopefully they now terminate properly through the roof.

The staining on the siding above the lower vents is consistent with the buoyancy of the air from the vents.

Staining above the soffit vents on the underside of the roof sheathing is consistent with the upper vent’s previous termination at the soffits.

I think the codes need to clean this up a bit and require minimum distances to vented soffits.  Any current guidelines are at best “vague.”

Until then we should resort to good sense.

Charles Buell, Real Estate Inspections in Seattle


Limp as a Geoduck at low tide–it can be exhausting work at times fighting the tides!!

I have posted many times about bathroom exhaust vents improperly terminating in attics.

When we first started installing mechanical ventilation in bathrooms, the requirements for where they terminated, merely stated that they needed to be ducted to the “exterior.” All around the country installers have assumed that the attic was “outdoors,” in terms of a place to terminate these vents.

We have learned over time that the attic is not only NOT outdoors, but that the attic can be a very bad place to vent a lot of warm moist air. This became even truer as we stopped “heating” our attics (because there was little to no insulation).

Here we can see a couple of bathroom vent ducts lying on an Attic floor like a couple of relaxed Geoducks at low tide.


Can you see where moisture has been staining the plywood at the ends of the ducts?

The roof sheathing can get very cold and moisture can and will condense on these cold surfaces. It will also contribute to ice-damming in areas prone to that problem.

It is not uncommon to see staining of the wood and even mold or mold-like fungal growth present in the area where the vents terminate, as can be seen at this vent terminating near bird-blocking.


Can you see where moisture has been staining the roof sheathing?

Inspectors will point out vent ducting that is aimed at gable vents, roof vents, and soffit bird-blocking.


The vents actually may work for a while, but these vents have their own job to do and should not be compromised by stuffing vent pipes in them. The screens at these vents will become clogged with lint rendering them less than fully functional for either their own job or the job of the exhaust fan vents.


Can you see the staining around the nail points from excessive moisture in the attic?

Sometimes there is no duct at all and sometimes the duct runs out through the soffits.


Venting out through soffits is not a good idea as the moist air can just find its way back into the attic if there are any other soffit vents or bird blocking nearby.


This is a new installation and already there is some staining of the wood above the vent.

If you look back at the second picture, where the vent terminates in the attic at the bird-blocking, and then look at the following picture, you can see the kind of staining and mold or mold-like fungal growth that is common in the overhangs of homes that have vents terminating at the bird-blocking.


In time the screens will plug up and more of the moisture will then stay in the attic.  Sometimes this staining tells me the vents terminate at the soffits even before I actually see it in the attic.

Generally speaking it is best to run the vents through the roof or through a side wall to a proper cap with a back-draft damper (as is required in most areas that are complying with Energy Codes).

Now wasn’t that exhausting?


By Charles Buell, Real Estate Inspections in Seattle

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Is your attic “exhausted?”

There was a period of time when the attic was considered “outdoors,” and in a sense that was true, in that the attic space does not “communicate” in any way with the house interior (if only this were actually true).  Unfortunately that is in an “ideal” world.

Lint beginning to clog a roof vent

Lint beginning to clog a roof vent

There are many ways the house/attic barrier is compromised:  missing access hatch weather-stripping, missing fire-stopping/draft-stopping, can lights, crappy insulation that does not provide an adequate air barrier around junction boxes, and exhaust vents that terminate in the attic, are some of the main ones.During that period of lack of awareness, many builders would terminate exhaust fans in the attic because they thought of this space as “outdoors.”  The reality is that it is only “partially” outdoors–a sort of “ventilation purgatory” where things may or may not be OK depending on how the offending component behaves–kind of like the more infamous purgatory I am told.

Another thing that makes the attic NOT outdoors is that its own little micro-climate is created when it is heated by the sun hitting the roof, when cold snow piles up on the roof, and when winds blow by the roof  creating a negative pressure on the attic.  As a Seattle Home Inspector, I routinely recommend that exhaust fans be properly vented to the true exterior of the building–instead of into the attic.  These vents carry moisture laden air into a potentially cold place where the moisture can condense on cold surfaces.  This is often responsible for staining of sheathing, rusting of metal components and it can result in decay/rot and mold growth on the wood roof structures.

Dryers vented into attics can actually do considerable damage to the roof structure–besides creating a fire hazard with all the lint.Many builders would attempt to “improve” this approach by merely aiming the vent pipe at one of the screened attic roof vents–not realizing that over time these screens would plug with lint and result in the roof vent becoming non-functional for the job it was put there to do (vent the attic) and also create a baffle to redirect the exhausted air back into the attic.  The following pictures are excellent examples of just how bad this condition can get.

Roof vent clogging device

Roof vent clogging device

Can you see the staining and rusting around the roofing nails that stick through the sheathing?  In this case, all of the home’s exhaust fans terminated in the attic and contributed to more moisture than the roof venting system could adequately vent away.  The one pictured is a normal everyday bathroom exhaust fan–not a dryer vent–as the amount of lint might make one think.

All exhaust fans from the interior of the home should be ducted directly to the exterior of the home to a cap with a proper back-draft damper.  Current regulations also require insulation on the duct work.


By Charles Buell, Real Estate Inspections in Seattle


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