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
