A frequent question we field throughout the roofing industry is: Can I use attic exhaust vents, such as box vents, low on the roof as attic intake vents? It is OK to use attic exhaust vents such as box vents—
also called can vents, pot vents, turtle vents, roof louvers— low on the roof acting as intake vents as long as the vent manufacturer tested the vent for that application. That’s the key: Has the vent been tested for use low on the roof?
Before we explore what that testing looks like, let’s dig into why someone would want to use an attic exhaust vent as an intake vent in the first place.
Some municipalities have code requirements calling for the attic intake vents to be more than 10 feet from the lot line. “Zero lot line homes” are being built with the edge of the home five feet from the lot line. In those cases, traditional intake vents such as rectangular undereave vents, soffit vents, vented drip edge, and our roof-top “edge of roof” style intake vents will not meet the requirement of being more than 10 feet from the lot line. But an attic exhaust box vent positioned low on the roof—but not at the edge of the roof— would satisfy the 10-foot requirement.
Water Entry is the Biggest Worry
The concern with using exhaust box vents situated low on the roof has been water entry. In a rain event, the volume of water and the velocity of the water traveling on the roof before hitting the vent is different when the vent is high on the roof (exhaust) vs. low on the roof (intake). There’s also the wind speed and direction to contend with. Installed as exhaust a few feet from the roof peak, every style of box vent (round, square, slant-back) safely handles the volume and velocity of rainwater. That is not the case when those vents are installed low on the roof. Vent shape and design come into play more significantly as the volume and velocity of rainwater increases.
We explored this very topic with Colby Cruz, product design engineer at Dallas-based AirVent, in a recent podcast episode. “The biggest concern is the accumulation of runoff water from the ridge of the roof,” says Cruz. “When an exhaust vent is installed close to the ridge there’s not enough time and not enough roof frankly above it for the water to accumulate and start to pool up. But once you allow space between the ridge and the placement of the top of the vent—such as using an exhaust vent low on the roof as an intake vent—water can pool up and start to gain momentum and volume. This is why vent geometry becomes far more critical when used as an intake vent.”
Round, Square and Slant-Back
In our testing, our attic exhaust box vents with a slant-back design handled the water on the roof—as opposed to the square design and the round design box vents.
“The slant-back profile creates a nice natural deflection surface for that water runoff,” says Cruz. “Imagine a high volume of water flowing down the roof. As soon as it hits that slant-back part of the vent’s dome it’s almost like a reverse drip edge. The water hits that angled portion of the vent and it’s directed away from the vent instead of directly into the vent like non-slant-back designed vents. As a result, you’re relieved of a lot of water volume potentially hitting the opening in the roof deck.
“Another nice feature of most our slant-back vents we tested is that they are either fully enclosed or almost fully enclosed. This means the slant-back dome just about covers all of the possible entry points from the base flange of the vents. This especially comes into play with omni-directional, wind-driven rain situations when the rain is coming from the side of the vent, the back of the vent, even from the eave of the roof itself. You’re getting some geometry there that deflects the water and protects the opening in the roof.”
The last line of defense with the slant-back design is the base with a tall, high collar. “If any water should get past the enclosed portion of the vent, the high collar is still protecting the opening in the roof,” says Cruz. “It’s the combination of all of these design features that lend themselves nicely for using box vents low on the roof.”
Put to the Test
Our in-house testing followed the Miami-Dade TAS-100(A) test standard to calculate a simulated rainfall rate which employs a standard rate of 8.8 inches per hour. We chose a more aggressive rate of 17.7 inches per hour exceeding the standard with winds up to 110 MPH. All in-house testing validates the use of these vents as attic intake vents from 3/12 to 12/12 roof pitches (Models SLP61, RVG55, SLA, RVGL50, and RVAL50).
“We’re not an official lab, of course, but this more aggressive testing gave us confidence that the slant-back design can be used low on the roof as intake,” Cruz says. “The total amount of water allowed to accumulate and infiltrate into the roof deck opening per Miami-Dade is 0.05%. We made sure to follow that standard even with the more aggressive rate of rainfall. I am fully confident in these vents in this application. My additional guidance to the industry would be to check your local building code requirements.”
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ABOUT THE AUTHOR: Paul Scelsi is marketing communications manager at AirVent, Dallas, and leader of its Attic Ventilation: Ask the Expert seminars (gibraltarbuildingproducts.com). He hosts the podcast, “Airing it out with AirVent,” and he’s the chairperson of the Asphalt Roofing Manufacturers Association Ventilation Task Force. For more information email [email protected].
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