Do Not Mix Attic Exhaust Vents

One of the most challenged and disagreed with tips that we offer the roofing industry is this: do not mix different types of attic exhaust vents. Agree with it or not, mixing exhaust vents is bad news.

There are five types or categories of attic exhaust vents on the market:

• Box vents or off-ridge vents (also called roof louvers or static vents)
• Wind turbines
• Ridge vents (4-foot stick or rolled)
• Gable vents
• Power fans (roof-mount & gable mount; traditional electric or solar powered)

The guidance from attic ventilation manufacturers is pretty direct and clear: Do not mix any two of those five types on the same roof above a shared, common attic. Speaking strictly on behalf of AirVent, this guidance is written inside the installation instructions for each of the five types of attic exhaust vents we make.

Building Code clearly states, “Ventilators shall be installed in accordance with the manufacturer’s instructions.” It’s inside chapter R806 of the International Residential Code (IRC).

I am the chairperson of the Ventilation Task Force under the umbrella of ARMA—the Asphalt Roofing Manufacturers Association. ARMA represents the industry’s shingle makers. The Task Force I chair includes the various vent makers on the market. Together we say: do not mix attic exhaust vents.

ARMA published a Technical Bulletin titled Considerations in Attic Ventilation System Selection which the Ventilation Task Force helped write. The Bulletin says the following: “Combining different types of exhaust vents on the same roof above a common attic space may cause short-circuiting of the attic ventilation system and adversely affect performance. Using different vent types together often is inconsistent with the vent manufacturer’s installation instructions.”

What catches my attention immediately—after the fact the vent makers say do not do it—is this statement… “it may adversely affect performance.” Why do anything that may adversely affect performance?

The Science behind the Caution

Here’s the science and rationale behind do not mix different types of attic exhaust vents:

Balanced attic ventilation or airflow is 50% intake vents and 50% exhaust vents. The intake vents are installed low on the roof’s edge or in the soffit/overhang. The exhaust vents are installed high on the roof at or near the peak of the roof. That one/two combination of intake and exhaust allows the air to enter the attic at the lowest possible location, flow through the entire attic low to high and exit through the exhaust vents. Along the way the air is flushing out any potentially damaging heat buildup and moisture buildup throughout the attic. The airflow starts low and exits high. And there are no airflow openings in between. None.

Airflow dynamics is such that air always follows the path of least resistance. Air is constantly on the lookout for the easiest path it can take from Point A to Point B. That means, in our well balanced vented attic, the path of least resistance the air will follow will be the distance between the intake vents low on the roof (or in the soffit/overhang) to the exhaust vents high on the roof at or near the roof’s peak. Perfect.

But if we were to mix a second type of attic exhaust vent into the picture…

•          Let’s say wind turbines mixed with box vents or off-ridge vents.

•          Or let’s say a gable vent with ridge vents.

•          Or let’s say ridge vents with either a roof-mount or gable-mount power fan…any combination of the five different types of exhaust vents…

…if we do that, it short-circuits the airflow meaning the path of least resistance is now the shortest distance between vent openings.

•          That’s the distance between the wind turbines and the box vents or off-ridge vents.

•          That’s the distance between the gable vent and the ridge vents.

•          That’s the distance between the ridge vents and the roof-mount or gable-mount power fan.

When different types of attic exhaust vents are mixed, the airflow is concentrated in the upper region of the attic failing to vent the lower half of the attic efficiently. Consequently, the potentially damaging heat and moisture buildup is not being vented away as it would if the attic was properly balanced with intake and one single type of exhaust.

Eyewitness Accounts from Roofers

In one of our podcast episodes I interviewed nine roofing contractors who shared what they eye witnessed after stepping into an attic that had pre-existing mixed types of attic exhaust. Here are a few of the problems they reported.

•          A running power fan pulled air in through the ridge vent from the outdoors causing it to become clogged with debris and allowing weather into the attic through the ridge vent along the way.

•          Uneven aging of the shingles and the roof decking because the majority of the airflow is concentrated between the two types of attic exhaust vents leaving other areas of the attic and roof with inadequate airflow.

•          Snow, rain, Mother Nature in general entered the attic through the second type of exhaust vent that has suddenly become an intake vent for the primary types of exhaust.

All three of those situations are bad news. The podcast episode is titled “2nd Most Common Attic Ventilation Mistake” because it literally is the 2nd most frequent mistake we see in attic ventilation across North America. I think hearing the roofers tell you what they witnessed is very compelling.

If you’re wondering why this tip “don’t mix attic exhaust vents” is often challenged or questioned, here are the two reasons told to me most often:

•          Reason #1: “Long before I ever heard that mixing exhaust vents was problematic, I was doing it and I have not seen any consequences.” While that may be true for those particular roofing contractors—knock on wood—I have witnessed the consequences and I have talked to roofing contractors and homeowners across North America describing the resulting damage and problems. Does it mean the problems will happen all the time? No. Is it worth rolling the dice to find out? I don’t think it is.

•          Reason #2: “I thought the attic could use more airflow so I added another type of exhaust in addition to what was already there thinking an exhaust vent is an exhaust vent and the type really didn’t matter.” An exhaust vent is not just an exhaust vent. The various types deliver performance based on their design. Mixed together with other types, they cannot deliver that performance. A power fan has a motor keeping it running. When the motor shuts off, the power fan is a box vent. Box vents rely on thermal effect or warm air naturally rising. Wind turbines spin with the wind to pull more air. When they are not spinning, they are a tall box vent. Externally baffled ridge vents with their “winged design” use the slightest wind speed to pull air out of the attic by creating low pressure above the vent openings similar to what gives lift to an airplane. And finally gable vents are an outlet or opening on the upper side of the house. The bottom line here: There are differences between each category. Stay within the same category per roof above a common attic.

You can find the ARMA Technical Bulletin Considerations in Attic Ventilation System Selection on ARMA’s website asphaltroofing.org. You can also find it on our website gibraltarbuildingproducts.com.