Attic Ventilation: It’s All About Balance


Inadequate attic ventilation leaves you vulnerable to condensation — and therefore rust, rot, and mold. Too much ventilation, on the other hand, can leave you with exactly the same problem. So how do you find a balance?


There are two basic elements to an attic ventilation system: intake and exhaust.


  • Intake
    Perforated soffit panels allow fresh air to enter the attic from outside. Soffit panels sit at the roofline — between the gutter and main wall — under the protection of the soffit overhang, which is usually between 12 and 18 inches wide.
  • Exhaust
    Just as your attic inhales, it must exhale. An exhaust vent — a ridge vent — is usually installed in a 2.5-inch cutout at the peak of the roof or a powered attic fan.  Ridge cap shingles are applied on top of the ridge regardless.


In a healthy attic, air flows in beneath and around the roof sheeting (which can be half-inch CDX plywood or cheaper OSB wafer board) and escapes through the exhaust vent above. As Mr. Miyagi told Daniel-Son in the “Karate Kid”, balance is everything.


The Ventilation Sweet Spot


Sometimes there isn’t enough ventilation. For example, solid soffit panels — or panels that are appropriately vented but are blocked by improperly installed insulation — restrict airflow into the attic. This can be fixed with a cardboard-and-styrofoam baffle, which can create continuous airflow into the attic.


But more ventilation isn’t always better, either.


For example, when an attic fan is mounted on the roof, usually 2 or 3 feet from the ridge vent, it can draw air from the vent instead of from the attic floor. This creates circulation only at the top of the attic, also known as ‘shortcircuiting’, which leaves the air at the lower end stagnant and susceptible to moisture buildup.


Gable vents can be another example of over ventilation. In a typical 2-story home, gable vents — which are about 2 feet tall and 1 foot wide — were commonly installed 2 to 3 feet down from the roof peak. If a ridge vent is installed without first removing or shutting down gable vents, a shortcircuit can result.


15 years ago — way back in the Stone Age of the late 1990s — builders and engineers didn’t have this knowledge. But as time has passed, standards have improved and modern homes enjoy a uniform ventilation ratio set by the Federal Housing Administration — 1:300 with a vapor barrier and 1:150 without.


The Moisture Menace


Whether you know it or not, your home has moisture in the air — and that moisture always migrates to the attic. Moisture comes from a lot of places: from cooking, from watering plants, from fish tanks — your body alone emits about a gallon of water into the air every day.


But some home designs go out of their way to make it worse. For example, all too often, exhaust fans from bathrooms lead directly into the attic. A simple flex line leading straight out of the roof fixes that problem before every mold spore in town gets the invitation. Until recently, it was standard homebuilding practice to actually direct warm, moist bathroom exhaust out through soffits.


Soffit for intake, Daniel-Son, never exhaust!

Attic ventilation is arguably the single most important element in roofing today — and one of the trickiest to get right. There is an endless stream of incorrect information available online, and even if your contractor gets everything perfect, an HVAC system can create humidity that a ventilation system can’t handle. See the MTS Sheffield professionals for further details on getting your attic breathing right.