We covered why ventilation matters in the Roof Maintenance series. This article gets into the actual components: what they are, how they work, and the differences between the options.

BHR crew installing a new roof in Layton showing the ridge line before ridge vent and ridge cap shingles are applied . ridge vent runs the full length of the peak for uniform attic exhaust . Bob Harvey Roofing

The exhaust side

Ridge vent. This is a continuous vent that runs along the peak of the roof. It is covered by the ridge cap shingles, so from the ground it is virtually invisible. Ridge vent provides the most uniform exhaust ventilation because it runs the entire length of the ridge, allowing hot air to escape evenly across the attic. This is what we install on most homes.

Box vents (static vents). These are individual vents, usually square or round, installed near the ridge of the roof. Each one covers a specific area of attic space. They are passive, relying on natural convection rather than any mechanical process. Box vents work, but they provide less uniform exhaust than ridge vent and require multiple units to cover the same area.

Turbine vents (whirlybirds). You see these spinning on roofs around Utah. They use wind to create suction that pulls air out of the attic. They move more air than static box vents when the wind is blowing, but they do nothing on calm days. In Utah, where we get wind regularly, they can be effective, but ridge vent is still the preferred option for most installations.

Powered attic fans. These are electrically powered fans that actively pull air out of the attic. They move a lot of air, but they can create problems. If the attic is not properly sealed from the living space below, a powerful attic fan can actually pull conditioned air from your home into the attic, increasing your energy costs. In most cases, passive ventilation with proper intake and exhaust is more effective and less problematic.

The intake side

Soffit vents. These are vents installed in the soffit, the underside of the eave overhang. They allow cool outside air to enter the attic at the lowest point, which then rises through the attic space and exits through the exhaust vents at the ridge. Soffit vents come in continuous strips or individual vents.

The intake side is where most ventilation systems fail. Either the soffit vents are too small, there are not enough of them, or insulation inside the attic has been pushed up against the soffit area, blocking the intake. Without adequate intake, even a perfectly installed ridge vent cannot do its job because there is no air entering the system to be exhausted.

Baffles. These are channels installed between the rafters at the eave end of the attic. They keep insulation from blocking the soffit intake and maintain a clear air path from the soffit vent into the attic space. Baffles are inexpensive and should be installed in every rafter bay that has a soffit vent. We install them on every job.

"Do not mix exhaust vent types on the same roof plane. Ridge vent combined with box vents will short-circuit the airflow . pulling air in through the box vents instead of from the soffits."

The balance equation

The system needs balanced intake and exhaust. The general rule is 1 square foot of net free vent area for every 150 square feet of attic floor space, split roughly evenly between intake and exhaust. Going heavy on one side without the other creates problems.

And one important rule: do not mix exhaust vent types. If you have ridge vent, do not also have box vents or turbine vents on the same roof plane. They can short-circuit the airflow, with the ridge vent pulling air in through the box vents instead of from the soffits.