What is Proper Ventilation?

Proper ventilation requires adequate air flow through the attic space, and it can be achieved naturally. Warm air rises; cool air descends. A natural ventilation system takes advantage of this natural convection by allowing cool air to enter the attic under the eaves and warmer air to exit the attic at the ridge. Wind also affects this system by forcing air into the attic under the eaves and drawing air out at the ridge.

Through a thoughtfully engineered combination of air intake and exhaust vents, natural ventilation should be planned into the home design. Intake and exhaust can be achieved in different ways, but according to the U.S. Department of Energy and Habitat for Humanity, “a combination of continuous ridge vent along the peak of the roof and continuous soffit vents at the eaves provides the most effective ventilation.”

The formula generally used to achieve adequate ventilation requires approximately:

50% intake under the eaves and 50% exhaust at or near the peak of the roof. Ideally, the system should allow more intake than exhaust. Both intake and exhaust are measured in Net Free Area (NFA), which refers to ventilation space, or the total amount of unobstructed area through which air can pass through a vent. Most building codes specify NFA for proper ventilation.

NOTE: In cooler climates, if a vapor barrier is used, the formula for achieving minimal ventilation is 1 square foot of ventilation space (NFA) for every 300 square feet of attic floor space. The amount of ventilation that is used on the roof then mandates the amount (NFA) of ventilation that is required under the eaves in the soffit. For example, if the average attic requires approximately 22 square feet NFA of exhaust on the roof, a soffit profile with at least 11 NFA on both sides of the house is necessary to achieve a combined NFA of 22.

Soffit ventilation is a dynamic system that has to work in conjunction with your roof vent strategy, and these specifications should be considered minimum standards and might not be sufficient for healthiest indoor air quality, since they were written before the current trend towards tighter house construction. The presence of vents in soffit can only guarantee proper air flow through the attic if the system is properly engineered. If a home’s vented soffit has less NFA than the roof vent system, it can actually prevent attic air from escaping.