How many air changes per hour does attic ventilation need?
The fan sizing rule
The Home Ventilating Institute's guidance for powered attic ventilators is to multiply the attic floor area by 0.7 to get the fan's airflow in cubic feet per minute, which is roughly 10 air changes per hour for a typical attic. For dark shingles or steep roofs, it raises the multiplier by about 15 percent.
- 1,500 square foot attic: about 1,050 CFM.
- Same attic with dark shingles: about 1,200 CFM.
The part people skip: intake
A fan can only move air that has a way in. HVI's guidance calls for at least 1 square foot of intake net free area for every 300 CFM of fan capacity. For the 1,050 CFM fan above, that is 3.5 square feet, or about 504 square inches of soffit vent area. Without it, the fan pulls replacement air from the house through ceiling gaps, light fixtures and the attic hatch, which wastes cooled air and can even backdraft gas appliances.
Why passive vents use a different yardstick
Passive vents move air with wind and rising heat, so their flow changes hour by hour. The residential code skips air changes entirely and requires net free area of 1/150 of the attic floor, or 1/300 when the vents are balanced between high and low. A well balanced passive system on a breezy day can move plenty of air; on a still, muggy night it moves far less, and that is normal.
Is more air always better?
No. In Georgia's humid summers, a fan pulling huge volumes of outside air does little for moisture and may raise your power bill. Most homes do better with balanced passive vents and good air sealing. Compare the options in roof ventilation basics and attic heat and shingles.