What size attic fan do I need?
Short answer. A common manufacturer rule is to multiply the attic floor area by 0.7 to get the fan's CFM rating and add about 15 percent for dark roofs, so a 1,500 square foot attic calls for roughly 1,050 to 1,200 CFM. Just as important, the attic needs about 1 square foot of net free intake for every 300 CFM, or the fan will pull cooled air out of the house.
Working through an example
- Attic floor area: say 1,800 square feet.
- Base airflow: 1,800 times 0.7 is 1,260 CFM.
- Dark or steep roof: add about 15 percent, for roughly 1,450 CFM. On a large or complex roof, two smaller fans on separate slopes can work better than one big fan.
- Intake needed: 1,450 divided by 300 is about 4.8 square feet, or roughly 700 square inches of net free intake at the soffits.
Check the fan maker's own sizing chart too, since some use slightly different factors.
Why intake decides everything
A fan can only move the air that can get in. If the soffits are blocked or too small, the fan creates negative pressure in the attic and pulls air from wherever it can: ceiling light cans, the attic hatch, plumbing and wiring holes, and gaps around ducts. In Georgia that means pulling out air you paid to cool, and it can even backdraft a gas water heater or furnace located in the attic. Some building officials and energy advisers discourage power fans for exactly this reason.
Settle these first
- No ridge vent on the same attic, and close or remove nearby box vents and gable vents, or the fan will pull through them.
- Set the thermostat where the maker recommends instead of letting the fan run all day.
- Ask whether a passive soffit and ridge system would do the job, which is often the simpler answer.