Powered Attic Ventilator Sizing

Fan CFM = attic floor area x ~0.7 CFM/ft^2 (about 10 air changes/hr), with a ~15% increase for a dark roof, and the required intake (soffit) net free area of about 1 ft^2 per 300 CFM so the fan pulls outdoor air rather than starving. A 1,500 ft^2 attic needs a 1,050 CFM fan and 3.5 ft^2 (504 in^2) of intake; a dark roof pushes the fan to 1,208 CFM. Balanced passive ridge-and-soffit ventilation is often preferred and some codes restrict powered fans. A sizing aid; the fan manufacturer's data and the local code govern.

fan_cfm = attic_area x cfm_per_ft2 x (dark_roof ? 1.15 : 1.0); intake_ft2 = fan_cfm / 300; intake_in2 = intake_ft2 x 144. Default cfm_per_ft2 = 0.7.

Powered attic-ventilator sizing, standard roofing / attic-fan manufacturer practice (~0.7 CFM per ft^2, ~1 ft^2 intake per 300 CFM) by name; first-principles airflow.

The CFM-per-square-foot and intake-per-CFM rules of thumb are published free in attic-fan manufacturer sizing guides.

Estimate. AHJ and licensed professional govern.

Field names used by the API: attic_area_ft2, cfm_per_ft2, dark_roof, fan_cfm, intake_ft2, intake_in2

• Fan airflow ~0.7 CFM per ft^2 of attic floor (~10 ACH), +~15% for a dark roofattic-fan sizing practice
• Intake area ~1 ft^2 of net free intake per 300 CFM so the fan is not starvedattic-fan sizing practice
• Passive preferred balanced passive ventilation is often preferred; some codes restrict powered fansscope of this tile