Powered Attic Ventilator Sizing
The fan size and matching intake for a powered attic ventilator: 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.
Formula and source
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.
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