Calculate required attic net free ventilation area (1/150 vs 1/300 ratio)

Steps

1. Start from the IRC/IECC baseline: 1 square foot of net free ventilating area (NFVA) per 150 square feet of attic floor area (the 1/150 ratio) applies by default.
2. Check whether the job qualifies for the 1/300 exception, which requires both: a Class I or II vapor retarder on the ceiling's warm-in-winter side (mandatory only in Climate Zones 6, 7, 8), and 40-50% of ventilation area placed in the upper attic (within 3 ft of the ridge) with the remainder low/soffit.
3. Compute NFVA by dividing attic floor square footage by 150 (or 300 if the exception applies) — e.g., a 1,500 sq ft attic needs 10 sq ft NFVA at 1/150, or 5 sq ft at 1/300.
4. Split the calculated NFVA between high (ridge/gable-upper) and low (soffit/eave) vents per balanced-ventilation guidance in PNNL's Building America Solution Center (basc.pnnl.gov/information/calculating-attic-passive-ventilation).
5. Confirm the locally adopted IRC/IECC edition and any local amendments before finalizing, since jurisdictions occasionally amend ventilation ratios or vapor-retarder triggers.