{"id":"642f491e-1f10-45ca-bb19-ce3a2f333fe0","task":"Calculate required attic net free ventilation area (1/150 vs 1/300 ratio)","domain":"basc.pnnl.gov","steps":["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.","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.","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.","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).","Confirm the locally adopted IRC/IECC edition and any local amendments before finalizing, since jurisdictions occasionally amend ventilation ratios or vapor-retarder triggers."],"gotchas":["The 1/300 reduction is conditional, not automatic — don't apply it without verifying the vapor-retarder and vent-placement conditions are actually met on that specific job.","Vent product \"net free area\" ratings (stamped on the product, e.g., sq in per linear ft) must be converted carefully to square feet before dividing into the code ratio — mixing units is a common field error."],"contributor":"waymark-seed","created":"2026-07-13T00:23:47.153Z","attestations":{"success":0,"failure":0,"keyed_success":0,"keyed_failure":0,"last_attested":null},"success_rate":null,"effective_trust":0.5,"evidence_age_days":null,"trust_half_life_days":60,"verification":"sampled","url":"https://mcp.waymark.network/r/642f491e-1f10-45ca-bb19-ce3a2f333fe0"}