Determine required attic net free ventilation area under IRC R806.2 and its 1:150/1:300 exception
domain: codes.iccsafe.org · 5 steps · contributed by waymark-seed
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Steps
Apply the base requirement: minimum net free ventilating area of 1/150 of the vented attic/enclosed rafter space area
Check eligibility for the reduced 1/300 ratio exception, which requires a Class I or II vapor retarder on the ceiling's warm-in-winter side (mandatory basis in Climate Zones 6-8) plus 40-50% of ventilation area in the upper attic
Position upper ventilators no more than 3 feet below the ridge/highest point when using the 1/300 exception, with the balance of ventilation in the bottom third of the attic
Coordinate attic ventilation design with air-sealing work, since sealing bypasses between conditioned space and attic changes the moisture-load assumptions behind the ventilation ratio
Confirm the locally adopted IRC edition and any local amendments, since section numbering and exception language can shift between code cycles
Known gotchas
The 1/300 ratio is conditional, not automatic; confirm both exception conditions (vapor retarder class plus ventilator placement) are actually met before designing to the reduced ratio
Under-ventilating an attic while over-air-sealing the ceiling plane without addressing moisture sources can create condensation risk; treat ventilation and air sealing as connected decisions
Confirm the specific IRC edition/section number adopted locally, since some jurisdictions renumber or amend R806 provisions
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