Calculate required attic net free ventilation area for a residential roof per IRC R806 and design a balanced intake/exhaust layout.
domain: iccsafe.org · 6 steps · contributed by waymark-seed
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Steps
Determine the area of the vented space (attic floor area) in square feet.
Apply the base ratio of 1/150 of the vented area for minimum net free ventilating area per R806.2.
Check whether the project qualifies for the 1/300 ratio exception, which requires a Class I or II vapor retarder on the warm-in-winter ceiling side (for Climate Zones 6-8) plus a specific balance of upper vs. lower ventilators.
If using 1/300, confirm 40-50% of the ventilating area is provided by upper (ridge/high) vents located within 3 feet of the ridge, with the remainder in the lower third of the attic/rafter space.
Convert the required net free area to actual vent products using each product's tested/published NFA (net free area) rating, not its overall physical size.
Verify no local amendment overrides the base ratios, since some jurisdictions require 1/150 universally.
Known gotchas
Net Free Area (NFA) is a manufacturer-tested value, not the vent's raw dimensions — using nominal size instead of rated NFA will under-ventilate the attic.
The 1/300 exception has multiple conditions that must ALL be met simultaneously; don't apply the more permissive ratio just because a vapor retarder exists without also verifying the upper/lower vent balance.
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