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Attic Ventilation Requirements — The 1/150 Rule & When 1/300 Applies

Most people quote 1/300 as the attic ventilation rule. It is actually the exception. The IRC starts at one square foot of net free vent area for every 150 square feet of attic, and you only earn the reduction by meeting two conditions at once.

Requirement	Model Code Standard	Section
Baseline ratio	Net free ventilating area of 1 sq ft per 150 sq ft of the space being ventilated	IRC R806.2
Reduced ratio	The 1/300 reduction requires both conditions to be satisfied: a Class I or II vapor retarder on the warm-in-winter side of the ceiling in climate zones 6, 7 and 8, and 40 to 50 percent of the vent area located in upper ventilators positioned within 3 ft below the ridge, with the remainder in the lower third of the space	IRC R806.2
Opening size	Openings must be covered so the least dimension of the gap falls between 1/16 in. and 1/4 in.	IRC R806.1
Opening location	Ventilation openings go in the soffit, the eave, the gable end or the roof itself, positioned to open directly to outside air	IRC R806.1
Insulation clearance	Where eave or cornice vents are installed, keep at least 1 in. of open air above the insulation at the vent path	IRC R806.3
Unvented option	Unvented attic and rafter assemblies are permitted under a separate set of conditions, with minimum air-impermeable insulation R-values set by climate zone in Table R806.5	IRC R806.5

Worth knowing

The reduction to 1/300 is conditional, not the default. Both the vapor-retarder condition and the vent-distribution condition have to be met; meeting one of them is not enough. Earlier editions read “one or more,” and the change to requiring both appears to have landed in the 2018 IRC.
Net free area is not the same as the hole you cut. Screens, louvers and baffles cut the effective opening down, so use the manufacturer’s published net free area for the vent, not the rough opening dimension.
Table R806.5 was not updated between the 2018 and 2021 editions, so its insulation values no longer line up with the envelope table in Chapter 11. If you are designing an unvented assembly, confirm which table your jurisdiction enforces.
Balance matters as much as total area. Intake at the eaves paired with exhaust near the ridge is what moves air; two ridge vents and no soffit intake can satisfy the arithmetic and still fail in practice.

Sections cite the 2021 International Residential Code and 2023 National Electrical Code as model standards. This is BuildSpec’s own summary — not ICC or NFPA text; read the authoritative language at codes.iccsafe.org. Your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.

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