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Home / Code Guides / Attic Ventilation

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.

RequirementModel Code StandardSection
Baseline ratioNet free ventilating area of 1 sq ft per 150 sq ft of the space being ventilatedIRC R806.2
Reduced ratioThe 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 spaceIRC R806.2
Opening sizeOpenings must be covered so the least dimension of the gap falls between 1/16 in. and 1/4 in.IRC R806.1
Opening locationVentilation openings go in the soffit, the eave, the gable end or the roof itself, positioned to open directly to outside airIRC R806.1
Insulation clearanceWhere eave or cornice vents are installed, keep at least 1 in. of open air above the insulation at the vent pathIRC R806.3
Unvented optionUnvented 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 R806.5IRC 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.

Common questions

What does the code say about baseline ratio for Attic Ventilation?

Net free ventilating area of 1 sq ft per 150 sq ft of the space being ventilated. (IRC R806.2)

What does the code say about reduced ratio for Attic Ventilation?

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)

What does the code say about opening size for Attic Ventilation?

Openings must be covered so the least dimension of the gap falls between 1/16 in. and 1/4 in.. (IRC R806.1)

Adopted edition by state

Base-code year from BuildSpec’s adoption records. States often amend the base code, and your city or county may differ, so confirm with your building department.

Related sections and guides

This guide is BuildSpec’s own summary, not code text. It cites the 2021 International Residential Code and the 2023 National Electrical Code as model standards. The IRC 2021 text itself is in the Code Library for signed-in members; BuildSpec publishes no NEC text, which is at nfpa.org.Neither is an adoption record: your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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