What does the IRC code say about attic ventilation?
Short answer. The International Residential Code covers attic ventilation in Section R806. R806.1 requires ventilation for enclosed attics and rafter spaces, R806.2 sets the minimum vent area at 1/150 of the attic (or 1/300 with properly placed upper vents), R806.3 keeps insulation from blocking vents, R806.4 requires installation per the manufacturer, and R806.5 allows unvented attics under specific conditions.
The sections in plain English
- R806.1 Ventilation required. Enclosed attics and enclosed rafter spaces need cross ventilation for each separate space, with openings protected against rain and snow. Openings larger than 1/4 inch need screening such as corrosion resistant wire mesh or perforated vinyl.
- R806.2 Minimum vent area. Net free area of at least 1/150 of the vented space. The 1/300 exception applies when 40 to 50 percent of the required area is in upper vents no more than 3 feet below the ridge, measured vertically, with the balance low in the attic. A Class I or II vapor retarder condition applies only in Climate Zones 6 through 8, so it does not affect Georgia homes.
- R806.3 Vent and insulation clearance. Insulation, blocking, and bridging cannot block airflow, and a minimum 1 inch space is required between insulation and the roof sheathing at the vent location.
- R806.4 Installation and weather protection. Vents are installed per the manufacturer's instructions.
- R806.5 Unvented attic and unvented enclosed rafter assemblies. Allowed when its conditions are met, most commonly air impermeable insulation applied directly to the underside of the roof deck, with the attic inside the building's thermal envelope.
Related sections worth knowing
R807 covers attic access openings. The mechanical chapters require bath and dryer exhaust to terminate outdoors rather than in the attic. Exact wording and numbering can shift between code editions.
How it applies in Georgia
Georgia's residential construction code is based on the IRC with state amendments, and each city or county building department enforces the version currently adopted. Check with your local office before relying on a specific edition. More in roofing permits in Georgia and roof ventilation basics.