Roof and attic ventilation

Required ventilation areaR806.2#

Enclosed attics and enclosed rafter spaces require cross ventilation. The minimum net free ventilating area is 1/150 of the vented space. The ratio may be reduced to 1/300 when a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling, or when the balance rule below is met.

Balanced soffit and ridgeR806.2#

The 1/300 ratio is also permitted when at least 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic, with the remainder provided by eave or cornice vents. The upper vents must be at least 3 feet above the eave vents.

Opening protection and blockingR806.1, R806.3#

Ventilation openings must be covered with corrosion-resistant mesh sized to block pests while preserving free area. Where eave or soffit vents are installed, insulation must not block the required airflow. A baffle extending above the top of the insulation is required at each rafter bay.

Unvented roof assembliesR806.5#

Unvented attic and unvented rafter assemblies are permitted when air-impermeable insulation is installed in direct contact with the underside of the roof deck, or when a combination of air-impermeable and air-permeable insulation meets the minimum R-value for condensation control for the climate zone. Check the adopted value for your specific zone before specifying.

Common inspection flags#

Typical roof ventilation failures caught at insulation and final inspection:

• Blown insulation covering soffit vents with no baffles installed
• Ridge vent installed without any soffit intake
• Net free area calculated from the gross vent size instead of the manufacturer rating
• Unvented cathedral ceiling with only fiberglass batts against the deck
• Gable vents screened with hardware cloth too coarse to block insects