The IRC 2012 for Roof Ventilation will read in part as follows:
R806.1 Ventilation required. Enclosed attics and enclosed rafter spaces formed where ceilings are applied to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain and snow…
R806.2 Minimum vent area. The minimum net free ventilation area shall be 1/150 of the area of the vented space.
Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided one or more of the following conditions are met:
- In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
- 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 or rafter space… with the balance of the required ventilation provided by eave or cornice vents.
R806.3 Vent and insulation clearance. Where eave or cornice vents are installed, insulation shall not block the free flow of air. A minimum of a 1-inch (25mm) space shall be provided between the insulation and the roof sheathing and at the location of the vent.
R806.4 Installation and weather protection. Ventilators shall be installed in accordance with manufacturer’s installation instructions…
- The exception for reducing the attic ventilation area from the 1/150 ratio to the 1/300 ratio when a vapor retarder is installed on the ceiling now only applies to cold-weather climates.
- The exception for reducing the attic ventilation area based on cross ventilation now requires no less than 40% and no more than 50% (previously 50% and 80%) of the required ventilating area to be placed in the upper portion of the roof.
- The code now clarifies that vents must be installed in accordance with the manufacturer’s installation instructions.