Proper attic ventilation can be an important performance consideration when designing and installing steep-slope roof systems. Typically, building codes include minimum requirements applicable to attic ventilation. The roofing industry also has practical guidelines for attic ventilation.
The International Building Code,® 2012 Edition (IBC 2012) requires enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof framing to have cross-ventilation for each separate space. The net free vent area (NFVA) shall not be less than 1:150 of the area of the space being vented. Blocking and bridging must be arranged so they do not interfere with air movement; air space no less than 1 inch must be provided between any insulation or obstruction and the roof sheathing. Ventilation openings must be protected to prevent rain or snow infiltration.
An exception to IBC 2012's ventilation requirement permits the NFVA to be reduced to no less than 1:300 as long as 50 to 80 percent of the ventilating area is provided at or near the upper portion of the space being vented or where a vapor retarder is provided on the warm-in-winter side of the ventilation space.