Replies

1. GBA Editor
   Martin Holladay | Mar 12, 2014 01:42pm | #1

   Scott,
   Code requirements for roof venting or attic venting are based on the area of the attic floor. Assuming that you split the ventilation opening between the soffits and the ridge -- the most common approach -- you will need 1 square foot of net free ventilation area for every 300 square feet of attic floor area, with about half of that ventilation area at the soffits, and about half at the ridge.

   Manufacturers of soffit vents and ridge vents usually specify the net free vent area of their products on product packaging or in specifications available online.

   For more information on attic venting, see All About Attic Venting.

   For more code details, you can look up code requirements for attic ventilation online. Here is the relevant section of the 2012 IRC:

   SECTION R806 ROOF VENTILATION
   R806.1 Ventilation required.
   Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilation openings shall have a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Ventilation openings having a least dimension larger than 1/4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, or similar material with openings having a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Openings in roof framing members shall conform to the requirements of Section R802.7. Required ventilation openings shall open directly to the outside air.

   Exception: Attic ventilation shall not be required when determined not necessary by the code official due to atmospheric or climatic conditions.

   R806.2 Minimum vent area.
   The minimum net free ventilating 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:
   1. 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.
   2. 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. Upper ventilators shall be located no more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically, with the balance of the required ventilation provided by eave or cornice vents. Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted.

   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 (25 mm) space shall be provided between the insulation and the roof sheathing and at the location of the vent.