How Much Venting Does My Attic Need?

International Residential Code

• Minimum requirements were further outlined in the U.S. by the 2009 International Residential Code (IRC) Section R806. This code placed the ventilation ratio at 1:150 unless the 1:300 ratio includes a Class I or II vapor barrier installed on the warm-in-winter side of the ceiling, or power ventilators are located 3 feet above eave or cornice vents to ventilate 50 to 80 percent of the space. The remaining space could then be ventilated by static eave and cornice vents.

Ridge Vents and Power Vents

• Ridge vents are well concealed after installation as they can be color-matched to your shingles. These vents are rated to operate efficiently over the life of your roof and provide excellent protection against the elements. Energy performance is good, and the vents are cost effective to buy and install. Power vents provide good energy performance and, unlike ridge vents, can be installed on any type of roof. Power vents are well concealed on gables but are noisier in operation and costlier than ridge vents.

Louvers and Turbines

• Louvers are fairly easy to install on new roofs and are a good choice for re-roofing. Louvers are well protected against animal, insect and precipitation entry. Energy performance is fair to good, and louvers operate very quietly. Turbines are more difficult to install but comparable in protection to louvers. Turbines are marginally less expensive and a little noisier than louvers but provide better energy performance overall. Louvers and turbines are similar to power vents in life expectancy but are not as long lasting as ridge vents.

Non-powered Ventilation Systems

• Ventilation systems using static vents with no power capability are based upon the 1:150 ratio. To calculate static venting requirements, multiply the length of the attic floor (in feet) by its width. Divide the result by 150. Divide the last answer by 2, which is the square footage required for intake vents. The same square footage is required for exhaust vents. For a 40 x 25-foot attic, the square footage of 1,000 divided by 150 equals 6.6 square feet of venting -- 3.3 square feet each for intake and exhaust.