Attic Ventilation Ratio

Ventsure inflow intake vent.

Attic ventilation ratio.

So in our example we will divide the 1 500 square feet by 300. Proper attic ventilation consists of a balance between air intake at your eaves soffits or fascias and air exhaust at or near your roof ridge. The recommended ventilation ratio to provide for vented attic assemblies when an air barrier is present is the 1 300 ratio as specified by most building codes. Ok so now we know the square footage of the attic area to be ventilated.

Ventilation of attic spaces is required by most building codes as well as by roofing material manufacturers and the national roofing contractors association nrca. Most building codes require a ratio of 1 150 ventilation space to attic floor space. Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents. Ventilation manufacturers assign an nfa value to the non motorized vents they make.

According to the national roofing contractors association nrca a power vent with an airflow rate of 1 0 cubic foot per minute per square foot of attic space measured at the attic floor is generally considered to be equivalent to a 1 150 ventilation ratio. This is based principally on good historical experience and simple psychrometric analysis handegord giroux 1984. Now let s figure out how much ventilation space we need. Choose one of the following products to meet the recommended intake nfva of 528 square inches.

1 square foot of nfva per 300 square feet of attic floor if a vapor barrier is installed on the ceiling below. Without exception you should talk to a professional to determine what your home requires. 1 square foot of net free vent area nfva per 150 feet of attic floor. The following product options will allow you to achieve an intake nfva of 528 square inches.

You can also contact your local municipality to check on building code in your area. Remember the minimum area requirement when using a continuous system is a ratio of 1 300. Nfa is the unobstructed area through which air can pass through a vent usually measured in square inches. Attic square footage 2 square inches of exhaust and square inches of intake net free area nfa needed.

What does the code require. Intake vents located at the lowest part of the roof under the eaves. Allows cool dry air to enter the attic.