Attic Ventilation Ratio

Ventilation of attic spaces is required by most building codes as well as by roofing material manufacturers and the national roofing contractors association nrca.

Attic ventilation ratio.

What does the code require. You can also contact your local municipality to check on building code in your area. Most building codes require a ratio of 1 150 ventilation space to attic floor space. So in our example we will divide the 1 500 square feet by 300.

Remember the minimum area requirement when using a continuous system is a ratio of 1 300. Choose one of the following products to meet the recommended intake nfva of 528 square inches. Ventsure inflow intake vent. The following product options will allow you to achieve an intake nfva of 528 square inches.

Now let s figure out how much ventilation space we need. Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents. That said air resistance and interference such as vent grates reduces the area of true ventilation. Federal housing authority recommends a minimum of at least 1 square foot of attic ventilation evenly split between intake and exhaust for every 300 square feet of attic floor space.

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. Nfa is the unobstructed area through which air can pass through a vent usually measured in square inches. Ventilation manufacturers assign an nfa value to the non motorized vents they make. 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.

Attic square footage 2 square inches of exhaust and square inches of intake net free area nfa needed. Without exception you should talk to a professional to determine what your home requires. Intake vents located at the lowest part of the roof under the eaves. 1 square foot of nfva per 300 square feet of attic floor if a vapor barrier is installed on the ceiling below.

1 square foot of net free vent area nfva per 150 feet of attic floor. Ok so now we know the square footage of the attic area to be ventilated. Generally speaking you need a ratio of 1 300 where for every 300 square feet of ceiling space you need 1 square foot of attic ventilation.