Attic Ventilation Ratio

Now let s figure out how much ventilation space we need.

Attic ventilation ratio.

Most building codes require a ratio of 1 150 ventilation space to attic floor space. 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. Ventilation manufacturers assign an nfa value to the non motorized vents they make. Ventsure inflow intake vent.

So in our example we will divide the 1 500 square feet by 300. Intake vents located at the lowest part of the roof under the eaves. Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents. Nfa is the unobstructed area through which air can pass through a vent usually measured in square inches.

1 square foot of nfva per 300 square feet of attic floor if a vapor barrier is installed on the ceiling below. Attic square footage 2 square inches of exhaust and square inches of intake net free area nfa needed. 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. 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.

Ventilation of attic spaces is required by most building codes as well as by roofing material manufacturers and the national roofing contractors association nrca. Without exception you should talk to a professional to determine what your home requires. Ok so now we know the square footage of the attic area to be ventilated. That said air resistance and interference such as vent grates reduces the area of true ventilation.

You can also contact your local municipality to check on building code in your area. 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 following product options will allow you to achieve an intake nfva of 528 square inches. Allows cool dry air to enter the attic.

What does the code require. 1 square foot of net free vent area nfva per 150 feet of attic floor. This is based principally on good historical experience and simple psychrometric analysis handegord giroux 1984.