Proper attic ventilation systems allow a continuous flow of outside air through the attic, protecting the efficiency of the insulation and helping to lower temperatures in the living space. A well designed system consists of a balance between air intake (at your eaves or soffits) and air exhaust (at or near your roof ridge). The FHA recommends a minimum of at least 1 square foot of attic ventilation (both intake and exhaust) for every 300 square feet of attic space. For example, if your attic is 900 square feet, you need a total of 3 square feet of ventilation. This amount is generally divided equally between intake and exhaust ventilation (i.e., 11/2 feet of each), to insure proper air flow through the attic.

Calculating Your Ventilation Requirements
Example:
Calculating the number of SSB960 exhaust vents needed for a 30’x 40’ attic:

Step 1)
Calculate attic square footage

How: Multiply length of attic (in feet) times width of attic (in feet)
30’ x 40’ = 1,200 square feet
Step 2)
Calculate NFA (Net Free Area) needed for this attic by using the “1 in 300” rule
How: Divide attic square footage by 300
1,200 sq. ft. 300 = 4 square feet of NFA needed
Step 3)
Convert square feet of NFA to square inches

How: Multiply square feet of NFA by 144
4 sq. ft. x 144 = 576 square inches of NFA needed
Step 4)
Split the amount of NFA needed equally between the intake and the exhaust
How: Divide square inches of NFA needed by 2
576 sq. in. 2 = 288 square inches of NFA needed equally for exhaust & intake
Step 5)
Calculate # of SSB960 vents needed
How: Divide the square inches of NFA needed at the ridge by the NFA of the SSB960 vent
(the SSB960 has 60 sq. inches of NFA)
288 sq. in. 60 = 4.8 vents (round up to 5 vents)

This attic requires five (5) SSB960 exhaust vents to meet FHA minimum requirements.

Intake ventilation MUST be at least equal to exhaust ventilation. To determine the number of intake vents needed, select your vent on page 12, and install enough vents so that their combined NFA is at least 288 square inches.