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Calculate attic square
footage How: Multiply length of attic (in feet) times width of
attic (in feet) 30’ x 40’ = 1,200
square feet
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
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
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
Calculate # of lineal feet of Cobra® Exhaust Vent
needed How: Divide the square inches of NFA needed at the ridge
by the NFA of the Cobra® Exhaust Vent (Cobra® Exhaust Vent has 16.9 sq.
inches of NFA per lineal foot) 288 sq.
in. ÷ 16.9 = 17 lineal feet
This
attic requires seventeen (17) lineal feet of Cobra® Exhaust Vent 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.
Remember: Always have a balanced ventilation
system. In no case should the amount of exhaust ventilation exceed the
amount of intake ventilation. 