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Net Free Air (NFA) is a standard of measurement for the unobstructed area of a ventilator through which air may flow.
A 1 ft. by 1 ft. hole is is 1 sq. ft. of NFA, while a 1 ft. by 1 ft. hole with a screen has the 1 sq. ft. reduced by the amount of screen over the hole. Depending on the gauge and pattern, this may reduce the available air by half. Almost all static vent products have a NFA rating.