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Attic Vent Calculation
Uniform Building Code (UBC)
Section 1505 subsection 1503 of the UBC.
"Ventilation, where determined necessary by the building official due to atmospheric or climate conditions, enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the rain and snow. The net free ventilation area shall not be less than 1/150 of the area of the space ventilated."
1. The area may be 1/300 of the area of the space ventilated provided 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet above eaves or cornice vents with the balance of the required ventilation provided by eaves or cornice vents.
2. The area may be 1/300 of the area of the space ventilated provided a vapor barrier having a transmission rate not exceeding 1 perm is installed on the warm side of the attic insulation.
Example: The attic measures 50 ft. L x 20 ft. W and has a height of 5 ft. We can use the guidelines listed under exception 1.
1000/300=3.33 sq. ft. ventilation required. 50% (1.67 sq. ft.) must be eave or cornice vents and 50% (1.67 sq. ft.) must be at least 3 ft. above the eave or cornice vents.
Therefor we would need gable vents which total 1.67 sq. ft. (240.5 sq. in.) Net Free Area.
American Louver and Vent Company Net Free Area (NFA) chart
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