A. Clayton DeKorne responds: Most codes rely on the old FHA Minimum Property Standards, which call for enough net free ventilation to equal 1/300 of the attic floor area. If the ceiling does not have a vapor barrier, you need enough ventilation to equal 1/150 of the attic floor. Ridge and soffit vents should be balanced (equal openings along soffit and ridge).
Net free area means the unobstructed openings of a vent. If you cover the holes in the soffit blocks with 8x8 screen (64 openings per inch), the vent area is reduced by 25%. This means your three 1 1/2-inch holes in each rafter bay equal about 4 square inches of net free vent area.
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