NFVA or Net Free Ventilation Area is used to describe the total amount of space unobstructed by screens, louvers, or other material. In some cases it will be listed as a total for entire assembly like a gable vent or how much per linear feet for items like a ridge or soffit venting. In order to calculate the required amount, one must refer to the codes adopted in their jurisdiction. By far the most common calculation is either 1/150 or 1/300 depending on conditions.
