What is soffit vent net free area?
Why the rating matters
Attic ventilation requirements are written in net free area: 1 square foot for every 150 or 300 square feet of attic floor, split between intake and exhaust. If you size soffit vents by their outside dimensions, you can end up with a fraction of the airflow you think you have. Vent makers test and print NFA on the package or spec sheet, either per vent or per linear foot for continuous strip vents and perforated soffit panels.
Things that shrink it in real life
- Screens: fine mesh added behind a louvered vent can cut its effective opening a lot.
- Paint: repainting perforated soffit or louvers can partly fill the holes. Check after every exterior paint job.
- Insulation: a vent with a great rating does nothing if insulation is packed against its back side.
- Dirt and pollen: Georgia's spring pollen coats screens and perforations.
- Mixed panels: some vinyl soffit lines come fully vented, center vented and solid, each with a different rating per foot.
Converting the numbers
There are 144 square inches in a square foot. A 1,500 square foot attic at 1/300 needs 5 square feet, or 720 square inches, of total NFA, and about half of that, roughly 360 square inches, should be intake at the soffits. Divide 360 by the rating of the vent you plan to use, per vent or per foot, and you have the count or the length you need, spread evenly along the eaves. For the exhaust side of the math, see ridge vent vs box vents, and for why the ratios exist, roof ventilation basics.