What is the difference between NFA and NFVA in roof ventilation?
Short answer. There is no practical difference: NFA (net free area) and NFVA (net free vent area, or net free ventilating area) are two names for the same measurement, the actual open area a vent lets air through after subtracting louvers, screens and filters. It is always smaller than the vent's gross size, and it is the number the building code and attic calculations use.
Gross area vs net free area
Gross area is the outside dimension of the opening. Net free area is what remains after everything that blocks air is accounted for: louvers, mesh, baffles, filter fabric and the vent's own structure. A rectangular soffit vent with a 16 by 8 inch face has 128 square inches of gross area but a much smaller net free area.
How the numbers are reported
- Ridge vents: square inches per linear foot.
- Box vents and turbines: square inches per unit.
- Soffit strips and vented panels: per linear foot or per panel.
- Gable vents: per unit, often well below what their size suggests because of louvers and screen.
Why it matters
The code's 1/150 and 1/300 ratios refer to net free ventilating area. Using gross area overstates ventilation, sometimes by half or more, and leaves the attic short.
Other terms you may see
- NFA per LF: net free area per linear foot.
- CFM: cubic feet per minute, used for powered fans, which move a volume of air rather than providing an opening.
Practical tips
Read the stamp on the vent or its product data sheet. Do not count painted over or blocked vents. When combining products, add their NFA values, then compare the total to the requirement. See roof ventilation basics.