How is the net open area of a Non-Engineered Flood Vent measured?
FEMA Technical Bulletin (TB) 1, Openings in Foundation Walls and Walls of Enclosures (2008), states that the term "net open area" refers to the permanently open area of a non-engineered opening.
The National Flood Insurance Program (NFIP) regulation permits flood openings that are equipped with coverings or dividers as long as they allow the automatic exit and entry of floodwaters.
The net open area of a flood opening is calculated by measuring the opening area where air flows through the vent.
Insect screens that do not impede the entry and exit of floodwaters are allowed and do not affect the determination of net open area. If a community has adopted the International Building Code (IBC) or International Residential Code (IRC), a screen is required to cover ventilation openings to keep out animals and insects.