How is the net open area of a flood opening ("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. When calculating the net open area of a flood opening with a cover, coverings that have solid obstructions such as grilles, fixed louvers, or faceplates must be considered. Figure 14 of TB 1 shows a standard air vent faceplate that provides 42 square inches of net open area. This number is derived by multiplying the 0.5-inch width of the openings times the 6.5-inch height of the openings times the 13 total openings. Such estimates may be used when no other data are available.

According to FEMA TB 1, manufacturers of devices intended for use as standard air vents typically indicate the number of square inches that each device provides for airflow (either stamped onto the metal frame or noted on the packaging). This number should be used for the net open area when these devices are installed as non-engineered flood openings. To qualify as flood openings that permit automatic entry and exit of floodwaters, openings must not have solid covers installed. Similarly, typical air vent devices used as flood openings designed to be opened and closed manually must be disabled permanently in the open position.

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. The IBC and IRC provide a list of acceptable covering materials. The commentaries that accompany the codes note that some covering materials may reduce the gross open area by up to 50 percent, in turn reducing the net open area. As a result of this reduction, in communities where floodwaters are expected to carry debris, local officials may determine that additional openings are required to compensate for the possibility that some openings may become clogged with debris.

FEMA TB 1, page 27, presents the equation that can be used to determine the area of an engineered flood opening. The equation is taken from American Society of Civil Engineers (ASCE) 24-05, Flood Resistant Design and Construction (2005).

References:

• FEMA. 2008. Openings in Foundations Walls and Walls of Enclosures in Special Flood Hazard Areas in Accordance with the National Flood Insurance Program. NFIP Technical Bulletin 1. Washington, DC. August 2008.
• American Society of Civil Engineers (ASCE). 2005. Flood Resistant Design and Construction. ASCE 24-05.