To be eligible for flood insurance through FEMA's National Flood Insurance Program (NFIP), foundations must be constructed with certain flood damage mitigation features. In coastal areas and riverine zones, where high velocity floodwaters can occur, homes must be elevated on open pile foundations, and enclosures below the living space must be located at an elevation higher than the 100-year base flood elevation and must be designed to break away in a storm surge.
Homes built in floodplains where floodwater velocity levels are lower than coastlines and riverine zones (less than 5 feet per second), on perimeter type foundations, should be designed so that exterior walls of the foundation automatically equalize hydrostatic flood forces by allowing for the free flow of floodwater. This minimizes structural flood damage by reducing the horizontal hydrostatic force that unbalanced water levels impose on foundation walls.
Foundation designs meeting NFIP requirements must include prescribed openings or an engineer's certification that the following minimum criteria have been met:
- A minimum of two openings (on different foundation wall elevations) having a total net area of not less than one square inch for every square foot of enclosed area subject to flooding shall be provided.
- The bottom of all openings shall be no higher than one foot above grade.
- Openings may be equipped with screens, louvers, valves, or other coverings or devices, provided that they permit the automatic entry and exit of floodwaters.
Any openings that allow the free flow of floodwater into or out of a foundation will meet NFIP requirements, subject to minimum size and location criteria. Even voids in the continuity of a block foundation, or "blocks turned sideways", as one Maryland Department of the Environment technical note refers to it, will suffice.
Dependent upon their method of operation, conventional crawlspace ventilation grills may also serve to equalize water around the foundation during a flood. However, some air vents that are thermostatically controlled may not disengage in cold weather to allow water entry. If crawlspace vents will also be used as floodwater vents, the number and location of vents to be used should be specified based on the floodwater vent criteria.
Several manufacturers have developed unitized, shuttered products that meet the minimum flood venting requirements of NFIP. Smart Vent, Inc., one of these manufacturers markets several types of vents, including the Smart VENT® that automatically opens and closes as flood waters rise and fall and features temperature-controlled louvers open in warm weather to provide 50 in2. of net free ventilation area, then close in cold weather. Each Smart VENT®, sized 8" x 16" is certified to protect 200 sq ft of foundation area. Another model, Flood VENT™, is used when flood protection, not ventilation, is needed. This model has a non-vented insulated door that minimizes heat loss, air leakage, and pest intrusion when the door is closed, but swings open during a flood.
Another manufacturer, Floodex, has obtained pre-certification of its floodwater vent.
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If floodwater constantly applies pressure to your foundation, there is a great chance that it will break the foundation walls. By equalizing the water level in the foundation, less pressure is placed transversley on the wall, reducing the chance it will collapse under the water pressure.
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Contractor and masonry supply yards, as well as, construction material retailers stock many types of foundation vents, although Smart VENT® and Flood VENTTM can be purchased directly from the manufacturer.
Flood vents cannot be used in place of breakaway walls in high velocity flood areas. Designers should calculate both the net free ventilation area and the floodwater minimum requirement and choose the greater number of vents required by the two methods when specifying ventilation vents to satisfy both design criteria. Using certified flood vents can minimize the number of openings required in the foundation design to meet NFIP minimum criteria.
Some foundation ventilation units, not all of which comply with the NFIP criteria, range in price from $10 to $50.
Smart VENT® retails in the range of $175 per vent. Because of its certified status, a 1200 sq. ft. foundation would only require six units to comply with the NFIP minimum criteria.
Not Applicable
The National Flood Insurance Program sets floodwater vent criteria as a requirement for a home's insurability under the program. Local code authorities are responsible for enforcement of the criteria at plan review and inspection. Development and climatological changes have resulted in recent amendments to predicted flood plain areas, and base flood elevations. Accordingly, existing homes may be required to meet provisions of NFIP, retroactively.
Not Applicable
The vents must be installed no higher than 12" above the final grade to meet NFIP minimum requirements. Many vents are suitable for both new construction and retrofit. Existing foundations can be modified for compliance by simply removing non-complying existing vents or block and replacing with compliant vents, or leaving appropriate voids in the foundation.
Smart VENT warrants their product for one year from purchase and installation. Others vary by manufactirer.
For homes in floodplains and various coastal and river regions, floods are a possible and sometimes probable occurance. If the foundation is compromised, the whole house can be lost. Flood vents reduce the chances that the foundation will be compromised, greatly offsetting their costs in the event of flood.
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