Code-Compliant Flood Vents

Requirements for flood opening sizes, location, number and other characteristics are primarily governed by NFIP regulations. The major requirements are outlined below, but the FEMA document that provides extensive details about meeting flood opening regulations is Technical Bulletin 1-2008, Openings in Foundation Walls and Walls of Enclosures. The American Society of Civil Engineers (ASCE) has developed the standard Flood Resistant Design and Construction (ASCE 24). This standard applies to buildings and site developments proposed in flood hazard areas. It is also referenced by the International Building Code. ASCE 24 Section 2.6.2.2 contains installation and design criteria for engineered openings. Local ordinances also refer to these three documents as a basis of their own floodproofing ordinances.

Design Solution: Add Freeboard

Adding an extra one or more feet above the BFE to a building space can significantly reduce insurance premiums in a flood hazard zone while providing additional space and functionality for the life of the building. For example, making a 4- or 5-foot-high crawlspace into a 10-foot enclosure might reduce insurance premiums by as much as 48 percent. Even adding one foot of freeboard might reduce premiums by over 35 percent. The additional space can add low-value storage areas, parking, better access or even just much improved headroom, and typically has a very quick payback of 2-3 years, since the cost in extra materials and labor is often minimal. Elevating a building a few feet above legally mandated heights has very little effect on its overall look, but it has significant benefits, including: Increased protection from floods and storms. Stormwaters can and do rise higher than shown on Flood Insurance Rate Maps (FIRMs). Freeboard helps protect buildings from storms larger than those that FIRMs are based on, and provides an added margin of safety to address the flood modeling and mapping uncertainties associated with FIRMs. Better preparation for ongoing sea level rise. For example, Massachusetts has experienced a relative sea level rise of approximately 1 foot over the past 100 years. Since elevations on FIRMs do not include sea level rise, freeboard will help keep structures above floodwaters as storm surge elevations increase. Greatly reduced flood insurance premiums. Recognizing that freeboard reduces flood risk, FEMA and NFIP provide substantial (sometimes more than 50 percent) reductions in flood insurance premiums for structures incorporating freeboard. These savings can rapidly accumulate, especially over the life of a normal mortgage.

The major requirements are as follows:

Openings must be in multiple walls. In order to allow water to flow freely in and out of the building, each enclosed area is required to have a minimum of two openings on exterior walls, located below the BFE. They should be installed on at least two sides of the enclosed area to allow for more even filling and draining of floodwater. If possible the openings should be reasonably distributed around the perimeter, unless there is a special justification for putting them on just two sides, e.g. in townhouses or buildings set into sloping sites.

The International Building Code (by reference to ASCE 24) requires a “minimum of two openings on different sides of each enclosed area.”

Wall location. The NFIP's definition of an enclosure is any portion below an elevated building that is fully shut in by four rigid walls. Basements are not allowed in Special Flood Hazard Areas. To avoid having an enclosure classified as a basement, the entire length of one wall must have its inside grade higher than or equal to the outside grade for that wall. This would qualify the enclosure as a walkout basement.

Opening location. The bottom of the opening must be no more than one foot above the grade that is immediately under the opening, either the adjacent ground level, or the interior grade, whichever is higher. The lower wall will experience hydrostatic pressure first, so most of the openings should be there. This alleviates the initial force of the water and then provides quick drainage when water begins to recede.

In practice, most communities require additional height, or “freeboard,” and insurance premiums will be substantially reduced by additional freeboard. (See sidebar.)

Materials. All materials below the BFE must be made of flood resistant material such as stainless steel or specially treated flood resistant lumber.

Opening size. ASCE 24 is referenced in all regulations, and it requires that a 3-inch sphere should be able to pass through the flood opening. This is to ensure that grills and louvers don't interfere with the passage of debris, or become so blocked that they are ineffective. It is well known that no flood water is crystal clear. Debris is a fact of flood, and can be devastating in itself.

Photo by Smart Vent Products, Inc. Debris is a fact of flood. Flood openings that are not sized properly, such as this air vent, can become blocked with debris and rendered ineffective.

Screening. ICC building codes require that openings be screened to prevent the entry of insects, rodents, birds, etc. Commercially available grates, louvers and grills are available for this function. However, as just mentioned, the louvers and screens can't interfere with the equalization of the water levels, and they must be selected to minimize potential blockage by debris. Any opening that includes a cover is an unacceptable measure according to FEMA TB-1.

Screens also affect the calculation of the required net opening area, discussed next.

Net open area. The NFIP's standard for non-engineered openings requires one (1) square inch of net open area for every square foot of enclosed area. Any part of a screen, grate or louver that impedes entry will be subtracted from the net opening area. Non-engineered openings also do not meet the standards if they are likely to be closed during the winter, as many air vents are. In fact, any air vent intended to be used as a flood opening is required to be disabled in the open position, so that it physically cannot be closed. Other commonly used air vent devices will not qualify as acceptable flood openings, such as devices with detachable covers, or temperature-controlled vents that will shut automatically when waters rise around the foundation.

Windows will not qualify as flood vents. Often window glass will remain in place even when a wall is dislodged from its foundation. Garage doors do not qualify in themselves, as they could be disabled in a power outage and would require human intervention even if useable. However, some code-compliant automatic flood vents can be installed in garage doors.

Automatic Operation. Wet floodproofing measures that satisfy NFIP requirements must be automatic. That means that the system must be strictly passive, operating with no human interaction required (e.g., opening or closing of vents).