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Use the filters below to find answers to frequently asked questions about Building Science guidance regarding:
If you need additional information, please email the FEMA Building Science Helpline or call at 866-927-2104.
Please visit the Building Science Resource Library to reference all hazard-specific publications and guidance documents.
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’s Technical Bulletin (TB) 1, Openings in Foundation Walls and Walls of Enclosures Below Elevated Buildings (FEMA, 2008) provides National Flood Insurance Program (NFIP) requirements and guidance for installing flood openings in enclosures located below the base flood elevation (BFE) in Zones A, AE, A1-A30, AR, AO, and AH for a variety of different foundation types. Check with the local jurisdiction for additional information on flood openings.
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.
FEMA’s Technical Bulletin (TB) 2, Flood Damage-Resistant Materials Requirements for Buildings Located in Special Flood Hazard Areas (FEMA, 2008, revised 2010), provides guidance on the National Flood Insurance Program (NFIP) regulations concerning the required use of flood-damage resistant construction materials for building components located below the base flood elevation (BFE) in Special Flood Hazard Areas in both Zone A and Zone V. TB 2 provides a classification scheme for flood damage-resistant materials, as well as a table of common building materials, and describes whether they are allowed for use in construction below the BFE. Check with the local jurisdiction for additional requirements for use of materials below elevated buildings.
References: FEMA. 2008, revised 2010. Flood Damage-Resistant Materials Requirements for Buildings Located in Special Flood Hazard Areas in Accordance with the National Flood Insurance Program. NFIP Technical Bulletin 2. Washington, DC, August 2008.
he National Flood Insurance Program (NFIP) requires that the heating, ventilation, and air conditioning (HVAC) system in new construction or Substantial Improvement in a Special Flood Hazard Area (SFHA) be elevated above the base flood elevation (BFE) or designed so that floodwaters cannot infiltrate or accumulate within any component of the system.
All structural and nonstructural building components at or below the BFE must be constructed of materials resistant to flood damage. Flood damage-resistant materials must be used for all building elements subject to exposure to floodwaters, including floor joists, insulation, and ductwork. Building components that do not use flood damage-resistant materials must be elevated above the BFE. A flood damage-resistant material is defined as any building material capable of withstanding direct and prolonged contact with floodwaters without sustaining significant damage. Prolonged contact is defined as contact with floodwaters for a minimum of 72 hours and significant damage refers to any damage requiring more than low-cost cosmetic repair (such as painting). The cost of repair should be less than the cost of replacement for the material or system. This requirement applies regardless of the expected or historical flood duration.
The NFIP does not recommend installing ductwork below the BFE in new construction or Substantial Improvement located in an SFHA. There is no known cost-effective technique for designing air ducts to keep floodwaters from entering or accumulating within the system components during a flood. If ductwork must be installed below the BFE, it should be minimized as much as possible. The ducts must be composed of impermeable and watertight material, such as welded seamless ductwork or large-diameter PVC pipe. Such material is expensive, but practical when a short length of ductwork descends below the BFE.
In a Coastal High Hazard Area, structures in the Coastal A Zone (areas in Zone A that are subject to wave heights more than 1.5 feet) and Zone V can be significantly damaged by wave action. In Zone V, the lowest horizontal structural member must be installed at or above the BFE. Anything below the BFE is regarded as an obstruction and can result in increased flood insurance premiums. Ductwork below the BFE should be avoided at all costs in Zone V and Coastal A Zones because it is difficult to design and construct a ductwork system that will sustain not only the hydrostatic loading, but also the additional hydrodynamic, wave forces and debris impact loads in these flood zones.
For more information on this topic, see:
FEMA’s NFIP Technical Bulletin (TB) 2, Flood Damage-Resistant Materials Requirements for Buildings Located in Special Flood Hazard Areas in accordance with the National Flood Insurance Program (FEMA, 2008 and revised in 2010), classifies the flood damage-resistance of materials. Table 2 of TB 2 lists common building materials and classifies whether they are acceptable or unacceptable for use below the base flood elevation (BFE) in Special Flood Hazard Areas (SFHAs). Keep in mind, however, that flood damage-resistance may also be affected by the material’s specific application and the characteristics of the floodwaters. Sound judgment and knowledge of probable contaminants in local floodwaters are needed to select the proper materials.
Materials and products that are not listed in Table 2 may be used if accepted by the local official. For materials not listed in Table 2, manufacturers’ literature (i.e., specifications, materials safety data sheets, test reports) should be evaluated to determine if the product meets flood damage-resistance requirements; however, at this time, there are no specific tests or protocols for a manufacturer to use to test their materials for “flood damage-resistance.” Acceptance should be based on sufficient evidence, provided by the applicant, that the materials proposed to be used below the BFE will resist flood damage without requiring more than cosmetic repair and cleaning after being inundated by floodwater.
Note that community or State requirements that exceed those of the NFIP take precedence over what is specified in TB 2. Design professionals should contact the community to determine whether more restrictive criteria apply to the building or site in question.
References: FEMA. 2008, revised 2010. Flood Damage-Resistant Materials Requirements for Buildings Located in Special Flood Hazard Areas in Accordance with the National Flood Insurance Program. NFIP Technical Bulletin 2. Washington, DC, August 2008.
Information regarding Substantial Damage and Substantial Improvements can be found in FEMA P-758, Substantial Improvement/Substantial Damage Desk Reference (FEMA, 2010). To participate in the National Flood Insurance Program (NFIP), communities must adopt and enforce regulations and codes that apply to new development in Special Flood Hazard Areas (SFHAs). Local floodplain management regulations and codes contain minimum NFIP requirements that apply not only to new structures but also to existing structures that have been Substantially Damaged or which are undergoing Substantial Improvement. FEMA P-758 provides practical guidance and suggested procedures to implement the NFIP requirements for Substantial Improvement/Substantial Damage.
In addition, FEMA has an NFIP Policy Index available online with basic information about Substantial Improvement, and Substantial Damage.
Definition: Substantial Damage means damage of any origin for which the cost of restoring the structure to its before-damaged condition would equal or exceed 50 percent of the market value of the structure before the damage occurred (FEMA, 2010).
Definition: Substantial Improvement means any reconstruction, rehabilitation, addition, or other improvement of a structure for which the cost of the work equals or exceeds 50 percent of the market value of the structure before the start of construction of the improvement. This term includes structures that have incurred “substantial damage” regardless of the actual repair work performed (FEMA, 2010).
References: FEMA. 2010. Substantial Improvement/Substantial Damage Desk Reference. FEMA P-758. Washington, DC. May 2010.
How to measure your bottom (lowest) floor and whether your home meets the BFE requirements is determined by your community’s building codes and floodplain management regulations, and the flood zone in which your home is located.
The term “lowest floor” is defined by the NFIP and building codes. It refers to the portion of the building that is raised above the ground on an elevated foundation. In Zone A, the elevation of the “lowest floor” is measured at the top of the floor of the elevated building. In Zone V, the elevation is measured at the bottom of the lowest horizontal structural member (beam) that supports the elevated building.
To comply with the NFIP and building codes, a building must have its lowest floor elevated to or above the specified elevation, usually the BFE or higher. The only way to know for sure if your home is elevated high enough to comply with the requirements is to have a licensed surveyor prepare an Elevation Certificate. If your home was built in accordance with floodplain management rules in effect at the time it was constructed, your community may have a record of the surveyed information.
References: FEMA. 2012. Quick Reference Guide: Comparison of Select NFIP and Building Code Requirements for Special Flood Hazard Areas. Washington, DC. March 2012.
Yes, FEMA has several publications for homeowners, design professionals, and builders that illustrate important concepts and best practices for constructing stronger, safer residential buildings in flood-prone areas in accordance with building codes and standards. All of these publications can be downloaded for free from FEMA’s Building Science Publications web site.
FEMA also produces a series of Technical Bulletins to provide guidance on the building performance requirements of the National Flood Insurance Program (NFIP). All of these publications can be downloaded for free from FEMA’s NFIP Technical Bulletins web site here.
In addition, FEMA offers several free publications that provide information on how to complete an Elevation Certificate. Although these publications are written for local building officials, surveyors, architects, and engineers who are authorized by law to certify elevation information on the certificate, the information can help homeowners understand the requirements for elevating buildings. Fore more information, visit National Flood Insurance Program Elevation Certificate and Instructions.
Please check with a local building official regarding the requirements for different flood zones and elevation in your area. Note that some communities may have more stringent requirements than the NFIP.
References: None.
Work on flood-damaged homes located in communities that participate in the National Flood Insurance Program (NFIP) is subject to specific requirements. If the flood-damaged home is in a floodplain and is substantially damaged (see definition below), the NFIP requires that reconstruction and repairs, termed substantial improvements (see definition below), be performed to bring the home into compliance with local floodplain management regulations. These regulations include elevating the home so that its lowest floor is at or above the base flood elevation (BFE). The local jurisdiction may require elevating the home higher than the BFE, to an elevation called the design flood elevation (DFE). Other requirements include using the proper type of foundation (depending on the flood zone in which the house is located), properly anchoring the home to the foundation, using flood-resistant materials, complying with limitations on the use of enclosed areas below the elevated home, and protecting utilities and equipment. Check with the local jurisdiction for additional requirements that must be met.
Reconstruction of a home that is destroyed or that has been so severely damaged that it must be rebuilt is considered construction of a new home, and new homes must comply with the local floodplain management regulations.
Definition: Substantial damage means damage of any origin for which the cost of restoring the structure to its before-damaged condition would equal or exceed 50 percent of the market value of the structure before the damage occurred (FEMA, 2010).
Definition: Substantial improvement means any reconstruction, rehabilitation, addition, or other improvement of a structure for which the cost of the work equals or exceeds 50 percent of the market value of the structure before the start of construction of the improvement. This term includes structures that have incurred substantial damage regardless of the actual repair work performed (FEMA, 2010).
References: FEMA. 2010. Substantial Improvement/Substantial Damage Desk Reference. FEMA P-758. Washington, DC. May 2010.
FEMA does not verify or certify design calculations for any product. The design professional who signs the certification attests that the product in question will meet the requirements specified on the certification. The design professional should be licensed in the state in which the product will be used. Note that any product must be properly installed for its intended use only.
