Flood vents in building foundations:
This article gives a definition of foundation flood vents and explains how they work and why flood vents are needed. For buildings located in flood zones - areas where flooding is likely, permitting water to flood a building basement or crawl space - even if it seems counter-intuitive, can prevent more drastic building damage such as foundation collapse. We also list flood vent brands, manufacturers, and sources of where to buy flood vents. We provide information on where and how flood vents should be installed.
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A flood vent or "flood port" is a permanent opening in a wall designed to allow the unobstructed passage of water automatically in both directions in to or out of the building interior.
Flood vents are intended to the risk of structural damage (from the forces of floods or flood waters) for buildings located in special flood hazard areas [or "flood plains"] in accordance with the National Flood Insurance Program (NFIP).
Flood vents may be solid hinged covers intended for use on occupied space (or a garage) or vented hinged covers for dual use as a crawl space foundation vent (for un-conditioned crawl spaces) and as a flood relief opening.
This document explains how flood vents are used in building foundations or other walls to reduce structural damage from flooding. Flood vents, also referred to as "flood ports" or "flood gates" by some authorities, permit water to flow in and out of the foundation or other areas of a flooded building to reduce pressure and damage to the structure.
Flood ventilation is required (in flood-prone areas) by various building codes (cited here) and can, if properly installed, not only reduce building damage during flooding, but also they may qualify the building owner for a reduced flood insurance rate for insurance provided by NFIP, the National Flood Insurance Program. Flood vents will encountered by building inspectors and home inspectors who need to know the basics of what's required and as well, how to properly inform building buyers or owners about these devices and what they mean.
We provide general installation instructions for flood vents, building code citations for flood vent requirements, paraphrasing of advice from flood vent experts and product suppliers, links to classes offered in flood damage protection, and illustrations of these devices. Flood venting is advised or regulated by FEMA, the NFIP, and several national and state building codes in the U.S. and in Canada cited here
While several companies produce or have produced flood venting products the product which we find most-often installed for structural protection in flood-prone areas is the ere's the Smart VENT. Here we outline that company's product description gleaned from several different pages at their website. Other alternative flood ventilation products and photos are provided FLOOD VENTING PRODUCT SOURCES.
Smart VENT's Flood Vent Model# 1540-520 is certified to provide insulated flood protection only. This model is used for a garage or conditioned space, where flood protection is required but ventilation is NOT desired. The flood door is constructed of solid stainless steel wrapped around an insulating foam core.
Flood Protection - The Smart VENT door is latched closed until flood water enters. Entering flood water lifts the patented internal floats which unlatches and rotates the door open. This allows the flood water to automatically enter and exit through the frame opening, relieving the pressure from your foundation walls.
The Smart VENT door is latched closed until flood water enters. Entering flood water lifts the patented internal floats which unlatches and rotates the door open. This allows the flood water to automatically enter and exit through the frame opening, relieving the pressure from your foundation walls.
SmartVENT does not rely on the louvers to let flood water in and out. Regardless of the louvers' position, opened or closed, when flood water flows into the door, the internal floats release the door to rotate open to relieve the hydrostatic pressure. The louvers and pest screen are rotated out of the path of the flood water. The temperature controlled louvers are for ventilation purposes only.
Smart VENT Model XXXXXXx is certified to provide flood protection AND ventilation. This model is used for a home with a crawlspace or built on a pony wall that requires seasonal ventilation of the crawlspace and protection from flooding. All stainless steel construction resists weather and pests.
Flood Protection:The Smart VENT door is latched closed until flood water enters. Entering flood water lifts the patented internal floats which unlatches and rotates the door open. This allows the flood water to automatically enter and exit through the frame opening, relieving the pressure from your foundation walls.
Ventilation: A bimetal coil (like a thermostat, no electricity is needed) automatically opens and closes the ventilation louvers as temperature changes. They will be closed when it is freezing outside and open when it is warm outside to provide natural ventilation.
Important note:SmartVENT does not rely on the louvers to let flood water in and out. Regardless of the louvers' position, opened or closed, when flood water flows into the door, the internal floats release the door to rotate open to relieve the hydrostatic pressure. The louvers and pest screen are rotated out of the path of the flood water. The temperature controlled louvers are for ventilation purposes only.
By recommending your customers flood-proof their homes with the Smart Vent family of products, they will dramatically reduce the chance of structural damage during a flood with its automatic, bi-directional vent relieving hydrostatic pressure on foundation walls during rising or falling floodwaters.
What the presence of flood vents on a building immediately suggests is the following:
1. Building located in a flood plain: The buyer had better know that their house is in a flood plain and to understand what that means
- they need flood insurance (offered by the NFIP) (the use of flood vents may allow a discount on the NFIP premium)
- when there is flooding they may be affected with risks of damage or mold .
2. What Flood Vents Do: The buyer should be given to understand that the function of flood vents is to avoid damage to the foundation during flooding by letting water pass through, in, and back out of the foundation wall. The vent opens to let water pass and automatically closes to keep pests out. Flood vents attempt to avoid structural damage but do not address other types of water damage such as mold contamination caused by building flooding.
3. Flood Vent Installation Adequacy: I'd want to know what actual, as-installed, installation capacity and the required schedule of installation - number and spacing of flood pressure relief vents. Probably on a new house they will be to code. On an older home vents may have been installed as a retrofit and may be improperly installed, inadequate, or blocked by debris (depending on what type of vent was in use.) There is a requirement that each vent can protect so many sq.ft. of enclosed area, ranging from 60' to 200' per vent depending on vent design. Also are there house elevation requirements or setback requirements in re flooding?
3. Flood History: I'd want to know what the flood history is for the area, whether or not this house has been flooded, and if so, what the flooding did to the home in re effects on structure or perhaps a mold risk.
4. Construction Type & Elevation: Was the house on a basement or crawl or slab? New construction in flood plains in most jurisdictions is required to employ slab or crawl space construction rather than providing habitable basement space. New construction in flood-risk areas specifies the lowest allowable level of living space and requires, when specified by the local building official, that an "elevation certificate based on finished construction" has been provided.
Text included below describes and compares recommended flood venting practices in several U.S. jurisdictions. Since each author, regulation, or technical bulletin varies, we note the sum of ideas from these different experts and different geographic areas may permit a cautious builder or building owner to exceed the minimum standards required for his/her immediate area if additional security against flood damage is desired.
The National Flood Insurance Program,NFIP, requires at least two flood venting openings for finished or enclosed space (e.g. basements or garages) or for crawl spaces. The vent opening required is a net area of at least 1" per sq.ft.. of enclosed space. The flood ventilation openings must be on different sides of the enclosed space.
Each individual enclosed area requires openings on its exterior walls so that flood water can enter or exit directly to and from that space. The bottom of flood vent openings cannot be higher than one foot above the grade where the vents are installed. Windows, doors, and garage doors are not considered openings for flood venting purposes.
DF comment: it's interesting that the requirement is in square feet not cubic feet, and is unadjusted for obstructions within the enclosed area.
The following text is quoted from "Foundations in Flood Hazard Areas" published by the Louisiana State University (LSU) AgCenter bulletin in January 2007. A link to the original source is at the bottom of this page.
In locations that fall in FIRM A zones, the building elevation is measured at the top of the finished floor. In V zones (at risk for storm surge and waves), the elevation is measured at the bottom of the lowest horizontal structural member. The Coastal Construction Manual of the Federal Emergency Management Agency (FEMA) advises applying V-zone standards to construction in A zones that are near open water or V zones where buildings could be exposed to wave action.
Slab on compacted fill: Fill dirt is often used to elevate building sites. Some communities prohibit placing fill in the floodplain. Others allow fill but require that an equal volume of flood storage capacity be generated in the same vicinity.
Limiting fill to 2 feet -- and only under the footprint of the home -- minimizes the loss of flood storage capacity. The fill should slope to natural grade within 2 feet of the foundation. This limited-fill technique has the added advantage of reducing the impact of fill on trees. Foundation fill must be compacted to have adequate bearing capacity and resist erosion; this may be referred to as engineered fill.
When additional elevation is needed, one of the other elevated foundation methods can be used instead of or on top of the minimal fill.
Slab cap on filled stem wall: To build on a slab when you need to elevate more than 2 feet above existing grade level, foundation stem walls can be constructed to the needed height and the interior filled with compacted soil. The stem wall can consist of a reinforced concrete block walls on footings or cast-in-place concrete foundation walls engineered for the soil conditions.
The top of the stem wall can be shaped to provide a form for the slab; L-shaped blocks are made for this purpose. Even though the slab cap is elevated, a durable moisture barrier of plastic sheeting is still needed over the soil and should extend over the stem wall to before the slab is poured.
Unfinished, floodable crawl space or level: A similar architectural look can be achieved with a framed floor system over a crawl space without the expense of adding and compacting fill within the stem wall foundation.
When a raised house is built with an enclosed crawl space (or any enclosure beneath the BFE), it must be designed to allow free entry and exit of floodwater. The interior cannot be finished, and all materials used below base flood elevation must be flood-resistant. The space below BFE can be used only for parking, access and limited storage.
For any enclosure below BFE, the NFIP requires openings (flood vents) on at least two sides, totaling 1 square inch of opening for each square foot of enclosed area. The bottom of these openings must be no more than a foot above the outside grade. Flood vents are not required if an engineer or architect certifies that the design will allow for the automatic equalization of flood water forces on exterior walls. This usually involves areas designed to open or break away with water pressure.
Openings may be fitted with closing vents if those vents conform to the building code for automatic flood-venting requirements. A window, door or garage door is not considered a flood opening, but flood openings can be located in them.
Pile, post, column or pier foundation: In areas of slow-moving floodwater, any of these raised foundation types is acceptable. The traditional pier-and-beam foundation with a frame floor system is generally the least-expensive elevated foundation option. Traditional architectural styles in most southern states often include pier and beam foundations left open, so this look can offer cultural appeal and community value.
Continuous pile, open foundation: The use of fill or stem wall foundations for structural support of buildings is prohibited in V zones. The code requires homes to be elevated on a pile or column foundation (a post is a wooden column), and an engineer or architect must certify that the foundation and structure are anchored to resist floatation, collapse and lateral movement. Because a pile/column joint may weaken the foundation, FEMA's Coastal Construction Manual recommends using continuous piles.
Free of obstruction or minimal obstruction: For buildings in V zones, the space below BFE must be free of obstruction, meaning left open. If the local ordinance allows, this space can be enclosed with non-supporting breakaway walls, open latticework or insect screening designed to collapse under wind and water loads without causing damage to structural supports or the structure.
Minimum code requirements do not take into account the damage that can be caused when breakaway walls do just that and slam into neighboring buildings. For this reason, it's best to avoid the use of breakaway components, even if allowed.
Foundations for homes in minimal risk zones (C and X) near A- or V- zone property
Lowest floor is at BFE for the nearest higher-risk zone. The flood zone lines shown on a FIRM are adequate for rating insurance, since the zone-boundary errors tend to cancel each other. However, they are not a reliable system for protecting your home.
If your property is near an A or V zones, check the elevation of your property against the BFE for the adjacent zones. If your community does not require you to elevate and your mortgage lender does not require flood insurance, it's wise to choose to protect yourself by elevating to or above the nearby BFE. It is not unusual for homesites outside of a FIRM flood zone to be lower than the flood zone. Also, flood patterns often change over time, reducing the accuracy of FIRMs, and floods can and do exceed the level of the BFE.
When designing and building a foundation, be sure you design to avoid problems with termites and moisture. More information can be found in the Stronger, Safer, Smarter - Durable section of this Web site.
This section of our document on flood vents copies a New York State Technical Bulletin from the New York State Division of Code Enforcement and Administration, Effective as of January 1, 2003.
This document is to provide information regarding the requirements of the Residential Code of New York State (RCNYS) and the Building Code of New York State (BCNYS) for flood venting in foundations and enclosures below the design flood elevation in flood hazard areas.
The National Flood Insurance Program (NFIP) has an important objective to protect buildings constructed in floodplains from structural damage caused by flood waters. In support of this objective, the NFIP regulations include building design criteria that apply to new construction and substantial improvements to existing buildings in flood hazard areas.
This is accomplished through the strict requirements of FEMA Technical Bulletin TB-1-93, entitled "Openings in Foundation Walls for Buildings Located in Special Flood Hazard Areas" and ASCE 24-98, entitled "Flood-resistant Design and Construction." These requirements are also incorporated into the International Residential Code and the International Building Code, and subsequently into both the RCNYS and the BCNYS. These requirements are provided later in this document with specific additional clarification.
All of the requirements for flood vents are designed to allow the floodwaters to automatically flow through the building walls that are below the design flood elevation so that the walls are not washed out from under the building. Thus the flood vents are provided to equalize the hydrostatic pressure of the floodwaters. Flood vents have to be bi-directional. One-directional flood vents do not allow the hydrostatic pressure to equalize and prohibit the flow of water either in or out of the enclosed area.
There is a significant difference between a flood vent and an air vent. Both the RCNYS and BCNYS by referencing ASCE 24 requirements use the term "net area." The net area is calculated by subtracting the area of any obstructions, such as louvers, from the gross opening size.
Thus the gross opening size of an air vent with louvers will be larger than that of an unobstructed flood vent depending upon the area of the louvers. Additionally, the potential for blockage by debris when louvers or screens are installed has to be evaluated. Flood vents are required for crawl spaces which are below the design flood elevations, even if RCNYS section R322 or BCNYS section 1202.3 do not require the crawl space to be ventilated.
If air vents are used as flood vents they must be disabled in the open position in order to meet the requirement to provide for the automatic flow of floodwaters into and out of the enclosed area. Vents which close automatically based on temperature are not permitted because it would block the flow of floodwaters.
In the winter there is an inclination to insulate and/or close vent openings, which is not permitted because it would also block the flow of floodwaters. When human intervention is necessary to open the air vent it no longer functions as a flood vent. It is also critical that the bottom of the flood vent be no more than one foot above the outside grade.
Be aware that RCNYS section R327.3.5, "Design certificate" requires that a registered design professional certify that the design and methods of construction to be used meet the applicable criteria of section R327 and BCNYS section 1612.5, "Flood hazard certificates," spells out the requirement that a certification, by a registered design professional, shall be provided to the code enforcement official that the design of engineered flood openings meets the minimum requirements of ASCE 24.
Enclosed areas, including crawl spaces, that are below the design flood elevation shall:
1. Be used solely for parking of vehicles, building access or storage.
2. Be provided with flood openings which shall meet the following criteria:
2.1. There shall be a minimum of two openings on different sides of each enclosed area; if a building has more than one enclosed area below the design flood elevation, each area shall have openings on exterior walls.
2.2. The total net area of all openings shall be at least 1 square inch for each square foot (275 mm for each square meter) of enclosed area.
2.3. The bottom of each opening shall be 1 foot (305 mm) or less above the adjacent ground level.
2.4. Openings shall be at least 3 inches (76 mm) in diameter.
2.5. Any louvers, screens or other opening covers shall allow the automatic flow of floodwaters into and out of the enclosed area.
2.6. Openings installed in doors and windows, that meet requirements 2.1 through 2.5, are acceptable; however, doors and windows without installed openings do not meet the requirements of this section.
The design and construction of buildings and structures located in flood hazard areas, including flood hazard areas subject to high velocity wave action, shall be designed and constructed in accordance with ASCE 24.
ASCE 24 provides two methods for flood venting, non-engineered openings and engineered openings. The first method is as follows:
Section 18.104.22.168 "Non-Engineered Openings in Enclosures Below the Design Flood Elevation"
Non-engineered openings shall meet the following criteria:
1. there shall be a minimum of two openings on different sides of each enclosed area; if a building has more than one enclosed area below the design flood elevation, each area shall have openings on exterior walls.
2. the total net area of all openings shall be at least 1 square inch for each square foot (275 mm for each square meter) of enclosed area.
3. the bottom of each opening shall be 1 foot (305 mm) or less above the adjacent ground level.
4. openings shall be at least 3 inches (76 mm) in diameter.
5. any louvers, screens or other opening covers shall allow the automatic flow of floodwaters into and out of the enclosed area.
6. openings installed in doors and windows, that meet requirements 2.1 through 2.5, are acceptable; however, doors and windows without installed openings do not meet the requirements of this standard.
The second method is:
Section 22.214.171.124, "Engineered Openings in Enclosures Below the Design Flood Elevation," with the following criteria:
1. Each individual opening, and any louvers, screens or other opening covers, shall be designed to allow the automatic entry and exit of floodwaters during design flood or lesser flood.
2. There shall be a minimum of two openings on different sides of each enclosed area; if a structure has more than one enclosed area below the DFE, each area shall have openings.
3. Openings shall be not less than 3 inches (76 mm) in diameter.
4. The bottom of each required opening shall be no more than 1 foot (305 mm) above the adjacent ground level.
5. The difference between the exterior and interior floodwater levels shall not exceed 1 ft during periods of maximum rate of rise and maximum rate of fall of the floodwaters, and at other times during the design, or lesser, flood events.
6. In the absence of reliable data on the rates of rise and fall, assume a minimum rate of rise and fall of 5.0 ft/h; where analysis indicates the rates of rise and fall are greater ...; where analysis indicates the rates of rise and fall are less than ...
7. The minimum total net area of the required openings in non-breakaway walls shall be calculated using the equation: ...
8. The minimum total net area of the required openings in breakaway walls shall be calculated using the equation: ...
The RCNYS incorporated the ASCE 24 non-engineered method in the body of the code text. The engineered design has the same requirements as items 1 through 5 of the non-engineered design.
The NFIP describes an engineered opening as an opening certified by a registered design professional to pass flood water automatically in both directions, within the rate of rise and fall per the strict requirements of FEMA Technical Bulletin TB-1 and ASCE 24.
The term "breakaway walls" is defined as any type of wall using materials and construction techniques approved by the authority having jurisdiction, which does not provide structural support to a structure, and which is designed and constructed to fail under specified circumstances without damage to the structure, or supporting foundation system.
ASCE 24 Table 2-2, entitled Flood Opening Coefficient of Discharge, provides coefficients of discharge based on the shape of the flood opening. The table provides a coefficient of discharge of 0.20 (the lowest value) w opening is partially obstructed. Additionally BCNYS section 1202.3.1, "Openings for under-floor ventilation;" section 1202.3.2 Exceptions, 5 states:
For buildings in flood hazard areas as established in section 1612.3, the opening requirements of ASCE 24 are authorized to be satisfied by ventilation openings that are designed and installed in accordance with ASCE 24.
Ronald E. Piester, R.A., Director
Division of Code Enforcement and Administration
Source Documents: 19NYCRR 1220 - Residential Code of New York State (RCNYS) and 19NYCRR 1221 - Building Code of New York State (BCNYS) see the NYS DOS web page: http://www.dos.state.ny.us/code/pdf/TBfldvent.htm
Critical defects which an inspector should not fail to detect when examining any building component or system are defects which form an immediate, significant safety hazard or defects which are quite likely to involve significant repair or replacement cost, and which involve components or systems which are necessary to occupy and use the building. Methods for detection and diagnosis of these defects are discussed in this document and in its references. Suggestions for inclusion or exclusion of items in this list are invited - see the link "Contact Us".
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