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Acrawl space orcrawlspace is an unoccupied, unfinished, narrow space within a building, between the ground and the first (or ground) floor. The crawl space is so named because there is typically only enough room to crawl rather than stand; anything larger than about 1 to 1.5 metres (3 ft 3 in to 4 ft 11 in) and beneath the ground floor would tend to be considered abasement.
In the United States, a crawl space is often built when building a basement would be impractical, or too expensive. A crawl space can also substitute for aconcrete slabfoundation that would hinderbuilding inspections.[1]
The crawl space's functions include providing access to repair plumbing, electrical wiring, and heating and cooling systems without the need for excavation.Building insulation can also be installed in a crawl space.[2] The crawl space can provide a protective buffer between the damp ground and the wooden parts of a home and, with adequate sealing, help withradon mitigation. Crawl spaces are also sometimes used for storage of items such as canned goods that are not particularly susceptible to destruction bymildew or unstable temperatures.
A crawl space foundation can be used to elevate the lowest floors of residential buildings located inSpecial Flood Hazard Areas above the Base Flood Elevation. TheFederal Emergency Management Agency recommends that the floor of the crawl space be at or above the lowest grade adjacent to the building.[3]
Crawl spaces are not usually an option in cold regions, such as thenorthern United States, where a full basement is needed to get the foundation below thefrost line. Another downside of crawl spaces compared to basements is that they offer less protection againstearthquakes,tornadoes, andhurricanes. Crawl spaces also tend to be more expensive than slab foundations. Problems with crawl spaces, such as leaks, moisture, potential wood damage, and foundation issues may not be noticed as quickly as problems with basements, since people typically do not go into their crawl space as often. A crawl space also may not be as well-suited to a sloped lot as a basement.HVAC equipment in unconditioned crawl spaces tends not to operate as efficiently as it would in a conditioned space such as a basement.[4]
Crawl spaces can be actively or passively vented, or closed. An advantage of a vented crawl space is that harmful gases such asradon orcarbon monoxide (e.g. fromgas furnaces orwater heaters) can escape or be diluted before they can enter the living space.[5] However, in regions with a humid climate, vents to the outside can also allow moist air to come in, which can then condense if temperatures (e.g. on cooler surfaces such as ductwork) drop below thedew point, creating a damp environment that is hospitable toindoor mold growth as well as infestations by rodents and insects, possibly including wood-damaging ones such astermites[1] orcarpenter ants. Even without condensation,relative humidity above 80% can support mold growth and rot wooden structural materials such as floorjoists.[6] Humidity in some sealed crawl spaces is controlled using adehumidifier.
Encapsulation is sometimes used to prevent the passage of air from the crawl space to the living environment, to save energy and improveindoor air quality, since air in the crawl space might otherwise tend to rise due to thestack effect. Encapsulation involves adding a vapor barrier to the floor, sealing off all openings to the outdoors, addingthermal insulation to the walls, and sealing off any remaining gaps and cracks (such as plumbing and wiring penetrations) between the crawl space and the floor of the home. A 2005U.S. Department of Energy study of homes in thesoutheastern United States found that closed crawl spaces with sealed foundation wall vents, sealed polyethylene film liners and various insulation and drying strategies had significantly reduced space conditioning energy use compared to traditional wall-vented crawl spaces with perimeter wall vents and unsealed polyethylene film covering the ground surface.[7] As a further encapsulation measure, crawl space access doors are sometimes located inside the home, or an airtight, insulated access door is built in the perimeter wall.
A crawl space can be susceptible toflooding, a risk that is sometimes mitigated by such measures as usingrain drainage such asrain gutters to conduct rainwater away from the house and sloping the earth away from the house.[8]
Crawl space wall materials may include, e.g., solid concrete orconcrete masonry units.[9]
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