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Crawl Space Foundations

By HHI Staff

Unlike basement foundations, a crawl space is not considered part of a home’s living space. Crawl spaces are usually accessed only to repair pipes, ducts or mechanical equipment.


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Moisture prevention is key when constructing and maintaining crawl space foundations. All crawl spaces must have a drainage system around the perimeter to prevent ground water from entering. Equally important is preventing moisture buildup from within the crawl space itself. 

Evaporation from a bare dirt floor is the most significant moisture source with the crawl space. To block evaporation from the floor, a layer of polyethylene plastic sheeting (4-6 mil thick) should be placed on the floor of the space. This will prevent dampness from entering by diffusion and capillary action. The plastic is often covered with sand or fine gravel to protect it from punctures when it’s walked or crawled on. It should be sealed around the perimeter to the foundation wall; seams should be sealed with long-lasting caulking or mastic.


Most building codes require that crawl spaces be vented. But recent thinking among building specialists and moisture-control experts suggests that venting crawl spaces may actually contribute to moisture build-up.

In traditional crawl-space construction it’s common practice to leave the lower-level floor of a house uninsulated. In this case, a crawl space will tend to be warmer than the outdoor temperature in the winter—warm enough to prevent freezing plumbing lines. To prevent excessive heat losses, homeowners routinely close the crawl-space vents in the winter. While this may save on energy costs, closing the vents also prevents the dry outdoor air from entering the space, resulting in moisture build-up.

In the summer, crawl-space vents are generally left open. But when hot, humid, outdoor air enters a cool crawl space, it condenses on cool surfaces, leading to mold growth or rot. While for purposes of energy conservation it makes sense to close the vents in the winter and open them in the summer, just the opposite is true for moisture control.

Unvented Crawl Spaces

If a crawl space is unvented year round, it is imperative that moisture be kept out. This means the crawl space should be constructed just like a shallow basement. It may not need a concrete floor, but there should be a continuous diffusion retarder (usually plastic sheeting) on both the walls and floor. The ceiling should be uninsulated and not contain a diffusion retarder.

All insulation should be sealed very tightly with tape, caulking, gaskets or other sealants. In some cases, it’s recommended that the crawl space be pressurized (with a forced-air heating/cooling system or a ventilation system) to prevent water vapor from the soil, radon or other contaminants. This type of crawl space will be near the temperature and humidity of the living space throughout the year. Effective installation of insulation will go a long way in preventing moisture build-up and minimizing growth of mold or other biological pollutants. Any moisture that does enter the crawl space will escape by diffusion into the living space.

Pipes and ducts in an unvented crawl space generally don’t need to be insulated, but should be well-sealed so you’ll have control over where the air is going.


Recommendations vary for the appropriate ratio of vents per square footage of crawl space.

One formula suggests that the vent area be 1/150 of the crawl-space area and the screening in the vents be no larger than 1/4". The ratio of 1/150 is usually recommended when a crawl space does not have a plastic diffusion retarder on the floor.

More recent thinking suggests that if you minimize moisture entry from both the soil and the living space, a 1/1,500 ratio is sufficient.

Using this formula, a house having a 3,000 sq. ft. crawl space should have 2 sq. ft. of vents (3,000 ÷ 1,500) spaced around the perimeter. The 1/4" screening can allow insects to enter, but if a finer screen is used, more vents are usually required.

Vents are rated by their free area. This refers to the effective area of a vent after subtracting the louvers or screen. This information should be provided with the vent itself, or it can be obtained from the manufacturer.
Vented Crawl Spaces

If a crawl space is to be vented year round, it’s still important to prevent moisture from entering—primarily by the use of well-sealed plastic sheeting on the floor and a perimeter drainage system. The ceiling of a vented crawl space (the upper level’s floor) should be insulated and it should have a diffusion retarder. Vent should be spaced evenly around the perimeter and not be blocked by shrubs.

For adequate cross ventilation, vents should be located as high as possible on crawl space walls and at opposite ends of the space. Placing vents within three feet of corners will reduce stagnant air. Installing vents on or near the rim joists is not recommended because it will prevent effective insulation of the crawl space floor.

Ducts that are located in a vented crawl space should be well sealed. They should also be insulated for energy efficiency and to prevent condensation. Plumbing pipes should be insulated if freezing is likely.



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Crawl Space Foundations:  Created on May 8th, 2010.  Last Modified on May 16th, 2010


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