Crawl Spaces: The Downsides of Basements With Few of The Benefits
Bird's eye view
Crawl spaces should be insulated and sealed
Many building experts recommend against crawl spaces because they have the water problems of a basement with almost none of the storage space, at much higher cost than a slab.
Still, sometimes a crawl space makes sense. Crawl space walls should be insulated with rigid foam or closed-cell spray polyurethane foam, and should be sealed rather than vented.
Crawl spaces should be treated as if they were miniature basements, which is exactly what they are. The best current practice is to make a crawl space a conditioned area like the rest of the house. This is permitted by newer versions of most building codes, and is much better for the house and residents.
See below for:
Choose a durable ground cover
The minimum covering for a crawl space floor is 6-mil poly with taped seams. Other, more durable options include:
Many crawl spaces include mechanical equipment and plumbing that needs regular servicing. The material used to cover the crawl space floor must be easy to clean and durable enough to survive visits by maintenance workers.
A crawl space floor should be above grade
Ideally, the floor of a crawl space should be a little above the exterior grade at the foundation perimeter. Although this raises the first floor, complicating the installation of a handicap access ramp, it reduces the chance that the crawl space will be damp.
Leave plenty of headroom.The higher the crawl space, the easier it will be for workers to do a good job installing and maintaining equipment located under the house. Building codes typically call for a minimum 18 in. distance between grade and floor framing, but boosting that to 24 in. will make it easier to work down there. It's a good idea to include a generous number of permanent lighting fixtures.
Sealed crawl spaces should be conditioned. Conditioned air can be introduced by installing a supply register and a return-air grille. Another approach is to install a supply register balanced by a small exhaust fan connected to the exterior. The Building Science Corp. outlines a number of options in a thorough study of crawl space issues at http://www.buildingscience.com/documents/reports/rr-0401-conditioned-cra... space.
Choose the right tape
Use an appropriate tape when sealing polyethylene seams. Choices include housewrap tape (for example Venture #938 or #1585CW), Film-Gard Tuff Tape, or fiberglass mesh embedded in mastic.
For more information on tapes, see Air-Sealing Tapes and Gaskets.
Section N1102.2.8 covers requirements for crawl space wall insulation.
Although the illustration shows a crawl space with a below-grade floor, a better designed crawl space has a floor that is several inches higher than the exterior grade.
Illustration:Code Check Building 2nd Edition. click to buy .
ABOUT CRAWL SPACES
Vents are counterproductive
There are two types of crawl spaces: vented and sealed. Vented crawl spaces are associated with moisture problems and mold. Although older building codes required crawl spaces to be vented, newer versions of the code allow sealed crawl spaces. In most cases, a home with a sealed crawl space will use less energy than a home with a vented crawl space.
It may seem like a good idea to allow fresh air to circulate through the crawl space, at least during the summer. Locating vents on opposite sides of the crawl space should let air waft through, removing moisture and keeping things dry, right? Not really. What actually happens is that water vapor in humid summer air comes into contact with cool surfaces under the house and condenses into water. The dampness can foster the growth of mold, leading to decay (and the need for repairs). This is an especially likely scenario in the southeastern United States, where crawl spaces are common and summer air quite humid.
When crawl space walls are sealed and insulated and the air in the crawl space is heated and cooled, the potential for condensation problems is greatly reduced. Perimeter rigid foam insulation can be applied to either the inside or outside of the walls.
MORE ABOUT CRAWL SPACES
All crawl spaces require a layer of 6-mil (or heavier) polyethylene plastic spread over the floor of the crawl space to help keep moisture and soil gases from getting in. The plastic should be continuous, taped at any seams, and mechanically attached and sealed at the perimeter. For a superior crawlspace, consider covering the polyethylene with a thin slab.
For more information on best-practice details for sealed crawl spaces, see Building an Unvented Crawl Space.
Where radon is a hazard, the crawl space can be safely vented by installing perforated plastic pipe in gravel beneath the polyethylene ground cover and running the stack up through the roof.
For more information, see All About Radon.
Crawl space walls should be damp-proofed just like a full foundation wall to prevent water from migrating inside. If the floor of the crawl space is lower than the exterior grade, the foundation must have perimeter footing drains.
Newly built sealed crawl spaces may contain high moisture levels, especially if the crawl space was open during a spell of rainy weather before the house was closed in. In such circumstances, it may be prudent to install a portable dehumidifier in the crawl space, at least temporarily, to remove moisture that accumulated during construction. After six months, the dehumidifier can probably be safely removed.
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