Crawlspace foundations come into play for homes that do not have basements but rather a crawl space underneath the floor system. Sometimes crawlspaces can be high enough to actually stand up in but are usually just roomy enough to crawl. Crawlspace foundation walls are usually made from poured concrete or concrete masonry units (CMUs) and usually exist where a foundation needs to be below the frost line, deep enough to negate a slab-on-grade foundation but not so deep as to warrant the added cost of a basement.
One of the most important considerations when constructing a crawlspace is ventilation. Historically, crawlspaces have been vented regularly (every 6 to 8 feet) to allow air flow and decrease the risk of mold or rot occurring due to sustained moisture on the wood framing of the house. Recently, however, engineers have moved away from simply venting crawlspace foundations -- although ventilation is still required by most building codes. However, ventilation alone rarely sustains a dry atmosphere because the soil itself evaporates moisture content from underneath the home. In answer, contractors now lay down a vapor barrier, or vapor retarder, on the soil between foundation walls. This prevents moisture from the soil from affecting the framing above.
Access to the crawlspace is essential and required. Crawlspaces are not only vented but must be insulated as well. An advantage to crawlspaces lies in access for plumbers and other contractors to repair to pipes and conduit that run through the floor system and to repair squeaky floors from underneath. In some cases, such as homes built on slopes, crawlspace foundations may provide some storage as well, albeit for a limited amount of items.
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