If your house has a raised floor and no basement, the crawl space is the area between the floor framing and the ground below. This space can be either vented (the most common practice until recently) or unvented (sealed).
Although codes have encouraged vented crawl spaces for decades, recent evidence shows that in humid climates, vented crawl spaces can be subject to mold, decay, condensation on ductwork, and termite damage. Hardwood floors above a vented crawl space can cup, and radon levels in the crawl space may be elevated. In some colder climates, vented crawl spaces can cause problems in winter when cold air causes pipes to freeze.
Newer building codes (e.g., IRC, Section R408.3) now allow unvented, conditioned crawl spaces, to address these problems. Unvented crawl spaces are generally drier than vented spaces. Extending the conditioned space may increase winter heating demand, but the cost is minor compared to the cost of moisture damage resulting from venting.
Climate Zones and Crawl Space Venting
Unvented crawl spaces work best in the midwestern, southern, and eastern United States, where hot-humid summer conditions encourage condensation and cold-moist winter conditions can freeze water in pipes and moisture in ducts.
Vented crawl spaces generally perform well in most of the western United States, where condensation is rarely a problem in the hot-dry and marine climates. Make sure that the floor is insulated above the crawl space, that the insulation is properly supported, and that there is no space between the insulation and the floor.
Research shows that in the West, homes with vented crawl spaces tend to use less energy than those with sealed crawl spaces, even in the marine climate of the Pacific Northwest. Although vented crawl spaces in western climates may have higher average humidity levels, this is not likely to be a problem if condensation risks are low—that is, if the average ground temperature is above the average seasonal dew point.
Outside the dry and marine climates west of the Rockies, a vented crawl space may work best in an extremely cold climate, such as that of northern Canada or even the cooler parts of western Canada, where warm conditioned air in a sealed crawl space can melt the permafrost beneath the home, causing the home to sink.
In cold climates, insulate all plumbing pipes in vented crawl spaces to keep the water in them from freezing. Heat tape can also help where insulation isn’t sufficient to avoid freezing.
In a flood zone or an area with a high water table, it can be useful to install operable vents in an unvented crawl space to allow for drying after flooding.
Sealing a Crawl Space
When an unvented crawl space is being sealed, insulation is placed around the perimeter walls of the space rather than in the floor above. This, along with a fan or air blower that provides conditioned air to the crawl space via ducting, couples the house with the cooler temperatures of the ground. This is particularly helpful in hot-summer climates.
Work with a professional trained in building science to make sure your crawl space upgrade addresses potential problems with moisture and air pressure. For a successful conditioned crawl space, make sure your contractor does the following things:
For the greatest energy savings, all ductwork should be brought into the conditioned space. Seal any ductwork located in the crawl space, and ensure that ducts and appliance vents do not terminate inside the crawl space. Any ductwork in the crawl space should be well supported.
If there is insufficient air exchange with the house above, consider ventilating the crawl space via mechanical exhaust or conditioned-air supply.
If radon is a concern in your area, make sure that the crawl space is properly vented
Crawl Space Drainage
When possible, the floor of a crawl space should be higher than the exterior ground level, to minimize drainage problems. Downspouts should also divert water away from the foundation. If the floor of your crawl space is lower than the exterior ground level, the most effective way to divert water is to install perimeter footing drains (draining to an appropriate outlet) and slope the floor of the crawl space toward a drain or sump.