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:

  • Cover a dirt or cracked-concrete crawl space floor with 6-mil polyethylene sheeting. Attach and seal the sheeting at least 6 inches up the walls, overlapping and taping the seams to ensure sealing along the full perimeter of the crawl space.
  • Seal perimeter walls to prevent infiltration of outside air; any access doors to the space must also be sealed. Seal at joints between the top of the foundation wall and the mudsill, between the mudsill and the rim joist, between the rim joist and the subfloor, and at any penetrations through walls or rim joists, such as ducts or pipe conduits. Use caulks, adhesive, air-sealing foam, gaskets, or adhesive membranes.
  • Insulate perimeter walls and/or foundation walls according to applicable building codes, using fire-rated, water-resistant rigid foam board or sprayed-foam insulation. Do not use fiberglass batts or spray-on cellulose insulation, which are air permeable.

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.

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Replies to This Discussion

Thank you for focusing on the regional nature of this issue. We often see information from across the country extolling the benefits of sealed and conditioned crawlspaces. Be we live and work in the Northwest, where vented crawlspaces can actually save energy, and do not typically have moisture related problems (if properly constructed and ventilated). It is great to see you recognize the research and experience that exists on this subject and to present it regionally. Thank you!

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