The building science community is singing a tune and America’s builders are listening. For years, design professionals have been advocating sealed and insulated crawlspaces. While properly sealed crawlspaces improve indoor air and reduce moisture problems, in most southern homes, crawlspaces are vented. In these traditionally vented crawlspaces, look closely and you’ll often find loosely hanging insulation, poor air sealing at the floor and longstanding mold problems.

An interesting alternative is to seal, insulate and condition the crawlspace, essentially constructing a short, conditioned basement. This lessens the requirements for heating and cooling the home, and it will be more comfortable in winter because the floor will be closer to room temperature. The danger from pipes freezing will also be all but eliminated, and, importantly, the crawlspace is less likely to contain radon—an odorless, cancer-causing gas.

Here’s a top ten checklist for your next crawlspace building project:

1. Moisture can enter the crawlspace when the home is being constructed, creating mold even before the home is completed. Proper drainage, sealing and insulating are important early in construction—before the floor joists are installed.
2. After construction, proper drainage is essential. Install a drainage plane on the foundation and a perimeter French drain that channels water away from the house.
3. Don’t install conventional combustion equipment in the crawlspace. Use a heat pump or a high-efficiency, 90+ AFUE furnace. These units are sealed, directing air into and out of the furnace through hard PVC pipe. The same requirements apply to gas water heaters inside the crawlspace – either relocate them or make sure they are direct vent.
4. Install continuous insulation on the walls. An example is PerformGuard, a borate-treated foam board that deters termites and will not deteriorate should it ever get wet.
5. Air-seal and insulate the band joist area with an R-19 or R-30 batt.
6. Install a continuous, sealed plastic vapor barrier on the ground. Experience has shown that standard “6-mil poly” isn’t strong enough to survive construction so consider thicker, reinforced plastic. Overlap the seams and use mastic to seal the plastic “liner” to the walls and piers.
7. Construct a weatherstripped, properly insulated access door or crawlspace hatch.
8. Either exhaust air from the radon vent system under the liner or supply a small amount of conditioned air to the crawlspace (but don’t install a return duct!) to satisfy the code venting exception. Typically, 2 cubic feet per minute for every 100 square feet of crawlspace floor is sufficient. That’s equivalent to a dampered 3- or 4-inch supply duct for most crawlspaces.
9. Consider installing a dehumidifier plumbed to the outside or to a sump pump. Install a relative humidity (RH) meter and monitor it to keep the space below 50 percent relative humidity.
10. Post a sign inside the crawlspace access that can be read easily. It should tell future occupants or contractors that no chemicals should be applied.

It is important that the builder, owner, code official, home inspector and pest control installer understand how this “short basement” approach to a crawl space works. The concept has been proven but folklore and myths about crawlspace venting still abound. For more information, consult www.crawlspaces.org and the factsheets and tech bulletins at www.southface.org.