For decades, building codes and conventional wisdom has hold us to vent crawl spaces with outside air to control moisture. However recent research indicates venting crawl spaces only make moisture problems—like mold, rot, and insect infestations—worse.

Advanced Energy, Raleigh, NC studied houses in a North Carolina subdivision testing the moisture levels in homes with vented crawl spaces and homes with closed crawl spaces. The research showed the vented crawl spaces’ moisture levels fluctuated with the outside humidity, while the closed crawl spaces remained dry.

The results from the crawl space research project show that, in humid climates, three things need to be done to keep crawl spaces dry:

  1. Fixing water entry problems
  2. Closing off the wall vents
  3. Installing a full-ground moisture barrier that can effectively reduce crawl space moisture levels.

In the real world it can be difficult to meet these three requirements on a given home. Strategies and costs differ for new construction and existing homes. Focusing on existing homes, a typical crawl space mold control plan involves the following elements:

Train job supervisors in crawl space moisture management work. Because this will be a new line of work, training, work specifications and information resources are limited at this time. In concert with the crawl space research committee, the Basement Health Association plans on publishing training materials and starting a new crawl space certification in 2013.

Conduct a detailed crawl space inspection to identify water entry problems, current moisture levels, extent of visible mold, pest control requirements, etc. Crawl spaces should not be closed off if either of the following conditions is present: unresolved major water entry problems or the presence of gas or oil furnaces or water heaters that are supplied with combustion make-up air from the crawl space. Combustion safety strategies can be employed that will allow these crawl spaces to also be closed. One item not to overlook is the hot water heater temperature and pressure relief line, as well as the drain pan. These overflow pipes should not terminate in the crawl space. Add piping to run these lines to outdoors to prevent a water heater overflow from flooding the crawl space.

Protect workers’ health with respirators, safety glasses and protective clothing. Mold air testing by the crawl space research project shows that mold levels in the air increase dramatically when people enter and move around in crawl spaces.

Seal off rising ground moisture from evaporating into the crawl space air. Remove all debris and materials from the crawl space so that all exposed ground can be fully covered with a layer of at least 6-mil polyethylene. Lap seams 12 inches. Greater details and better materials are being developed.

Install a crawl space drain. This will prevent plumbing leaks and house water spills from creating standing water on top of the ground-lining polyethylene. Install one or more drains to daylight or to a sump pump. A drain line to daylight should include a backflow water damper and rodent mesh screening at the discharge end.

Prevent moldy crawl space air from entering the house by sealing floor holes and duct leakage. Mold spores and gases (mycotoxins) readily become airborne. Sealing the floor and ducts creates a barrier that keeps moldy crawl space air out of the house.

Close the crawl space from outdoors. Seal wall vent openings with rigid foam blocks cut to fit and seal with caulk, mastic or expanding foam. Also, seal all plumbing, electric and other wall penetrations. Adjust the crawl space access panel/door so that it tightly closes. Sealing a crawl space can reduce moisture levels to the point that hardwood flooring shows gaps and sheetrock joints crack. On the other hand, some buckled hardwood floors have laid down.

After wall vents are closed, use a dehumidifier to forcibly dry the excess moisture in the crawl space. It is highly recommended that a permanent dehumidifier be installed in the crawl space to ensure that low moisture levels (45{32139241e4574d71739665e265206cc0241a6321f53c59be91c83ca1707ea719} to 55{32139241e4574d71739665e265206cc0241a6321f53c59be91c83ca1707ea719} RH) are maintained. Other moisture control strategies are under investigation.

Monitor the crawl space performance. Provide the homeowner with a remote bulb relative humidity meter located in the home that displays the crawl space relative humidity level. Inspect the crawl space periodically to ensure that water entry does not lead to standing water collecting on top of the ground poly. Install a radon canister in radon areas to ensure that crawl space radon levels remain at safe levels.

Thanks to the research conducted by Advanced Energy, more contractors are closing crawl spaces in the humid climates in the United States. Advanced Energy’s research in North Carolina was so substantial that it actually moved the officials to change building codes to include guidelines on closed crawl spaces.

Advanced Energy is a non-profit corporation located in Raleigh, NC, that serves as a state and national resource to help utility, industrial and residential customers improve the return on their energy investment. Its mission is to create economic and environmental benefits through innovative approaches to energy.