“Almost all vented crawlspaces have mold,” says Davis. “We call them MADD, or mold amplification and delivery devices. And because, in most homes, the crawlspace is connected to the house via leakage, we estimate that of all the air in the house, 50 percent had been in the crawlspace at one time. Mold is an asthma trigger, so a closed crawl can be a risk-reduction technique.”
On the other hand, the closed crawl-spaces in Groups 2 and 3 maintained an RH below 60 percent and actually became drier over the course of the summer. The results for the two types of insulation were so close as to be interchangeable.
The closed crawls also used less energy. “We found we were able to actually reduce energy consumption by about 15 percent to 18 percent, saving $87 to $100 per year,” says Davis.
As for cost, Davis says the rule of thumb was $2 per square foot for new construction. Systems that include a dedicated dehumidifier can bring costs up to $6.50 per square foot.
TO SEAL OR NOTWhat does this mean for the average builder? If you're currently using a crawl-space foundation system, look to see if you're having moisture problems, says Davis. Do you get complaints of buckling hardwoods? Or water in the floor insulation? Or water on the ductwork? Quite often, subcontractors point the finger at each other for these problems. The HVAC guy says the insulation is bad because it has moisture in it. The insulator blames the HVAC guy. But the real problem might be the crawlspace itself, and a closed crawl may be the solution.
If you decide you want to build an unvented crawlspace, expect some initial resistance. Davis says that one particular hurdle was the pest control industry. Pest control contractors traditionally vent crawlspaces, which they believe will reduce the possibility of termites by reducing moisture problems. “When you tell them you want to close crawlspaces, they think you're telling them the stupidest thing in the world, and they want to stop you,” says Davis. But he says they came around when they saw the evidence: “Because of our research data, we have managed to develop a positive perception on their part about closed crawlspaces.”
Davis and Dastur's research has already had an impact in North Carolina: The findings led to a change in the North Carolina building code that made it easier to build closed crawlspaces. But do the findings apply to other climates? To find out, they have started a project that will include three sites around the country: one in a Southeastern state, one in a Northern climate dominated by winter, and one someplace else, maybe the Northwest. The final results should be available in three years.
Charles Wardell is a freelance writer based in Vineyard Haven, Mass.