Q: When a home is built over a crawlspace foundation, what are the advantages, if any, of sealing the vents versus leaving them open?
A: ABOUT 250,000 NEW HOMES ARE built over crawlspaces each year, most of them in a band from Northern Florida to Southern Indiana and Ohio. The reasons usually have less to do with building science than with what the local market considers a better house. “In some places, people think slabs are indicative of a poor quality house, where in others, people think the same way about crawlspaces,” says Bruce Davis, research director at Advanced Energy, a Raleigh, N.C., building science consulting company.
Whatever the reasons for building them, crawlspaces are notoriously damp. Their high moisture levels can cause complaints including mold growth, high humidity, and buckled hardwood flooring.
Most crawlspaces are built with foundation vents, which are supposed to keep moisture problems to a minimum. Building scientists have long asserted that the vents bring in more moisture than they let out. Davis, along with building science associate Cyrus Dastur, just finished a study that looked at whether those assertions were true, and whether it would make more sense to enclose a crawlspace. “We wanted to see if we could do something different in the construction of crawlspace foundations that would solve moisture problems and not cost more energy consumption,” says Davis.
The study consisted of 12 houses being built on the same street in Princeville, N.C. Davis and Dastur divided the homes into three groups of four homes each and tried different crawlspace techniques on each group, as follows.
Group 1: The first four homes—the control group—were built with what Davis calls “the best vented crawlspaces possible.” The homes didn't have any drainage problems. A good plastic vapor retarder was laid on the ground, and R-19 insulation was installed between the first-floor joists.
Group 2: In the next four houses, they left the R-19 insulation between the joists, but sealed all the foundation vents, taped all the seams in the vapor retarder, and extended the vapor retarder up the perimeter walls. They ran a 4-inch duct from the home's HVAC supply trunk line to the crawlspace. The duct put 35 cfm of air into the crawlspace whenever the air handler was running.
Group 3: In the last four homes, they didn't install the R-19 insulation, but instead put 2 inches of R-13 foil-faced polyisocyanurate foam on the crawlspace walls. As with Group 2, they sealed the vents, taped the vapor retarder seams, extended the vapor retarder, and ran a duct from the supply trunk to the crawlspace.
In each home, small, battery-operated data loggers installed in multiple places took air samples every 15 minutes. They measured temperature and relative humidity in all crawlspaces, as well as inside and outside the house. Each house was conditioned by a packaged unit heat pump, so the researchers installed meters that measured the heat pumps' electrical use. The researchers also monitored wood moisture content, taking readings from 10 places in every crawlspace every 60 days.
WHAT THEY FOUND Davis and Dastur found that the vented crawlspaces in Group 1 were indeed moisture traps, with routine relative humidity (RH) over 70 percent from early spring to late fall and over 90 percent in the summer. “The temperatures in the crawls were actually cooler than the dew point of the outside air,” says Davis, which meant that any air brought into the crawl would condense on the cooler surfaces. This created a fertile ground for mold growth.
Learn more about markets featured in this article: Greensboro, NC.