Q: I read your online response about crawl space vents. I found it very informative. I reside on the coast. My house is located on a lagoon off the inner coastal waterway. The crawl space floor is concrete. There are 10 air vents that measure 8-by-16 inches. I use to open them in the summer and close them in the winter. I was thinking about leaving them closed in the summer to keep out the humidity. Keeping them closed may also diminish the amount of sweating with my HVAC metal ductwork located in the crawl space. The summer temps usually hit between 80 and 100 degrees with 70 to 100 percent humidity.
A: I’m asked this question all the time by both contractors and instructors and the answer depends on the weather conditions where you live. In areas with changing seasons and high humidity problems, the vents need to be opened in the winter (this allows humidity to escape the crawl) and closed in the summer (to keep humidity out of the crawl). If the home is located in a frigid region, the vents should be closed to prevent pipes from freezing and help the floors to stay warmer. If the ducts are sweating, they need to be wrapped with insulation unless they are insulated on the inside of the ducts. Make sure that all joints and seams for the supply and return ducts are taped and sealed airtight. Use either metallic tape – the peel and stick kind – or duct mastic. Other tapes and caulks will dry and fall off sooner or later. If at all possible, the crawl space vents should be permanently sealed and a dehumidifier installed in the crawl. Try to maintain the humidity at a constant level below 70 percent. Provisions will have to be made to drain or to pump away the water that is collected by the dehumidifier.
C. Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Reach him at d.Barnett@insightbb.com.