Q:

Although I’ve sealed the crawlspace where the ductwork is, my house is extremely humid. My AC unit can’t dehumidify because, I suspect, it’s too big. What can I do other than to replace the unit? Can I improve efficiency by lowering the speed of the air-handler fan or by restricting the air-handler inlet?

Hubert Dameron, via email, None

A:

Terry Norris, the senior HVAC/building-science specialist for Advanced Energy, replies: Before we get to the HVAC system, let’s discuss the house itself. Several factors may be causing the moisture problems. Sealing the crawlspace is an excellent start, but you need to check the rest of your house, too.

Air leakage from fans and natural flows may be introducing moisture into your home. A blower-door test performed by a home-energy rater (www.energystar.gov; click on the Find Local Homebuilders link) will identify any extraneous leakage. The test will reveal pressure differences created by the HVAC system when you open and close room doors. These pressure differences can introduce outdoor air into the house’s roof, walls, or floors. If this is the case, relief ducts or grilles should be installed in the offending room(s) to reduce pressure differences.

You said you’ve already sealed the crawlspace, but make sure the crawlspace is 100% sealed. Laying plastic on the floor may not be enough. Seal the seams in the plastic, and then seal the edges to the walls with a mastictype sealer. After the crawlspace has been sealed, a dehumidifier may need to be installed temporarily until the crawlspace and wood joists have dried.

A duct-blaster test also should be performed to test the ducts for leakage. If the ducts have return leaks, air from the crawlspace will be sucked into the ducts; and if the crawlspace isn’t sealed perfectly, humid air from outside will be pulled into the crawlspace, increasing its relative humidity (as well as the dew point) and creating even more water in the crawlspace.