Because they’re unvented, sealed crawlspaces require a terminal that brings in conditioned air.
Todd Detwiler Because they’re unvented, sealed crawlspaces require a terminal that brings in conditioned air.

While foundation vents are supposed to prevent crawlspaces from developing moisture issues, over the course of a couple of centuries of home building, it’s become apparent that this theory doesn’t always hold up. But smart builders have found a way to avoid the callbacks, discomfort, and energy loss that come with damp, humid crawlspaces; these days, of the 15% to 20% of U.S. homes that are constructed with crawlspaces, one-third are now unvented.

A report published in 2013 by the Department of Energy’s Building America Solution Center comparing 12 new homes—four with vented crawlspaces and eight with sealed and conditioned crawlspaces—showed a 15% to 18% reduction in heating- and cooling-related energy consumption with the sealed models. Some of those offsets are associated with routing ductwork through a conditioned air space. Other benefits include a 20% reduction in crawlspace humidity.

Sealed crawlspaces achieve these benefits by completely sealing the ground floor with a minimum 6-mil air and moisture barrier, along with sealing and insulating the foundation and perimeter framing and an air-tight access panel. Experts also suggest adding a conditioned air supply with back-feed dampers (but no returns). Other requirements include a 4-inch threshold at access doors and a gap along the top portion of perimeter foundation walls to facilitate inspections for insects.

“From my perspective, as a builder, there’s a degree of liability protection [in sealed crawlspaces],” explains Todd Usher, president of Greer, S.C.–based Addison Homes. “In certain areas, it isn’t a question of when vented crawlspaces will get mold and mildew, but how soon.”

In the years before founding his building company, Usher worked as a home inspector, where he says he found moisture issues in 99.9% of crawlspaces. As a result, for approximately the past six years, he has made sealed crawlspaces a standard feature on his homes, which he says adds roughly $2.50 per square foot of crawlspace.