Retrofitting a vapor barrier
My son’s post-and-beam house in Virginia Beach was built on a pier foundation without the installation of a vapor barrier in the crawlspace. Because of the ground’s high moisture content, I feel a vapor barrier should be installed.
Anthony J. Bruno, Arlington, VA
Daniel Milton Hill, an architect in Eugene, Oregon, replies: Let me first suggest the least expensive, but also the most physically difficult, way to retrofit a vapor barrier in a crawlspace. For your son’s post-and-beam home, I would cut strips of 6-mil black polyethylene 12 in. wider than the spacing of the piers; that is, if the house has piers on 4-ft. centers, you’ll need 5-ft. wide strips of plastic. Cut a lot of strips, then roll the plastic up and crawl into the space, taking a utility knife with you and some rocks or bricks to hold down one end of the plastic. As you unroll the plastic, cut 6-in. slits at each post and tuck the plastic around the post as tightly as possible. Repeat this task for each bay, lapping the plastic at least 6 in. over the previous sheet.
Then, repeat the process, going perpendicular to the first layer, thus creating a crisscross pattern. This barrier is not completely impervious because of the seams and the slits around the posts, but it will help keep down some of the moisture in the crawlspace.
A second idea is to spray a lightweight air-entrained concrete over the whole crawlspace, including the piers. Obviously this process is much more expensive, but it’s physically a lot easier. I talked to some local weatherization experts in my area, and they believed air-entrained concrete would provide a good moisture barrier. We also talked about using a spray-foam material, but depending upon local problems with pests, foam might create a nesting environment for unwanted house guests.