Q&A: Vapor Barrier vs. Exterior Air Barrier

A.Alex Wilson responds:With plywood or foam sheathing on the exterior and a properly installed vapor barrier on the interior (all seams taped or caulked), an outside air barrier is not strictly required and will probably not be cost-effective. However, if the vapor barrier is typically installed — with unsealed seams and full of holes — an exterior air barrier is a good idea.

The research on this issue is scanty. Manufacturers have commissioned studies comparing different brands of air barriers and comparing taped to untaped installations. But no objective, third-party tests evaluate air barriers when the interior vapor barrier is properly installed.

One series of tests that sheds some light on this question was conducted in September 1987, by Architectural Testing Inc., in York, Pa., under the supervision of the NAHB Research Center, for Dupont. Some of the tests were conducted on walls with taped drywall on the interior, kraft-faced fiberglass, and polyethylene taped over outlets and windows — approximating a tight interior vapor barrier. Under those conditions, properly installed Tyvek (taped at the windows and at the top and bottom of the walls) reduced air leakage by 36% under the equivalent of a 14-mph wind. How this relates to energy savings in a real home, however, is unclear.

So should you use an air barrier? I recommend one in almost all situations as an insurance policy. If the vapor barrier is poorly installed or if it breaks down, the air barrier will be there to keep infiltration down. At an average cost of about 12¢ per square foot, it is just slightly more expensive than conventional building paper. An air barrier will also keep the shell much tighter until the windows are in, which can make a big difference in cold-weather construction.

If installing an air barrier, spend the extra few hours necessary to tape the seams and seal around windows. With clad windows, tape the flanges to the air barrier. With wood windows, caulk the exterior casing to the air barrier, or apply a bead of caulk before setting the window in place. Finally, apply a bead of foam sealant between the jamb and framing to provide a second, more dependable seal.

Alex Wilson is the editor and publisher of Environmental Building News, a bimonthly newsletter on environmentally sustainable design and construction.