Q: I know some builders who install polyethylene vapor barriers on the inside surface of all walls, even though kraft facing would technically meet the code. But I've heard that in some cases poly can do more harm than good.

A: AN INTERIOR VAPOR BARRIER PREVENTS moisture generated inside the house from passing through drywall into wall assemblies—a process known as “diffusion”—where it can cause mold, mildew, and even rot.

But while vapor barriers sound good in theory, they've always been controversial. One of the most vocal naysayers is Joe Lstiburek, principal of Building Science Corp. in Westford, Mass. Lstiburek is a well-known building scientist and expert on moisture in buildings who has never been afraid to challenge conventional wisdom. And when it comes to moisture control, one of the most conventional pieces of wisdom is that all homes need a vapor barrier.

In fact, Lstiburek is planning a code change proposal that would replace the requirement for a vapor barrier in mixed and cold climates with a more flexible rule. If it passes, whether or not you install a vapor barrier would depend on the local climate and on how the particular wall was built. Builders would consult a series of tables in the code to determine their requirements.

The reason for the proposal is that many builders unthinkingly install poly, and many inspectors require it, even though kraft-faced fiberglass satisfies the code. But unlike kraft facing, poly is totally impermeable to moisture. At the same time, homes are more tightly built than ever and more of them are being covered with insulated sheathing, which is also impermeable. In such homes interior poly would likely trap moisture in the walls. “Only a lunatic would build a wall assembly that's not capable of drying to one side,” says Lstiburek.

Other offenders are walls that allow moisture to come in from the outside such as above-grade walls in air-conditioned climates and finished basements. These walls need to dry to the inside, and poly would keep them from doing so. Lstiburek guesses that 30 percent to 40 percent of interior insulated basements suffer from mold because of this practice. And with air conditioning now dominating most of the country, interior poly is becoming a bad idea everywhere. “I've seen more buildings trashed by interior vapor barriers based on misguided information on energy conservation requirements than any single failure,” he says.

Not every expert blames mold and moisture on vapor barriers. In fact, Lstiburek is getting some flak from his fellow building scientists for shortchanging the role of air leakage in bringing moisture into walls. “Diffusion is relatively easy to address, but no attention is being paid to air leakage,” says Pat Huelman, coordinator of the Cold Climate Research Center at the University of Minnesota.

In turn, Lstiburek thinks most building scientists put too much emphasis on air leakage. “It's true that in extreme climates, like Minnesota and Florida, air leakage provides some wetting. [But] everywhere else, it provides drying,” he says. And in a high-performance wall with insulating sheathing, “the only way of drying is diffusion. If you have a vapor barrier on the inside and water gets in the walls, the wall is at risk of rot.”

Some building scientists don't disagree with Lstiburek but want him to postpone his proposal. “The background for the change hasn't been made clear enough,” says Bill Rose, a building science researcher at the University of Illinois in Urbana–Champaign, who wants to see computer models of different wall assemblies. “If we're coming up with a new rule, let's first put a tool in place to determine if it's a good rule.”

Lstiburek says field evidence of moisture problems is enough to push the proposal ahead, and he is trying to muster support. One crucial ally would be the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), which means getting manufacturers on board whose products might be disadvantaged by the proposal. But he remains confident: “I think ASHRAE will step up to the plate.” He is also trying to get support from the Department of Energy.

Lstiburek hopes to submit the proposed change to the International Code Council, in Falls Church, Va., sometime this year. Meanwhile, builders should be careful not to build walls that trap moisture. “It would be wise to use a kraft-faced batt rather than poly,” says Anton TenWolde, supervisory research physicist with the U.S. Forest Products Lab in Madison, Wis.

But what about inspectors who disagree? While he tentatively supports Lstiburek's proposal, TenWolde thinks the problem can be addressed without tables, by having the code clearly state that poly isn't required. “A simple change in language would be a lot easier to implement.”

Charles Wardell is a freelance writer based in Vineyard Haven, Mass.