Few topics in building science seem to have caused as much confusion as the use of a polyethylene vapor barrier in exterior walls.
Once routinely used by builders to prevent the migration of interior moisture into wall cavities, polyethylene is no longer recommended for houses unless they’re built in extremely cold climates.
But related questions keep coming, including this query from Ed in Chicago, which was posted in the Q&A forum at GreenBuildingAdvisor: “It’s well known that improper use of vapor retarders can prevent walls from drying out properly and leading to moisture related issues such as mold and rot,” Ed writes. “Is it uncommon to have moisture issues in walls due to air leaks or vapor diffusion if the vapor retarder is left out? Would there be a concern with moisture issues for a standard construction wall (siding, plywood sheathing, 2×4 or 2×6 framing, fiberglass or cellulose insulation, drywall without vapor retarding paint) if the vapor barrier is not used in cold climates?”
The house under construction (not Ed’s, but the house of someone he knows) won’t include a poly vapor barrier — but the builder doesn’t plan any special effort to make it airtight, either.
“It seems like every story I hear about wall moisture problems is because of improper use of vapor barriers, not because it was left out,” Ed wrote. “Is there concern that leaving out the vapor barrier without making the house reasonably airtight lead to a moisture problem?”
That’s the subject of this month’s Q&A Spotlight.
Don’t forget, vapor barriers had a purpose
Malcolm Taylor reminds Ed that problems associated with the movement of interior moisture is what prompted builders to begin using vapor barriers some 40 years ago, and that walls built in the manner Ed describes may indeed experience problems.
“You need to go back to why vapor barriers were introduced into exterior walls in the first place,” Taylor writes. “Indoor moisture making its way into wall cavities really wasn’t much of a problem until the 1970s, when the push to conserve energy lead to widespread use of (mainly) batt insulation. In Canada this was pushed by the CMHC [Canada Mortgage and Housing Corporation], and it was the widespread wall failures of the houses they had sponsored that was largely responsible for that adoption of poly vapor barriers as a way of mitigating air movement into the exterior walls.”
Taylor says other strategies are available, including the Airtight Drywall Approach and the use of exterior insulating sheathing. “But,” he adds, “I don’t think anyone is advocating building without some method of limiting the movement of air through exterior walls.”
The problem with vapor barriers, Taylor says, crops up when they are placed in such a way that the wall does not have a means of drying out. “The reason the wall would have moisture to dry in the first place would in most cases be that there was not an effective vapor barrier.”
Later, Taylor adds this: “I’m not advocating for their use, but at present, outside the rarified world of green building and it’s one-off projects, sealed poly represents a viable method of stopping air movement that doesn’t interfere with common building practices as they are done throughout most of this continent.”
No, the problem is air leakage
GBA senior editor Martin Holladay suggests Taylor may be confusing air barriers and vapor retarders. “If you pay attention to air tightness (by following the Airtight Drywall Approach), there is every reason to believe that your proposed wall would function very well in Chicago, even without vapor retarder paint,” Holladay writes.
“However, your local building code probably requires the use of a vapor retarder on the interior side of your wall — so you should probably go ahead and install kraft facing (the kind that accompanies fiberglass batts) or vapor retarder paint (if you choose cellulose — the better choice), just to keep the building inspector happy.”
More to the point, Holladay says, is to choose a builder who understands how to make a building airtight. Ideally, that would extend to the drywall contractor. “There’s no reason to build a brand-new house with contractors who don’t understand these principles,” he says. “‘How risky is this?’ is the wrong question. The right question is, ‘Why would I want to choose contractors who don’t know how to do a good job?’”
As to Taylor’s assertion that a poly vapor barrier, when installed correctly, is a viable means of controlling moisture, Holladay says: “Malcolm’s advice is appropriate for many locations in Canada, but is not recommended in the U.S., especially for buildings that will be air conditioned. Leave out the polyethylene unless you have a maple leaf on your flag.”
Vapor diffusion from the interior toward the exterior is an unlikely threat
Dana Dorsett points out that the major moisture threat is wind-driven rain leaking past improperly flashed windows and doors — what’s called “bulk leakage.”
That’s followed by air leakage, he adds, “with vapor diffusion at an extremely distant third place, as much attention as vapor-retarders get in code. If an assembly can be designed to work without poly vapor barriers, it SHOULD be built without them, in my opinon, since (a) In practice the poly is rarely airtight, and (b) poly severely inhibits drying, whether airtight or not.”
Dorsett continued,“In most of the lower-48 of the U.S., the rampant misapplication of poly in locations where it can just as easily be designed-out has caused as many real-world problems as it was intended to solve, and that would include Chicago. Summertime dew points in Chicago are high enough that a ‘drying toward the exterior only’ approach can create mold problems in air-conditioned houses with interior poly and fiber insulation, with or without ai-tight sheathing (or airtight wallboard), especially those with stucco or brick cladding. (Brick cladding and interior poly can be an issue for air conditioned buildings in Winnipeg, too.)”
Our expert’s opinion
Here’s how GBA technical director Peter Yost looks at it:
There are four ways that buildings can get wet: a bulk water leak, capillary suction or wicking in porous materials, convective moisture (air leaks), and vapor moving by diffusion. You should worry pretty much in that order, regardless of climate. And yes, an assembly getting wet by diffusion alone is largely a cold climate issue, or an issue in buildings with inherently high interior relative humidity (like an indoor swimming pool), or both. For all other situations, reliance on just that continuous air barrier is the best way to keep interior or exterior moisture from getting into assemblies.
We avoid Class I vapor retarders (0.1 perms or less, like polyethylene) if we can because they restrict drying and because, when used to keep moisture out of a wall or ceiling, are usually complete overkill. But other factors matter too: for example, the indoor and outdoor relative humidities, as well as the vapor permeability of all the OTHER layers in the wall or ceiling assembly. (For more information on these topics, see Vapor Profiles Help Predict Whether a Wall Can Dry and Are Dew-Point Calculations Really Necessary?)
Walls can get wet from mechanisms other than those already mentioned: for example due to solar-driven moisture or wicking from mortar continuity in veneer masonry. In general, you want to take all the drying potential you can find, rather than unnecessarily restricting drying with less vapor-permeable retarders than required by wintertime interior relative humidities.
I would add that if your building inspector pushes you to install a dedicated interior vapor retarder, “smart” materials such as or should be considered in cold climates.
There is some evidence that asphalt-impregnated kraft paper facings do not really stand up to repeated wetting and drying — at least in terms of their variable vapor permeability — like the other “smart” retarders do.