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BackPage Primer from Environmental Building News

September 1, 2008

Vapor Retarders and Air Barriers:
Managing Moisture in Building Envelopes

In this wall assembly with fiberglass insulation, there are no impermeable vapor retarders, allowing drying in both directions.

When one side of a wall, roof, or foundation assembly is colder than the other, moisture in the air can condense on a cold surface inside the assembly, potentially causing mold problems and structural decay. Moisture from air can get into a wall cavity through air leaks or, in smaller quantities, by diffusing through a permeable material such as drywall. Many people think in terms of vapor barriers addressing both of these problems, but there are two distinct functions: preventing air leakage, accomplished with an air barrier; and controlling moisture diffusion, which calls for a vapor retarder.

The air barrier can be installed toward the inside, toward the outside, or on both sides of an assembly (see EBN Vol. 17, No. 6). It has to be continuous because even a small hole can leak lots of moist air. With vapor retarders, on the other hand, complete coverage is less important than placement.

In cold climates, vapor generated by indoor activities tends to migrate outward through exterior walls. As it does, it encounters colder air and may condense. A vapor retarder installed on the inside of the insulation prevents that movement and therefore the condensation. In hot climates, on the other hand, humid outdoor air migrating inward through a wall assembly can encounter a surface cooled by air-conditioning, and condense.

Preventing condensation inside the assembly is not always as straightforward as installing a vapor retarder on the warm side of the wall, however. Many regions see significant heating as well as cooling needs; the warm side of the wall changes in different seasons. Also, common building materials, such as plywood, function as vapor retarders, even if they were not installed for that purpose. Supplements added in 2007 to both the International Energy Conservation Code (IECC) and the International Residential Code (IRC) recognize a more complex understanding of vapor retarders. The requirements, though not perfect, offer guidance that specifically accounts for climate, cladding and sheathing type, and wall and insulation thickness.

The 2007 updates also reflect the varying degrees of vapor permeability exhibited by building materials, as measured by their perm ratings. Materials with less than 0.1 perms (like polyethylene) are classed as impermeable. Materials with 0.1 to 1.0 perms (like kraft paper) are semi-impermeable. Materials with 1.0 to 10 perms (like latex paint) are semi-permeable. Materials with more than 10 perms (like Tyvek) are permeable.

Although builders most often focus on preventing wetting, the vapor retarder’s ability to allow drying can be more significant. Whether moisture enters the building through air leaks, vapor diffusion, wind-driven rain, curing concrete, or damp framing lumber, a good design allows that moisture to escape through relatively permeable materials. Analyzing all the layers of the building assembly to avoid trapping moisture between impermeable materials—by determining an assembly’s vapor profile—is critical to achieving a healthy and durable building envelope.

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