Building Shell : Air & Vapor Barriers Explained

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Our friends at buildingwell.org were kind enough to share some of their building science expertise on air and vapor barriers. 

Building Shell : Air & Vapor Barriers

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If the scope of your retrofit is such that you are rehabbing the walls, it is important to understand the role of vapor barriers and their cousins, vapor retarders and air barriers. The distinctions between these three wall assembly components are often confusing. Since each serves a specific, and sometimes unwanted, purpose, making deliberate decisions about which to include in your building and where to locate them is critical.

Air Barriers

Air barriers are critical for creating a sealed building envelope. Gypsum board, OSB, or plywood are generally considered appropriate air barriers. It is critical that the air barrier is continuous, which means that all seams need to be sealed, as well as spaces around windows and electrical outlets, the space where the walls meet the roof, and any other gaps that may be present in the envelope of the building.

Vapor Barriers

Vapor barriers and vapor retarders control the movement of airborne water vapor through the wall assembly. Physics tells us that water vapor moves from areas of heat to cold and from areas of greater water concentrations to drier areas. That means that, depending on the climate and the building's use of air conditioning, the necessary location of vapor barriers varies.

The problem arises when a vapor barrier is built on the appropriate side of the wall for one season (i.e. on the inside of the assembly for summer in the midwest), but in the opposite season (in this case, winter), the vapor barrier is now on the wrong side, causing water to condense as it tries to move toward the warm inside of the building but is inhibited by the vapor barrier and prevented from drying, potentially causing mold and other moisture problems. For this reason, the use of vapor barriers in walls is generally discouraged.

Due to the potential for moisture condensation and mold, it is recommended that you not use a vapor barrier in the walls so that they can breathe both ways.

Vapor Retarders

Although the use of vapor barriers is discouraged, vapor retarders are required by some codes to be on the interior surface of the insulation in insulated wall and floor assemblies where the permeance of the exterior sheathing is greater than 1.0 perm OR requires a Class II (or lower) vapor retarder on the interior surface of insulation in insulated wall and floor assemblies where the permeance of the exterior sheathing/cladding assembly is less than or equal to 1.0 perm and greater than 0.1 perm.

For example, where plywood or OSB exterior sheathing is used, an unfaced fiberglass batt can be installed within the wall cavity and then the gypsum board can be painted with latex paint (Class III vapor retarder) on the interior of this assembly. Where a Class II vapor retarder is required on the interior you could use a kraft paper faced fiberglass batt to meet the code requirements. You can still use a vapor retarder to meet code requirements, but avoid using a vapor barrier because of its likelihood of causing mold issues.

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