Know your barrrers

Understand the difference between air, vapor and water-r€sistive barriers

f, In sennteRs ARE Nor to be confused with vapor barrila.ers or water-resistive barriers-although individual materials may provide all three functions. Each barrier has a different function to perform in a building assembly. Dealers and distributors should understand the separate functions their materials provide to sell the right product for each job.

Vapor barriers are materials used to slow or reduce the movement of water vapor through a material (water vapor is also transported by air leakage but this can be resolved by installing an air barrier). Vapor barrier materials are typically installed on the warm side of the insulation in a building assembly.

The position of the vapor barrier in a building assembly will be determined based on the climatic conditions. In warm climates, it will typically be on the exterior and in cold climates, it will be on the interior.

Vapor barrier materials can be self-adhered membranes (depending on composition), fluid-applied materials, rigid cellular thermal insulation board, or medium density spray polyurethane foam (SPF). The thickness of the material will impact whether it is vapor penneable or not.

Water vapor transmission is the measurement of water vapor through a material. In most cases, the results are expressed as vapor permeance. The most common test method for measuring water vapor permeance is ASTM E96. It can be tested using either the desiccant or water method, and typically the results are reported in the units of ng/(Pa.s.m2) or US Perms. Keep in mind that these test methods are different and results from both methods of testing are not comparable. Check your local building code for the method that is referenced in the glossary.

The test measures the water vapor that passes through a material. Proper specimen preparation is critical and experience has shown that if there are pinholes in the material or if the specimen has not been properly sealed into the apparatus using the correct mixture of wax, the results are not repeatable or reproducible.

Wuter-resistive barriers are materials on the exterior of a building that are intended to resist liquid (bulk) water that has leaked, penetrated or seeped past the exterior cladding from absorbing into the exterior sheathing or concrete wall (depending on the application) and further into the wall assembly.

Water-resistive barrier materials can be mechanicallyfastened building wraps, self-adhered membranes (depending on composition), fluid-applied materials, rigid cellular thermal insulation board, medium density spray polyurethane foam, or any other material that has been designed to resist liquid water.

Water-resistive barriers are combined with flashing and other supporting materials to ensure that there is a shingled effect to direct liquid water away from the exterior sheathing.

Testing the water resistance of a material is normally done using the "boat method" (ASTM D119), the "water ponding" method (CCMC Oi 102 section 6.4.5), or the hydrostatic head method (AATCC t21).

Air barrier materials are used anywhere in a building assembly to stop the movement of air into or out of the conditioned space (water vapor can also be transported by air). Air barriers can be mechanically-fastened building wraps, self-adhered membranes (depending on composition), fluidapplied materials, rigid cellular thermal insulation board, medium density spray polyurethane foam, poured concrete. metal. or a host of other materials.

Air permeance is the amount of air that permeates through a material , whereas air leakage is the air that passes through holes or gaps. Any material that has an air permeance no greater than 0.02 L/(s.mz), at a pressure difference of 15 Pa (0.004 cfm/sq. ft. at a pressure difference of 1.56 lb./sq. ft.) when tested in accordance with ASTM E 2118, is an air barrier material.

Air barrier assemblies are a collection of air barrier materials and air barrier accessories (such as sealants, tapes and transition membranes) assembled together to lorm a continuous barrier for air filtration into and out of the environmental separator.

The performance of an air barrier assembly is of far greater importance that the air permeance of the material. When materials and accessories are installed proficiently, the benefits of the assembly have positive benefits for the entire life cycle of the building.

Multi-Function Materials

Materials can provide an air barrier, a vapor barrier, a water-resistive barrier, or any combination of these three functions. Some materials can provide all three, plus the function of a thermal insulation. Other materials may provide all three functions, and some materials may provide two functions. It would be unusual for a material to only provide one function.

By understanding these principles, an effective building separator can be provided to professionals to create effective barriers that help increase building occupant comfort, reduce energy bills, and prevent future moisture and air-related concerns.

- Colin Szewaga, C.E.T., is technical services advisor for the Air Barrier Association of America. He can be reached via www.airbarrie r.ory.

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