Using building wraps as air barriers

Photos courtesy Typar Construction Products

by Bijan Mansouri
Controlling the movement of air between the conditioned interior environment of a building and the exterior is a core function of building science. However, if you ask someone the best method of doing this, you are likely to get dozens of different answers. The best route depends on several factors—from building schedules and regional climate to building codes and industry best practices. There are also many product and installation factors to consider. One potential option for air barrier assemblies involves using building wraps and other weather-resistant components IN ORDER to guard against airflow without preventing moisture escape.

Quite simply, an air barrier is a material or system of materials designed to control airflow between conditioned and unconditioned spaces. It serves as the primary air enclosure boundary separating indoor and outdoor air. Within multi-family construction, the air barrier system also separates the conditioned air from any given unit and adjacent units. Air barriers also typically define the building enclosure’s pressure boundary.

The requirement for continuous air barriers was added to the International Energy Conservation Code (IECC) for the 2012 version, and there are a number of ways to meet this requirement. As part of this mandate, the code requires all new construction and additions to be visually inspected, as well as pressure-tested, as standard operating procedure.

To know whether a given material can serve as an effective air barrier, it is first important to understand an air barrier can be defined as either a material or an assembly—each category is subject to a specific set of tests. For an individual building material to be classified as an air barrier, its air permeance (i.e. amount of air migrating through materials rather than holes or gaps) must be equal to or less than 0.02 L/(s-m2) @ 75 Pa (0.0004 cfm/sf @ 1.57 psf) when tested in accordance with ASTM E2178, Standard Test Method for Air Permeance of Building Materials. Gypsum board, liquid-applied membranes, sprayed polyurethane foam (SPF) insulation, and this article’s focus—polyethylene or polypropylene building wraps—are all examples of air barrier materials.

The requirements for an air-barrier assembly are somewhat less stringent, and are measured in terms of air leakage. When tested in accordance with ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies, the subject must be 0.20 L/(s-m2) @ 75 Pa (0.04 cfm/sf @ 1.57 psf) in both directions (i.e. infiltration and exfiltration), which works out to 10 times greater than a material alone. This method is intended to simulate the performance of various air barrier materials and accessories when combined into an assembly. This ‘air barrier assembly’ is defined as a group of materials assembled and joined together to provide a barrier to air leakage through the building envelope. For example, use of a weather-resistive barrier (WRB), combined with properly installed flashing and tapes, would be considered an air barrier assembly.

Air barriers can also be defined through whole-building testing in accordance with ASTM E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization. This test method is intended for the measurement of the airtightness of building envelopes of single-zone buildings. For the purpose of this test method, many multi-zone buildings can be treated as single-zone by opening interior doors or inducing equal pressures in adjacent zones.

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