Air Barrier in details

ESSENTIAL KNOWLEDGE

Air infiltration barriers, or ˇ°housewraps,ˇ± as they are known in the industry, have grown in popularity since their appearance in the 1970s in the wake of the energy crisis. DuPont, one of the first companies to introduce such a product, came out with Tyvek? in the late 1970s. Today there is a variety of similar products that reduce air infiltration and improve energy performance.

The primary attribute of housewraps is their ability to operate as air infiltration barriers while not forming an impervious vapor barrier. When placed over the exterior surface of the wall sheathing, the material allows moisture vapor to escape from the frame wall cavity while reducing convective air movement in the insulation, thereby helping to maintain the composite R-value of the wall. The greater the exterior air movement, the greater the benefit.

The ten biggest selling housewrap products fall into one of two basic categories: perforated and nonperforated. Perforated products are either woven polyethylene, woven polypropylene, spun bonded polypropylene, or laminated polypropylene film. These materials are more impervious to moisture vapor migration than nonperforated wraps, thus are provided with ˇ°micro-perforationsˇ± to allow vapor migration and diminish their vapor retarding properties. With the exception of the polyethylene films, all the perforated housewraps are further coated with either polyethylene or polypropylene for added air infiltration resistance. In contrast, nonperforated housewraps are either spun bonded polyethylene or fiber-mesh-reinforced polyolefin. The structure of these materials allows water vapor to pass through, while inhibiting air infiltration.

In addition to their primary functions as air infiltration barriers and water vapor transmitters, some (but not all) of the major housewrap brands are code approved as substitutes for required moisture protection barriers. To gain national code approval as a substitute for No. 15 felt, the product manufacturer must apply to each of the three major model building codes, or CABO, and supply specific testing data on water penetration resistance. With code recognition, the product can be used under all siding applications, including stucco and masonry veneer. Currently, at least four products are listed by all three model codes as acceptable moisture protection barriers: Amowrap, Pinkwrap, R-Wrap, and Tyvek. Tyvek also produces a product, StuccoWrap?, that is specifically intended for use with traditional and synthetic stucco, and is code listed for that application. Other housewraps are acceptable to some codes as weather resistant barriers. Before using a particular product as a weather barrier, its approval should be verified with the governing code.

In addition to air leakage resistance, permeance, and moisture resistance, two other material characteristics are worth considering: UV sunlight resistance, and strength. All major housewrap brands have a manufacturer's rated UV exposure time ranging from 120 days to more than 1 year. Some products are manufactured with antioxidants and UV stabilizers, while others are naturally more resistant by their composition. In the field, however, covering the housewrap as quickly as practicable is recommended, as some UV degradation will occur even over a short period, and other unrelated damage to the membrane can be avoided.

Strength of the housewrap can be critical, as wind conditions or adverse job site handling can tear or puncture the material during and after installation. Even small holes can negatively affect overall performance. The inherent strengths of housewrap can be judged on three levels: tensile strength, tear strength, and burst strength. Respectively, these are the material's ability to withstand damage from pulling and stretching; withstand tearing at nail and staple locations; and to withstand separation of material fibers, fabrics, or films. Unfortunately, testing procedures and standards vary between manufacturers, so product comparison is difficult. Generally, the spun bonded products have good tensile and burst strength but tear easily; woven and fiber-reinforced have good tear and burst strength, but are susceptible to diagonal tensile loading; laminated film products tend to be weakest of all and can lose strength significantly, making a tight installation more difficult.