Buildings shield us from the natural elements, yet they too require protection to ensure their performance and integrity. To this end, the many layers of materials comprising a building envelope work together in keeping it—and us—healthy.

Within the building envelope, weather barriers “physically separate the interior conditioned space from the unconditioned exterior environment,” says Ben Meyer, a building-science expert at the DuPont Building Knowledge Center. Typically made from polymer-based materials, building wraps can serve as air barriers, vapor retarders or barriers, and secondary water-resistive barriers to the exterior cladding, which are required by code. Although the different types of barriers are not interchangeable per se, many building wraps can perform a combination of either two or all three functions.

Water-resistive barriers prevent rain that has penetrated the exterior cladding from entering the wall assembly. These barriers are sometimes referred to as weather barriers or weather-resistive barriers, both of which are generic terms, “like facial tissue,” says Peter Barrett, a product manager at Cosella-Dörken Products. Typically between 5 and 15 mils thick, water-resistive membranes tend to be vapor permeable to allow buildings to dry. Some may have a textured surface that “provides a continuous drainage space with claddings, like siding, that would traditionally be installed directly against the air and water barrier,” Meyer says.

Air barriers, meanwhile, restrict unintended airflow through the building envelope, minimize air leakage, and thus improve the building’s overall energy performance. The infiltration and exfiltration of unconditioned and conditioned air, respectively, “require the HVAC mechanical system to be oversized to compensate for these losses,” Meyer says. An air barrier can limit these losses, saving between 7 percent and 43 percent in energy usage, depending on climate. Air barriers are tested in accordance with ASTM E2178 “Standard Test Method for Air Permeance of Building Materials” and defined by the Air Barrier Association of America as having an air permeance of less than 0.004 cfm/ft 2 at the reference pressure difference of 1.57 lb/ft 2.

Air barriers must be continuous to keep wall systems dry. Because air transports moisture, its unregulated movement through the building envelope can indirectly compromise the structure’s durability and adversely impact indoor air quality, Barrett says. As air cools, it loses its ability to retain moisture. If it reaches the dew point while passing through a wall, condensation can form on interior building materials, expediting deterioration and creating an ideal environment for mold growth. Differences in the exterior and interior air pressure also mean that “air leakage can transport VOCs released by off-gassing of different building materials within the building enclosure or pollutants from outdoors,” Meyer says.

Vapor retarders and vapor barriers, the third category of building wraps, protect buildings from moisture by resisting the diffusion of water vapor through a wall. Products are evaluated following ASTM E96 “Standard Test Methods for Water Vapor Transmission of Materials” and given a perm rating to indicate their level of water vapor permeance. The lower the number, the less permeable it is to vapor diffusion—that is, less vapor can pass through it.

Although vapor retarder and vapor barrier are often used interchangeably, the latter typically refers to materials with a perm rating of 0.1 or less; these are considered impermeable and categorized as Class I by the International Residential Code. Class II refers to semi-impermeable vapor retarders with a perm rating between 0.1 and 1. Class III materials are semi-permeable with a permeance level between 1 and 10 perm.

While residentially focused, these classifications can help guide the selection of a vapor retarder and its position in a wall assembly, both of which are informed by climate. Because water vapor migrates from areas of high pressure to low pressure, vapor retarders are generally situated on the warm side of the wall. In areas with more heating days, this means placing them on the inside of a building envelope, “right under the drywall,” to impede moisture movement from the more humid interior through the wall and outside, Barrett says.

In southern climates, vapor retarders may be installed toward the exterior of the wall or even left off altogether. In fact, “many people argue you can do a building without a vapor barrier so long as you’re airtight,” Barrett says. One reason for this is that a far greater amount of moisture is transported by air than through diffusion. Consequently, unlike air barriers, which are only effective when continuous, vapor retarders can withstand a little imperfection. Furthermore, vapor permeability is not always a bad thing: “The key to sustainability for a wall isn’t necessarily to keep it from getting wet but to allow it to dry as quickly as possible, which dramatically reduces the risk of rot and mold,” Barrett says.

Building wraps can be installed in four ways: mechanically fastened, self-adhered, fluid-applied, and spray-foamed. Cost, climate, and the wall system design can help determine which method a project should use.

Mechanically fastened building wraps are the most cost-effective and can be installed in many weather conditions, Meyer says. “Most are vapor-permeable, so they can be used in any climate and wall design.” They are not, however, ideal for use on high-rise buildings, as strong winds can cause pumping and billowing and “they tend to blow off,” says Joseph Lstiburek, an ASHRAE Fellow and principal of Building Science Corp. in Somerville, Mass. Moreover, fasteners represent potential leak points, Barrett says.

Self-adhered sheeting, which is unaffected by wind loads, may be a better option for high-rises. Substrates should be primed for optimal bonding. Because cold temperatures and humidity can affect installation, designers should specify an adhesion field test. On the downside, self-adhered building wraps are difficult to “wrap around openings with complex geometries,” Lstiburek says.

Fluid-applied building wraps also boast fewer potential leak points by eliminating fasteners and minimizing seams. Rolled or sprayed on, they are fully adhered to the substrate and can be applied quickly, saving on labor costs. The substrate must be clean to improve adhesion. However, because many wraps are water-based, they may be less effective in cold temperatures, Lstiburek notes. “Results are highly dependent on good-quality workmanship during installation.”

Like self-adhered sheeting, fluid-applied wraps are typically vapor impermeable, limiting their use in certain climates. Recent products in both types, however, boast higher vapor permeances, which promote faster drying.

Finally, spray polyurethane foams resist bulk water penetration, offer an air and thermal barrier and, like their fluid-applied counterparts, easily handle CMU and other irregular surfaces. Vapor impermeability limits their use in certain climates where drying is desirable. Meyer notes that weather conditions can also hinder their installation. Because applying spray foam involves hazardous chemicals, it requires specialized equipment, certified installers and, if indoors, the use of protective gear.

Beyond choosing the appropriate building wrap, project teams must detail and install air and water barriers as complete systems. Designers should specify proper flashing details and integrate manufacturer-approved accessories to ensure wall penetrations are tight. Moreover, the air and water control layers in walls, decks, and roofs as well as around windows and doors must be continuous, Lstiburek says. The absence of such connections in architectural drawings is “the most common mistake.”

Such oversights should become a thing of the past as emphasis shifts to a systems approach to the application of air and water barriers, Meyer says. “Putting all the products together properly can help achieve the system performance designers should expect.” As a result, our buildings will work better and last longer and, maybe, so will we.