Section 4: The Right WRB for the Job

WRBs are available in dozens of varieties marketed as the same product, but they don’t offer the same level of performance. All of them should create a barrier behind exterior cladding while protecting the sheathing during weather events, provide a vapor-permeable membrane that allows moisture trapped in sheathing to escape, and be an energy-efficient air barrier to stop air infiltration into the building. But beyond that, which WRBs are best?

The International Code Council Evaluation Service (ICC-ES) evaluates a variety of key characteristics of WRBs, including water resistance, durability, air resistance, vapor permeability, drainage, flammability and others.

Water resistance

A WRB should be able to pass both water ponding tests in this category. The first measures its resistance to a pond of 25-mm (1-inch) water over two hours, and the second is a hydrostatic pressure test where it is subjected to a pressurized column of water for five hours.

The International Residential Code requires builders to install a layer of number 15 asphalt or paper-backed stucco lath over the wall sheathing or studs in new construction. This requirement can be met in a number of ways because it allows for use of a housewrap or another approved water-resistive barrier—in other words, there is a little room for interpretation. Several materials meet ICC-ES requirements as a substitute including plastic housewraps, liquid-applied WRBs, grade-D building paper and some wall assemblies incorporating rigid-foam insulation.

Durability

The WRB must endure the handling and application process without compromising its water-resistance capability. Its tear resistance, or tensile strength, is examined as well as UV and cold resistance. (Many WRBs recommend siding installation within 90–180 days, but in most cases, it should happen within 30 days if possible; the cold resistance test ensures there are no cracks at low temperatures.)

A superior WRB will withstand handling and application while maintaining its resistance to water, sun and/or cold temperatures.

Air resistance

An air barrier must continuously stop air movement across the building enclosure and have its air permeance equal to or less than 0.02 L/(s-m²) @ 75 Pa when tested in accordance with ASTM E2178, “Standard Test Method for Air Permeance of Building Materials.” The wall system should be able to withstand positive and negative combined wind and HVAC pressures on the envelope without sustaining damage or displacement.

Vapor permeability

The WRB’s vapor permeance, as well as any structural sheathing (for example, plywood) may accelerate inward vapor drive in some cases, but the degree of vapor restriction is unknown and many of those conditions haven’t been studied extensively. When a WRB is used in cold climates where outward vapor flows may occur, more problems may present themselves. A low-permeable WRB with no insulating sheathing with vapor-permeable cavity insulation results in decreased sheathing temperatures, which may result in condensation. To counteract this, either the sheathing temperature must be elevated through the use of exterior insulation or a more vapor-permeable WRB must be utilized in order to encourage outward drying.

Vapor diffusion is the movement of water vapor through materials when driven by variations in vapor pressure, brought on by differences in air temperature, among other things. That vapor diffuses through materials from the higher pressure to the lower pressure and this usually means from the warm side to the colder side. In colder climates, vapor drive generally moves from inside where it is warmer to outside.

The concern with vapor is when the higher-density moisture in the warm air becomes trapped inside, pushing in all directions toward the wall, ceiling, and flooring assemblies. High-perm WRBs will allow for drying if there is continuous insulation outside of the sheathing; this arrangement can also shift the location of the dewpoint outside and past the sheathing. High-perm WRBs aid in vapor diffusion and drying potential moisture intrusion.

In short, 10–20 perms should achieve the desired balance of protection from moisture and drying capability.

Drainage

Drainage is essential to the WRB so it helps keep walls dry and inhibits moisture. It has been determined that a 1-mm drainage gap will sufficiently drain water faster than rainwater is expected to penetrate behind cladding even in most extreme conditions. Manufacturers, builders and contractors are focused on WRB products designed to provide an integrated gap and drainage plane.

Flammability

When it comes to flammability, look for a Class A fire rating since it is the industry standard for WRBs. The IBC rating here means the flame-spread index must be less than 25 and the smoke development index (SDI) must be less than 450 per ASTM E 2178, ASTM E 2357, ASTM E 84 and National Fire Protection Agency (NFPA) 285.

Other WRB attributes

Surfactants (surface active agents) reduce the tension of liquid so it can penetrate deeper into a material. They occur naturally in cedar, stucco, stone mixes and other materials. Water that gets into a building system carries some surfactants and can help them move deeper into the walls. So, does the WRB in question resist surfactants?

Cladding is important. Tightly fastened cladding, such as cedar siding or fiber cement board, might allow water trapped between the siding and a smooth WRB to collect and could eventually make its way into the framing. Reservoir claddings (e.g., brick, stucco and stone) hold water, so once they get wet, the water can migrate elsewhere and cause problems. In these cases, the installer needs to separate the cladding from the rest of the assembly with a capillary break in the wall.

Climates are another factor to consider and was addressed in Section 1.

Manufacturer reputation and experience matters a lot to builders and contractors. Builders consider products’ certification and acceptance in associations along with any warranty.

WRBs need to be comprehensively and meticulously integrated with other elements of the building envelope in order to retain system integrity, and they need to include compatible tapes for seaming and adhesive flashings for openings in order to improve air and moisture resistance.

WRBs need to be very carefully combined with other elements of the building envelope in order to retain system integrity, which includes compatible tapes for seaming and adhesive flashings for openings in order to improve air and moisture resistance.

The various wall components need to be evaluated for performance in specific temperature and humidity profiles and in location-specific indoor and outdoor conditions so the right configuration can be found.

After all of these aspects have been considered, a WRB that passes these tests is the right one for the job—the one that will have the most positive impact on health, safety and wellness for occupants of a building.