Moisture management means not only getting water out of the wall, but also allowing air into the wall so it can dry quickly and completely. Since water infiltration poses a significant danger to walls, it is wise to take a redundant approach to moisture management. Redundancy means there are multiple planes of defense against moisture intrusion.
These multiple planes include first the watershed at the face of the cladding or veneer. Behind that is an air space encouraging water to drain out of the wall, breaking the directly connecting path for water to enter the wall. The third redundancy is the use of a highly water-resistant, continuous insulation layer such as extruded polystyrene (XPS), which will shed rather than absorb any water that makes it to the board’s face. (Another insulation option would be polyisocyanurate [polyiso]. Expanded polystyrene [EPS], sprayed polyurethane foam [SPF], and mineral wool could also be used as continuous insulation, but they are not as water-resistant as XPS.) The final line of defense is the water-resistive barrier itself, often installed behind the continuous insulation and over the exterior-grade gypsum sheathing. All of the redundant layers are a natural part of masonry veneer construction.
Air- and water-resistive barriers are often a single product, the same layer in the wall, which resists bulk water penetration and wind-driven rain penetrating the exterior cladding. This contrasts with vapor, which either enters the wall system by permeation or is carried into it by air leakage. In a complete wall system, depending on the regional design considerations, the functions of the air barrier, vapor barrier, and WRB are sometimes combined in one product—frequently, a liquid product that is roller- or spray-applied. Greater efficiencies can be achieved if only one trade is involved in applying the all-in-one type of product instead of multiple trades applying each of the air-, vapor-, and water-resistive barriers.
Air barriers have a strong influence on energy efficiency. It is estimated air leakage is responsible for about six percent of total energy used by commercial buildings in the U.S. About 15 percent of primary energy consumption in commercial buildings attributable to fenestration and building envelope components in 2010 was due to air leakage. (For more, visit www.airbarrier.org/wp-content/uploads/2017/06/Buildings-XIII_OnlineAirtightnessCalculator_V5.pdf.) Air barriers are often also weather-protective and water-resistant. They allow the building envelope to prevent accumulation of water in the building and establish a drainage plane inside the wall.
Vapor barriers control the rate at which moisture moves in and out so the wall can dry. Many variables go into choosing and placing the correct barrier. For example, should it be located on the warm or cold side of the cavity? Since vapor will always move into the wall from the high-vapor-pressure (moister)
side of the wall, and migrate to the low-pressure (drier) side, the rule of thumb is the barrier always goes on the high-pressure side. This generally means the barrier goes on the interior or ‘heated’ side in northern locations, and on the exterior ‘high humidity’ side in the south. In the middle states, vapor barrier placement and the question of whether one should be used are a bit ambiguous. In such situations, further hygrothermal evaluation should be done by a qualified expert—often consultants or insulation manufacturers—using tools considering climate, building materials, HVAC systems, and building function.
In addition to placement, it is equally critical to decide between high- or low-perm barriers. Part of the vapor management consideration also involves the absorptive capability of the other components in the wall itself. All building materials absorb water, reservoir it, and then release it as conditions change, so one must account for these conditions as well.
A good place to start researching vapor barriers is the International Building Code (IBC) Section 1405.3, “Vapor Retarders,” which has definitions of and perm ratings for vapor barriers. The higher a material’s perm rating, the more permeable it is to water vapor. A Class I vapor barrier is a material with a perm rating of less than 0.1, which is at the level of polyethylenes or trilaminates like foil scrim kraft materials. Class II barriers have a permeance of greater than 0.1, but less than or equal to one, which is typical of fiberglass facers like a foil or kraft paper facer. Finally, there are the Class III barriers, which include all barriers with a perm rating greater than one and less than or equal to 10, such as common wall paint.
When it comes to placing the vapor barrier, IBC says a wall with continuous insulation is more tolerant of moisture because it stays warmer; therefore, condensation inside the wall becomes less of a possibility. If the cladding is back-ventilated, as it is in a masonry cavity wall, the wall can dry faster and more completely, which influences the vapor barrier choice. Given there are so many interdependent variables, and because each building and region creates a dynamic and unique set of conditions, a hydrothermal analysis, such as can be provided by WUFI software, is often helpful.
WUFI allows realistic calculation of the transient coupled one- and two-dimensional heat and moisture transport in walls and other multilayer building components exposed to natural weather, enabling a full understanding of how all the layers of the wall perform together to manage vapor and air movement under thermal conditions that vary by hour over years.
In addition to understanding the way vapor barriers handle moisture, it is necessary to consider their flame spread ratings. Typically, steel stud/brick veneer construction is classified by IBC as Type I or II construction and its insulation must use a facer with a flame spread less than or equal to 25 when tested in accordance with ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials.