November 6, 2017
by Herbert Slone, RA, and Art Fox
A masonry cavity wall system must successfully perform multiple functions throughout the life of the building. A proper wall is expected to manage moisture, air, and heat, contain fire, and hold up the structure itself. For a wall to perform all these functions, specifications should include all the products necessary for the components to work together.
For the contractor, building a masonry cavity wall is just as challenging as specifying it is for the architect. Contractors rely on the architect for highly precise drawings and specifications so they can produce an accurate bid. They want to be able to build with familiar, proven methods and materials that are compatible and readily available through distribution.
For these reasons, specifying a complete wall system with all the components tested and warrantied together can offer many advantages to the design professional, such as helping support risk management. The design professional’s ability to thrive depends on his or her ability to provide timely documentation for the building’s performance.
Components of a masonry veneer wall system
The structural components forming the basis of the substrate may be steel or wood studs or concrete masonry units (CMUs). On the outside is the weather-resistant component—the cladding or masonry veneer. Between those are three functional component categories that complete the wall system and make the wall perform: moisture/air, thermal, and structural management.
Moisture/air management relies on:
Thermal management involves:
Structural management depends on:
Having all of the right components in the wall is not enough. A true wall system must have passed extensive testing proving the components, as a system, meet the code-mandated performance criteria and are physically and chemically compatible. Further, the system must pass industry-standard tests, such as:
Individual product components of the system can also provide the protection of a warranty that covers them against defects. In the event there is a problem, unified and cooperative solutions are best rather than multiple companies acting separately.
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.
Through-wall flashing is a three-part moisture management path consisting of a flashing membrane, mortar dropping collection, and weep vents. When properly specified and installed, this system captures and directs water out of the wall, no matter where or how water penetrates it.
A flashing membrane must:
The membrane should also include a drainage mesh to prevent the possibility of mortar damming. Including a drip edge is highly recommended in order to prevent ultraviolet (UV) exposure from degrading the flashing membrane edge.
A mortar dropping collection device is the second component of the three-part moisture management path. Various techniques have been employed over the decades, but specially designed mesh products were introduced in 1992 and have become mandatory in a well-designed wall.
These products should be:
Weep vents are the third part of the complete moisture management path. They prevent the weep holes located in masonry veneer head joints—both low at the flashing level and high near the top of the cavity—from being clogged by insects or debris. Located low, they allow water to run freely off the flashing and out of the cavity. Located high and low, they allow air to move through the cavity to enable drying. Weep vent inserts are available made from mesh that is slightly compressible so they fill the head joint, replacing the mortar so the mason need not apply mortar to fill gaps between the vent insert and masonry. They are also available in colors to match mortar choices.
It is extremely important to specify weeps that do not block the flow of water off the flashing membrane. Weeps such as tubes and some rigid weeps form a barrier at the flashing level, which means water has to rise behind them in the cavity before it can run out. Rope weeps are not recommended because they provide no ventilation and can rot over time. It does not matter how good the rest of the moisture management path is—if water cannot get out through the weeps, it will not work.
Sealants must be compatible with all materials to which they are applied and must also be function-specific. For example, while butyl sealants are perfect for sealing overlapping materials such as flashing membrane joints because they are extremely long-lasting and aggressively adhesive, they must never be used in vertical butt joints because butyl never ‘sets’ and will ooze out of the joint. Butyl also remains ‘tacky’ throughout its lifetime, so it is not paintable, and should not be used where it is exposed and visible because it has the potential to hold dirt and debris. (The specific sealant will be dependent on the materials specified for each project. For assemblies like the one with which the authors are most familiar, a silyl terminated polymer [STP] works well due to its moisture-cure properties, color availability, and flexibility while still being compatible with any incidental asphaltic-based products in surrounding areas. Butyl will also work with many of the system’s installation procedures, including on the brick ledge, at panel overlaps, and at points where membranes overlap corners and end dams, and on top of the termination bar. Other sealant types, such as modified polyether, may also work well, but their effectiveness and compatibility can be material-dependent. A designer should always check with the manufacturer of the sealant they want to specify to ensure the chosen sealant is compatible with the other components.)
Depending on the design, one to three types of insulation may be needed in complete masonry veneer wall systems.
Framed wall insulation
If the structural backup wall is framed wood or steel, then insulation can be installed between the studs. It can be fiberglass, mineral wool blankets or batts, or sprayed polyurethane foam (SPF). Which to specify depends on the needed performance and the jobsite conditions. It is important to keep in mind all framing members—but steel studs in particular—act as thermal bridges between the inside and outside of the building, and will reduce the framing insulation’s effectiveness by up to 50 percent. This is one of the reasons why energy codes such as International Energy Conservation Code (IECC), American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, and ASHRAE 189.1, Standard for the Design of High-performance Green Buildings, prescribe continuous insulation over the studs as well as inside the cavity.
Continuous insulation can be semi-rigid (in the form of fibrous boards), rigid (in the form of plastic boards), or SPF. As water will get into the cavity, all continuous insulation should be highly water-resistant and not rely on facers to keep water out of the insulation.
It is important to note not all rigid continuous insulation is alike. In the Brick Industry Association’s (BIA’s) Technical Note 28B, “Brick Veneer/Steel Stud Walls,” it specifies a water-resistant, closed-cell, rigid foam insulation to keep water from penetrating into the sheathing. Although many types of insulation are said to be ‘closed-cell,’ when water absorption levels, molecular structure (i.e. hydrophobic versus hydroscopic), and cell structures are compared, there are differences. Extruded polystyrene is one good option for minimizing water penetration and absorption, because:
Specialized fasteners, including corrosion-resistant screws, are required to install rigid continuous insulation, plus specialized washers with enough surface area to pull the insulation tight against the substrate and create an air- and watertight seal around the screws. Since the wrong fasteners can pull out and the wrong washers can allow air and moisture infiltration, these components should be specified rather than left to the insulation contractor or mason.
SPF is also an effective insulator when properly applied, but because it is essentially manufactured in the field, it may have variable insulation quality, both between the studs and as continuous insulation. The installer must ensure the application temperature, nozzle pressure, rate of wand speed, and foam thickness are all correct to deliver the specified R-value. Since semi-rigid and rigid continuous insulations are factory manufactured, quality is consistent and reliable.
Fire safing insulation
Fire safing insulation is highly fire-resistant mineral wool insulation and is often required at the floor line on buildings designed with curtain wall systems, especially mid- and high-rise structures where smoke and fire must be prevented from spreading between floors. In situations where the floor assembly must be fire-resistance-rated, one route for the fire to spread is through and up the exterior curtain wall.
Fire safing insulation and smoke-sealed joints must be installed to limit fire spread up into and through the exterior curtain wall. Fire safing insulation is installed between the floor slab edge and the curtain wall insulation to contain the fire, and must be compression-fit to ensure a tight seal. As part of a firestopping system, this insulation does not have a fire rating on its own. Only complete firestopping systems have fire resistance ratings determined by ASTM E2307 and define the perimeter fire containment standard specifically designed to extend the rating of the floor assembly out to the exterior skin.
Structural components anchor the masonry veneer to the structural wall. These critical elements must:
Concrete masonry unit walls often employ ‘hook and ladder-style’ joint reinforcing, which also includes eyes into which wall ties connecting to the masonry veneer can be inserted. For steel and wood stud wall systems, the best anchor types are barrel-style. The barrel makes a single penetration that creates a very stable connection, and includes a self-drilling head and a thick shoulder that engages the face of the stud and stabilizes the anchor.
When using a combination of anchors, ties, and washers, it can be useful to include a barrel-style anchor with a specialty sealing washer around it and a thermal break where the brick tie attaches to the anchor. This combination creates just one penetration, is sealed air- and watertight with the washer, and the thermal break reduces thermal conductance by breaking the steel-to-steel connection. Experiments show this system increases thermal efficiency by one to three percent. (The thermal benefit of the thermal clip, and the estimation of thermal benefit, was determined by ad-hoc testing conducted by a manufacturer. The masonry anchor is a three-component assembly—the barrel, a low-conductivity thermal clip, and a wire tie. The steel barrel that screws into the structural stud has a loop on the outer end to receive a wire tie, or, to receive the optional thermal clip. The thermal clip is an engineered plastic structurally capable of holding the pintle legs of the wire tie; it is low in conductivity, breaking the steel to steel connection that reduces thermal transmission compared to when a clip is not used between the barrel and the wire tie.)
Since it is difficult, if not impossible, to achieve true building performance by specifying individual components, it is important to think holistically and look for a wall system tested and warranted to work together as a whole. Building and energy codes are replete with requirements for structural performance, multiple aspects of fire performance (e.g. containment, resistance, and propagation limits), as well as energy, water, and air management performance requirements.
All these code requirements are mandatory, and based on system performance. No single component can be tested to ensure overall system performance requirements are achieved. Therefore, it is incumbent on specifiers to ensure all the components they choose have been tested together to document system performance requirements. It is very difficult for a design professional to research data verifying a given collection of components has been tested together. It is much more efficient and reassuring to work with manufacturers that cooperate and collaboratively test complete wall systems to make certain a complete wall system meets mandatory building and energy code requirements.
Herbert Slone, RA, is Owens Corning’s chief architect and senior manager of commercial building systems. As a registered architect with more than 45 years of experience in construction, he provides leadership in building envelope systemization. Besides working as an architect, he has also worked as a building official and university instructor, authored articles, and chaired industry committees, and was appointed by Ohio’s Governor to the Ohio Board of Building Standards. Slone can be reached by writing to firstname.lastname@example.org.
Art Fox has been the head of marketing and communications at Mortar Net Solutions since 2012. He was also the chief operating officer of the company when it was initially formed 25 years ago. Fox has been involved in the building trades since he was a contractor specializing in new home and light commercial construction in New Mexico in the 1970s. He can be reached via e-mail at email@example.com.
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