Assembly 6 is a rainscreen design approach with concrete masonry unit (CMU) structure and thin-set masonry veneer over cement backer board. The components of this assembly, from interior to exterior, are described in Fig. 6-1. This assembly is appropriate for many applications including low- or mid-rise residential or commercial buildings. An example application of this assembly is shown in Fig. 6-2. Benefits and special considerations for this assembly are discussed in Table 6-1.
As noted in the Introduction, an above-grade wall assembly should provide control of water, air, heat, vapor, sound, and fire to serve as an effective and durable environmental separator. Control of these elements is provided by critical barriers such as a water-shedding surface (WSS), water-resistive barrier (WRB), air barrier system (AB), thermal envelope, and vapor retarder (VR). Refer to Fig. i-8 of the introductory chapter for a list of primary building enclosure control functions and associated critical barriers.
Fig. 6-3 illustrates the critical barrier locations for this assembly. The critical barriers for typical Chapter 6 assembly details are also provided adjacent to each detail at the end of this chapter.
As shown in Fig. 6-3, the WSS critical barrier occurs at the adhered masonry veneer with most watershedding occurring at the wall face, while a minimal amount of water will be stored within the masonry veneer to be released at a later time. The WRB, AB, and VR critical barriers are all depicted at the same location at the exterior face of the CMU wall structure. As a result, a single membrane is used to serve as the WRB and AB (and may serve as the VR); this membrane is commonly referred to in this chapter as the air and water-resistive barrier (AB/WRB). The thermal envelope barrier includes the exterior insulation between the wall structure and masonry veneer.
The following sections provide more information and discuss best practices for the specific critical barriers of this assembly.
The water-shedding surface is a critical barrier that controls water.
The adhered masonry veneer cladding—including both grouted joints and masonry veneer units—is the primary WSS of this assembly. Additional components include sheet-metal flashings and drip edges, sealant joints, and fenestration systems as shown on the details included at the end of this assembly chapter.
To promote water shedding at the masonry cladding, grouted joints between veneer units should be appropriately installed with a tooled concave (preferred) or “V” shape.
When finished, the WSS critical barrier should be free of gaps except where providing drainage. Movement joints and joints around fenestrations and penetrations should be continuously sealed with a backer rod and sealant joint or counterflashed with a sheet-metal flashing to deflect wind-driven rain and shed water away from the rainscreen cavity.
The water-resistive barrier is a critical barrier that controls water.
In this assembly, the WRB is a sheet-applied or fluid-applied membrane (that also functions as an AB and may function as a VR). Either a fluid-applied or self-adhered sheet-applied membrane is depicted in the details at the end of this chapter. An example of a fluid-applied WRB membrane is shown in Fig. 6-4. This membrane may be either vapor-permeable or vapor-impermeable product because it is located interior (warm side) of the assembly’s thermal envelope. Physical properties such as vapor permeability of WRB products are discussed in detail in the introductory chapter.
The WRB layer must be continuous across the wall face to serve as an effective critical barrier. In addition to the field membrane, the WRB layer of this assembly also includes fluid-applied or flexible flashing membranes, sealants, sheet-metal flashings, and interfaces with fenestration systems (e.g., windows and doors) as shown in the detail drawings that follow this chapter discussion. Where sheet-metal flashing components occur, the back leg of the sheet-metal flashing is lapped into the fluid-applied membrane to encourage water at the WRB layer to drain toward the building exterior.
Cladding support clip fasteners in this assembly will penetrate the AB/WRB critical barrier and should be detailed based on the WRB manufacturer’s installation requirements. Typically, fasteners may be required to be set in a compatible sealant or fluid-applied flashing product or be attached through a self-adhered membrane patch.
The AB system is a critical barrier that primarily controls air, heat, and vapor. The AB system also controls water, sounds, and fire.
In this assembly, the AB system critical barrier is the same self-adhered sheet or fluid-applied field membrane that also serves as the WRB critical barrier. The components described in the above Water-Resistive Barrier (WRB) section are also part of the AB layer, except sheet-metal flashings.
The thermal envelope is a critical barrier that controls heat and assists with controlling vapor, sound, and fire.
In this wall assembly, the exterior insulation provides the thermal envelope. At transition details, the thermal envelope includes exterior insulation across bond beams, peripheral floor lines, and roof assembly insulation as well as slab and foundation insulation. Windows and doors that penetrate this wall are also part of the thermal envelope.
Exterior insulation provides the following benefits:
The CMU (or concrete) in this assembly is also a thermal mass; thus, may provide thermal mass benefits as discussed in the introductory chapter.
Additional thermal envelope discussion is provided in the Thermal Performance and Energy Code Compliance section of this chapter and the introductory chapter.
Where exterior insulation is provided, semi-rigid mineral fiberboard insulation or moisture-tolerant rigid board insulation products (e.g., polyisocyanurate or XPS) may be used. Refer to the introductory chapter for a discussion on various insulation types and considerations.
The VR critical barrier is a layer that retards or greatly reduces (e.g., vapor barrier) the flow of water vapor due to vapor pressure differences across enclosure assemblies. Unlike the other critical barriers presented in this guide, the VR is not always necessary or required to be continuous.
For this assembly, a VR is not necessary. The risk of condensation development or damage to the structure due to outward vapor drive and condensation is unlikely due to all of the assembly’s insulation being located exterior of the wall structure and the AB/WRB.
Note that Fig. 6-3 identifies the VR at the exterior face of the CMU. This represents the exterior-most plane of the CMU wall structure, which has some vapor resistance. It would also represent the location of a VR if an impermeable AB/WRB membrane is used.
Although masonry is defined as a noncombustible cladding material, a combustible air and water-resistive barrier or foam plastic insulation within a wall cavity can trigger fire propagation considerations and requirements. Depending on the local jurisdiction, IBC Section 1403.5 regarding vertical and lateral flame propagation as it relates to a combustible water-resistive barrier may require acceptance criteria for NFPA 285. The use of foam plastic insulation within a wall cavity should also be addressed for IBC Chapter 26 provisions.
The adhered veneer cladding is expected to shed most water it is exposed to; however, some moisture is expected to penetrate the cladding and enter the rainscreen cavity. This moisture is drained through the cavity created by the continuous Z-girts that support the cladding and through the drainable, semi-rigid insulation.
The rainscreen cavity is created by 1-inch minimum continuous Z-girts typically spaced at 16 inches on-center. These Z-girts should be broken at horizontal joints where movement joints occur or where cross-cavity sheet-metal flashings occur; typically at every floor line for structures 3 stories or taller.
The rainscreen cavity should be open at the top and bottom to encourage ventilation and should be protected with an insect screen. This can be achieved by wrapping the insulation and base of the Z-girt. Insect screen should be placed at all locations where the rainscreen cavity is open to the exterior (e.g., base of walls, window head flashings, parapets, and cross-cavity flashings at floor lines).
Sheet-metal components for this assembly are reflected throughout the details located at the end of this chapter. Cross-cavity sheet-metal components are located at the head of a penetration (e.g., a window head) and at cross-cavity floor line locations similar to that shown in Fig. 6-10. These flashings assist with draining the rainscreen cavity. Counterflashing sheet-metal components assist with watershed and are located at window sill and parapet cap to protect the cavity from water ingress while still allowing for cavity ventilation.
Sheet-metal flashing components that bridge the exterior insulation degrade the thermal performance of the assembly; however, they are a necessary element for the rainscreen design approach.
Refer to the introductory chapter for general recommendations on sheet-metal flashing products, including design considerations and materials.
In this assembly, the thin masonry units are bonded to a crack isolation membrane over cement backer board. If using clay masonry units, they will expand over time, whereas manufacturer concrete veneer products and grout joints between units will shrink. Movement of the thin masonry veneer is accommodated within the grout, cement backer board, and crack isolation membrane.
The cement backer board and cladding support intermittent Z-girts and Z-furring will experience some movement along with the CMU backup structure which is expected to shrink over time due to initial drying, temperature fluctuations, and carbonation. As a result, both horizontal and vertical movement joints are needed to accommodate differential movement between the structure, cladding support system, and veneer.
Horizontal gaps within the veneer and cladding support system are recommended at every floor line for buildings taller than three stories. These gaps are typical and provided at cross-cavity sheet-metal flashing locations and should be continuous across all elevations of the building. Gaps above and below penetrations (such as windows) and below structure projections (such as parapet blocking) should also be provided. Locations where this gap should occur are indicated with an asterisk (*) in the details at the end of this chapter. At each horizontal gap, place either a backer rod and sealant joint or a cross-cavity sheet-metal flashing. The sizing and location of vertical movement joints will vary depending on the expected differential movement between the wall and veneer. It is the Designer of Record’s responsibility to appropriately locate and size each joint. In general, a minimum gap dimension of 3/8 inch should be provided.
Vertical joint recommendations vary throughout the industry and should be confirmed with the veneer unit manufacturer for the project-specific application. This guide recommends that vertical movement joints be located throughout the veneer system and that horizontal-to-vertical placement relationships are also considered. Refer to the Joint Location section of the introductory chapter for more information on locating joints. For vertical joints, provide a minimum gap dimension of 3/8 of an inch.
Adhered masonry veneers rely on adhesion to secure the masonry units and should be designed to comply with local building codes and ACI 530.
The code requires that adhered veneers be applied over concrete or masonry backings and, traditionally, adhered masonry was applied directly over these wall types. However, recent code cycles requiring exterior insulation have dictated that adhered veneers over a CMU wall include some insulation at the exterior face of the backup wall and WRB plane.
Adhesion between adhered veneer units and the backer board must have a minimum shear strength at of at least 50 psi in accordance with ASTM C482. The units should be adhered in a thin-set mortar adhesive application to form a continuous bed free of voids. It is best practice to adhere veneer units with a modified mortar adhesive over a crack isolation membrane and water-resistive cement backer board.
When exterior insulation is required, the thin masonry assembly is supported by an intermittent cladding support clips and continuous vertical Z-girts such as those shown in Fig. 6-11. The spacing of the supports and the sizing of the girts will need to be designed by the Designer of Record to resist building loads and limit out-of-plane deflection of the wall to less than L/360. Limiting this deflection will reduce the likelihood of flexural cracking. Minimizing the cladding support spacing may be considered to limit out-of-plane deflection but should be considered for impact on the effective thermal performance of the assembly. As shown in Fig. 6-12, smaller cladding support clip spacing is necessary to resist greater wind loads. As clip spacing is reduced, the effective thermal performance of the assembly is also reduced. Using lower conductivity structural supports can reduce the effect that cladding support clips have.
To avoid premature cladding replacement, the durability and longevity of metal components within this assembly should match that expected of the masonry veneer cladding system. Metal components within this assembly include intermittent Z-girts (when constructed of metal), continuous Z-furring, sheet-metal flashings, and fasteners such as screws and anchors. Where available, metal components should be manufactured of Type 304 or 316 stainless steel, which is non-staining, resistant to the alkaline content of mortar materials, and tolerant of the high humidity conditions that can exist within a rainscreen cavity. Where stainless steel components may not be available, minimum G185 hot-dipped galvanized steel products should be considered.
Whereas the use of stainless steel sheet-metal flashing components is not always economically feasible or aesthetically desirable, prefinishing sheet metal may be considered. Where used, the base sheet metal should receive a minimum G90 hot- dipped galvanized coating in conformance with ASTM A653 or minimum AZ50 galvalume coating in conformance with ASTM A792. The exposed top finish of the sheet metal should be coated with an architectural grade coating conforming to AAMA 2605.
Cement backer board used within this assembly should be exterior-grade water-, mold-, and mildew-resistant, which meets ASTM C1325 Type A (exterior applications) or ANSI 118.9. The cement backer board should be attached to the continuous vertical Z-girts as required by the backer board manufacturer and project-specific design loads. The attachment method used should be appropriate for the Z-furring and intermittent cladding support design.
Joints of the cement board should be staggered and treated with a mesh tape bed in the veneer bonding material. Cement backer board product should be installed in conformance with the manufacturer installation instructions and set to provide a maximum 1/4-inch per 10 feet of tolerance. The cement backer board should not span joints within the veneer that are expected to accommodate movement.
A crack isolation membrane, like that shown in Fig. 6-13, is a flexible fluid-applied membrane used in thin masonry veneer applications where the veneer is adhered to a cement backer board. The crack isolation membrane is applied following installation of the cement backer board and treatment of the board joints. This membrane assists with:
Traditionally, this membrane may have been installed to protect the primary structure from moisture exposure. However, in this rainscreen assembly, the crack isolation membrane is not a replacement for the AB/WRB membrane, which is located on the exterior face of the CMU wall.
It is best practice to use a crack isolation membrane over cement backer board in thin masonry veneer applications. Some manufacturers may require this membrane to achieve a warrantable cladding installation.
There are several types of adhered masonry veneer products that may be used with this assembly. Those most typical within the Northwest include thin veneer brick units made of clay or shale or manufacturer stone masonry veneer units.
Thin veneer brick used for this assembly should comply with ASTM C1088 and should be exterior-grade. Manufacturer stone masonry veneer units should comply with ASTM C1670.
For thin-set applications over cement board, as shown in this assembly, modified mortars at-minimum should conform to ANSI A118.4.
Appropriate product selection of masonry veneer unit and mortar materials is necessary to provide a durable and water-resistive cladding system. The veneer units and mortar bed and joints should also be installed in conformance with industry standard best practices and manufacturer requirements and should comply with ASTM C1780. The specifics of architectural characteristics and structural properties of the veneer system, including mortar and cladding support systems, should be designed and reviewed by a qualified Designer of Record.
Various industry resources are available to assist with veneer design and are listed in the Resources section.
A clear water repellent should be applied to the adhered masonry veneer of this assembly. Refer to the introductory chapter for more information on selecting an appropriate clear water repellent and best practice installation guidelines.
This chapter assembly is typically classified as a “mass” above-grade wall for energy code compliance purposes. Prescriptive energy code compliance values for this assembly are summarized in Table 6-2 and describe:
When a non-prescriptive compliance option (e.g., a trade-off strategy or whole-building modeling strategy) is used for energy code compliance, this assembly’s effective thermal performance will need to be calculated; however, it may or may not be required to meet the prescriptive values shown in Table 6-2.
Fig. i-17 of the introductory chapter describes the typical process of navigating energy code compliance strategies and options. Thermal modeling results demonstrated within this chapter may be used to assist with estimating insulation thickness and cladding support clip type/material to achieve a target thermal performance value. Options for thermally optimizing this assembly, as determined through the modeling results are also provided.
Claddings support clips, such as intermittent Z-girts or fiberglass clips as shown in Fig. 6-5, penetrate the exterior insulation in this assembly and create areas of thermal bridging (i.e., heat loss). An example of thermal bridging is described by Fig. 6-6 and Fig. 6-7, which show the relative thermal gradient of this assembly when thermally modeled with an intermittent Z-girt. The lighter blue thermal gradient color at the attachment describes a warmer temperature than the adjacent darker blue insulation face—an indicator of heat loss at the penetration through the insulation. This thermal bridging reduces the assembly’s effective thermal performance.
To demonstrate the this assembly’s effective thermal performance with various insulation thicknesses, insulation R-values, and cladding support clips/materials, three dimensional thermal modeling was performed. A discussion on the modeling performed for this guide is included in the Introduction Chapter and the Appendix.
The following are modeling variables specific to this assembly—CMU wall with adhered masonry veneer:
The results of this modeling are shown in Table 6-3, Fig. 6-8, and Fig. 6-9 (see page 6-10 and page 6-11)and demonstrate the assembly effective R-value under various conditions; Fig. 6-8, and Fig. 6-9 are graphical representations of the results summarized in Table 6-3. Discussion of these results is provided below and key points for thermally optimizing this assembly are italicized in boldface.
Project-specific thermal performance values for the opaque above-grade wall assembly of this chapter should be used for energy code compliance and should be determined from a source that is approved by the local governing jurisdiction. Sources may include the Appendices of the WSEC and SEC, ASHRAE 90.1, COMcheck, thermal modeling, or other industry resources.
A pricing analysis for this assembly is provided in Table 6-4. Pricing demonstrates the relative price per square foot and is based on a 10,000-square-foot wall area with easy drive-up access. Pricing includes all components outboard of the CMU wall structure and provides no evaluation for interior finishes or CMU wall structure.
Pricing is valued for the 2015–2016 calendar year.