The Chapter 8 assemblies are a rainscreen design approach with wood-framed wall structure and an adhered masonry veneer. The components of this assembly, from interior to exterior, are described in Fig. 8-1 for two assemblies, Option A and Option B. The thin masonry cladding is either applied with a thick bed method (scratch and bond coat) or thin bed method (thinset over cement backer board) and may be either fired clay masonry or manufactured stone. The thick bed method of application is identified in this chapter as Option A and is most appropriate for low-rise structures. The thin bed method is identified within this chapter as Option B and is most appropriate for low- to mid-rise structures. Both assemblies are appropriate for residential and commercial applications. An example application of this assembly is shown in Fig. 8-2. Benefits and special considerations for this assembly are discussed in Table 8-1.
As noted in the Introduction, an above-grade wall assembly should provide control of water, air, heat, vapor, sound, and fire to serve as 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. 8-3 illustrates the critical barrier locations for this assembly. The critical barriers for typical Chapter 8 assembly details are also provided adjacent to each detail at the end of this chapter.
As shown in Fig. 8-3, 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 and AB barriers occur at the same location exterior of the wall sheathing. As a result, a single membrane is used to provide these two critical barriers and is commonly referred to in this chapter as the air and water-resistive barrier (AB/WRB). The thermal envelope includes the cavity insulation. The VR layer is located at the interior (warm side) of the wood-framed structure.
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 watershedding 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 liquid water.
In this assembly, the WRB is a vapor-permeable mechanically attached sheet membrane, self-adhered sheet membrane, or fluid-applied membrane (that also functions as the AB). A vapor-permeable AB/WRB membrane allows this assembly to dry to the exterior. Drying ability to the exterior is not only beneficial during the service life of the building but also helps relieve construction-related moisture that may occur at the wood framing or wood-based sheathing products. A vapor-permeable mechanically attached sheet membrane is depicted in the details at the end of this chapter. An example of this WRB membrane type is shown in Fig. 8-4.
The AB/WRB layer must be continuous across the wall face to serve as an effective critical barrier. In addition to the AB/WRB field membrane, the WRB critical barrier 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 AB/WRB field 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, cladding support clips may be required to be set in a compatible sealant, fluid-applied flashing product, or attached through a self-adhered membrane patch.
The air barrier is a critical barrier that primarily controls air, heat, and vapor. The AB also controls water, sounds, and fire.
In this assembly, the AB system critical barrier is the same 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.
Mechanically attached sheet-applied AB/WRB barrier materials should be attached per manufacturer recommendations to avoid membrane displacement and damage during wind events. For Option A of this assembly, manufacturer-recommended fasteners may be washer head nails or fasteners. The installation of drainage matrix and cladding components will also assist with holding the AB/WRB membrane in place. For Option B of this assembly, furring strips serve as the mechanical attachment as shown in Fig. 8-4. Note that furring strips should be installed immediately following AB/WRB membrane installation. Where furring strips are not immediately installed, manufacturer-recommended washer cap fasteners should be installed.
The thermal envelope is a critical barrier that controls heat and assists with controlling vapor, sound, and fire.
In this wall assembly, the cavity insulation provides the thermal envelope. At transition details, the thermal envelope also includes parapet cavity insulation and insulation at the roof assembly, slab, and foundation elements. Windows and doors that penetrate this wall are part of the thermal envelope.
Exterior insulation may also be used with this assembly, as shown in Fig. 8-5, to increase the assembly’s effective thermal performance.
Additional thermal envelope discussion is provided in the Thermal Performance and Energy Code Compliance section of this chapter and the introductory chapter.
The cavity insulation in this assembly is typically a vapor-permeable fiberglass or mineral fiber batt insulation product.
Exterior insulation is not recommended with Option A due to the difficulty of fastening insulation, drainage matrix, and metal lath materials and the difficulty in supporting final veneer components with this system.
Where exterior insulation is desired, Option B of this assembly should be considered. The adhered masonry veneer is supported with a cladding attachment support system made of intermittent clips and Z-girt furring as shown similarly in Fig. 8-5 and Fig. 8-6. Refer to Chapter 7 for additional discussion on insulation selection, cladding attachment supports, and cladding discussion when exterior insulation is to be used.
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.
The VR of this assembly is located on the interior (warm side) and is typically at the face of or just behind the interior gypsum board. The VR for this assembly should comply with Section 1405.3 of the governing International Building Code (IBC). In the Northwest, typical VR products include PVA vapor-retarding primer, asphalt-coated kraft paper, or a polyamide film retarder membrane. These products are further discussed in the introductory chapter.
Although masonry is defined as a noncombustible cladding material, the use of 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 exterior sheathing of this assembly is typically a wood- or gypsum-based product and is designated by structural requirements. Where wood-based products are used, plywood is recommended for its moisture tolerance. Where gypsum board is used, a moisture-resistant product with fiberglass facer is recommended.
The adhered veneer cladding is expected to shed most water exposure; however, some moisture is expected to penetrate the cladding and enter the rainscreen cavity. This moisture is either drained through the drainage matrix layer of Option A (Fig. 8-10) or through the cavity created by the furring strips of Option B (Fig. 8-13). Water that drains through the rainscreen cavity is deflected back to the exterior of the cladding or evaporates by way of ventilation behind the cladding.
For Option A, the rainscreen ventilation and drainage cavity is created by the drainage matrix. An example of entangled-filament drainage matrix and filter fabric is shown in Fig. 8-10. A plastic-dimpled drainage may be used as an alternate to the filament type of drainage matrix. This drainage material allows drainage of liquid water and some ventilation. A minimum drainage depth of 3/4 of an inch is recommended, while 1/2 an inch is acceptable in non-marine areas. A moisture-tolerant filter fabric between the drainage matrix and the mortar bed protects the drainage cavity from mortar droppings. This fabric is typically factory-adhered to the drainage matrix and replaces the need for a separate installation of building paper prior to scratch coat application.
For Option B, the rainscreen cavity is created by 3/4-inch-thick preservative-treated furring strips that align with stud framing as shown in Fig. 8-13. Furring strips allow for more ventilation within the cavity than drainage matrix or mat products. Furring strips should be broken at floor lines to allow for building movement.
In both assemblies, the rainscreen cavity should be open at the top and bottom to allow ventilation and should be protected with an insect screen. This can be achieved by wrapping the drainage matrix perimeter in Option A and wrapping the ends of the furring strips in Option B as shown in Fig. 8-13. 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. These flashings assist with draining the rainscreen cavity. Counterflashing sheet-metal components assist only with watershed and are located at the window sill and parapet cap to protect the cavity from water ingress while still allowing for cavity ventilation.
Refer to the introductory chapter for general recommendations on sheet-metal flashing products, including design considerations and materials.
Some wood preservative treatments may react with certain fastener products; thus, care should be taken to select compatible fasteners and wood preservative treatment.
For both options of this assembly, the wood-framed structure will undergo shrinkage. For Option A, the cementitious-based scratch and bond coat, and mortar joints will shrink. Clay masonry veneer units will expand over time, whereas manufacturer concrete veneer products and grout joints between units will shrink. While the reinforcing mesh of Option A will assist with crack control, movement within the veneer and between the veneer and backup wall structure should still be accommodated with horizontal and vertical movement joints. Similarly, with Option B volumetric changes will occur between the veneer and wood-framed wall and must be accommodated. For additional discussion regarding movement of adhered veneers in cement backer board applications refer to the Movement Joints section of Chapter 7.
For both Options A and B, horizontal gaps within the veneer and cladding support system should be provided at every floor line for buildings taller than three stories. These gaps are typically provided at cross-cavity sheet-metal flashing locations and should be continuous across the elevation 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. Either a backer rod and sealant joint or cross-cavity sheet-metal flashing should be placed at each horizontal gap. 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, provide a minimum gap dimension of 3/8 of an inch.
Vertical movement 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 are 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.
Example locations for an adhered masonry veneer application are shown in Fig. 8-14.
For Option A, the masonry veneer is adhered with a bond coat to the mortar scratch coat. Metal lath reinforces the mortar and is attached back to the wood-framed structure. The thin masonry veneer should be designed to comply with local building code and ACI 530 requirements. The code requires that adhered masonry veneers be applied over concrete or masonry backings and, as such, there are special requirements for installing adhered veneer over wood-framed walls.
In Option A, the metal lath reinforces the scratch coat of mortar and controls cracking that may occur due to mortar shrinkage. Metal lath for this assembly is recommended to be a minimum 3.4 lbs/sq yd complying with ASTM C847 and should be installed in conformance with ASTM C1780. Fasteners used to secure the metal lath should be stainless steel nails with a minimum 3/4-inch embedment directly into the wood-framed structure. Fasteners should be spaced no more than 6 inches on-center in the vertical direction. An example of the Option A veneer is shown in Fig. 8-15.
The wood-framed wall itself should be designed to limit the out-of-plane deflection of the wall to less than L/360 to reduce cracking in the veneer. Framing should provide a minimum 1/4-inch per 10 feet of tolerance.
For the Option B Structural Considerations, Cement Backer Board, and Crack Isolation Membrane discussion, refer to these sections within Chapter 7. As discussed in Chapter 7, where exterior insulation is provided and cladding support clips are used, minimizing the cladding support spacing may be considered to limit out-of-plane deflection, but should also be balanced with the impact on the effective thermal performance of the assembly. As shown in Fig. 8-16, 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 impact 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 metal lath, sheet-metal flashings, and fasteners such as nails, screws, and staples. Where available, metal components should be manufactured of Type 304 or 316 stainless steel, which is non-staining and resistant to the alkaline content of mortar materials. Where stainless steel components may not be available, minimum G185 hot-dipped galvanized 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. It is recommended that the exposed top finish of the sheet metal be coated with an architectural-grade coating conforming to AAMA 2605.
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 thick bed applications as shown in Option A, the scratch coat should comply with ASTM C270 for site mix applications or ASTM C1714 for preblended mortar and should be Type S or N. Setting bed mortar should also conform to these standards or may conform to ANSI 118.4.
For thin-set applications over cement board as shown in Option B, 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. For Option A, veneer installation methods may also be referenced from the Masonry Veneer Manufacturers Association Installation Guide and Detailing Options for Compliance with ASTM C1780. The specifics of architectural characteristics and structural properties of the veneer system, including mortar and cladding support system, 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 of this guide.
Applied clear water repellents are recommended for this assembly whether using Option A or Option B. Refer to the introductory chapter of this guide for more information on selecting an appropriate clear water repellent and best practice installation guidelines.
This chapter assembly is typically classified as a “wood-framed and other” above-grade wall for energy code compliance purposes. Prescriptive energy code compliance values for this assembly are summarized in Table 8-3 and describe:
Wood-framed walls are typically constructed with 16-inch on-center stud spacing for standard framing or 24 inches on-center stud spacing for advanced framing methods. Nominal 2×6 framing typically accommodates up to an R-21 fiberglass or R-23 mineral fiber batt insulation and nominal 2×8 framing typically accommodates up to an R-30 mineral fiber batt insulation.
When continuous insulation requirements are to be met, this assembly will have insulation exterior of the wood frame structure and AB/WRB field membrane, as shown similarly in the Chapter 7 assembly.
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 8-3.
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 (for an exterior insulated option) to achieve a target thermal performance value. Options for thermally optimizing this assembly, as determined through the modeling results are also provided.
When exterior insulation is used with Option B of this assembly, cladding attachments supports such as intermittent Z-girts or fiberglass clips will penetrate the exterior insulation and create areas of thermal bridging (i.e., heat loss). An example of the thermal bridging is described by Fig. 8-7 and Fig. 8-8 which show the relative thermal gradient of this assembly when thermally modeled with an intermittent galvanized steel 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 isolated heat loss at the penetration through the insulation. This thermal bridging reduces the assembly’s effective thermal performance.
Three-dimensional thermal modeling demonstrates the effective thermal performance of the Option B assembly with and without exterior insulation (refer to Fig. 8-5). Also demonstrated through modeling is the effective thermal performance of the Option A assembly with various cavity insulation. A discussion on the modeling performed for this guide is included in the Introduction Chapter and the Appendix.
The following are assembly-specific modeling variables:
The results of this modeling are shown in Table 8-2, Fig. 8-11, and Fig. 8-12 (see page 8-12 and page 8-13) and demonstrate the assembly effective R-value under various conditions; Fig. 8-11, and Fig. 8-12 are graphical depictions of Table 8-2 results. Discussion of these results is provided below and key points for thermally optimizing this assembly are italicized in boldface. Results discussions are separated by assembly option.
A pricing analysis for this assembly is provided in Table 8-4 and Table 8-5. 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 exterior wall sheathing and provides no evaluation for interior finishes (including VR), framing/sheathing, or cavity insulation. Where Option B may be used with exterior insulation and a cladding attachment support system, refer to the Chapter 7 Pricing Analysis.
Pricing is valued for the 2015–2016 calendar year.