The Chapter 2 assembly is a rainscreen design approach with steel-framed wall structure and anchored masonry veneer. The typical components of this assembly, from interior to exterior, are described below in Fig. 2-1. This assembly is appropriate for many applications including low-, mid-, or high-rise residential or commercial buildings. An example application of this assembly is shown in Fig. 2-2. Benefits and special considerations for this assembly are discussed in Table 2-1.
As noted in the Introductory chapter, 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. 2-3 illustrates the critical barrier locations for this assembly. The critical barriers for typical Chapter 2 assembly details are also provided adjacent to each detail at the end of this chapter.
As shown in Fig. 2-3, the WSS critical barrier occurs at the anchored masonry veneer with most watershedding occurring at the wall face while some water will be stored within the masonry veneer to be released at a later time. The WRB and AB critical barriers occur at the same location exterior of the wall sheathing. As a result, a single membrane is typically 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 exterior insulation and wall cavity insulation. The VR layer is located at the interior (warm side) of the steel-framed structure.
The following sections provide more information and discuss best practices for critical barriers specific to this assembly.
The water-shedding surface is a critical barrier that controls water.
The anchored masonry veneer cladding, including both mortar 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, joints between masonry 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 and/or ventilation. Movement joints and joints around fenestrations and penetrations should be continuously sealed with a backer rod and sealant 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 critical barrier is primarily a self-adhered sheet- or fluid-applied membrane that also functions as the AB system; thus, it referred to as the AB/WRB membrane. Either a fluid-applied or self-adhered sheet AB/WRB field membrane is depicted in the details at the end of this chapter. An example of a fluid-applied AB/WRB membrane is shown in Fig. 2-4. The AB/WRB membrane of this assembly may be:
Refer to the Introductory chapter for a discussion on the physical properties of both vapor-permeable and vapor-impermeable AB/WRB membranes.
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.
Masonry veneer ties in this assembly will penetrate the AB/WRB critical barrier and should be detailed based on the membrane manufacturer’s installation requirements. Typically, plate ties are bed in a compatible sealant or fluid-applied flashing product or attached through a self-adhered membrane patch, whereas screw ties with gasketing washers typically do not require any detailing at the AB/WRB plane.
The air barrier system is a critical barrier that primarily controls air, heat, and vapor. The AB system also controls water, sound, 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 system, 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 cavity and exterior insulation provide the thermal envelope. At transition details, the thermal envelope also includes exterior insulation across floor lines, parapet cavity insulation, and insulation at the roof assembly, slab, and foundation elements. Windows and doors that penetrate this assembly are part of the thermal envelope.
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.
The exterior insulation in this assembly is typically semi-rigid mineral fiberboard insulation (R-4.2/inch), which is hydrophobic, tolerates moisture, and has free-draining capabilities. Its vapor permeance allows it to be acceptable for use exterior of a vapor-permeable WRB/AB membrane without inhibiting assembly drying. An example of this insulation is shown in Fig. 2-5. The semi-rigid properties of the insulation allow it to be fit tightly around penetrations such as masonry veneer ties.
A vapor-impermeable rigid board insulation such as XPS or moisture-resistant polyisocyanurate may be appropriate when a vapor-impermeable AB/WRB membrane is used. Refer to the Water-Resistive Barrier section of this chapter for discussion regarding AB/WRB permeability.
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.
When a vapor-permeable AB/WRB critical barrier is used within this assembly (see the Water-Resistive Barrier section of this chapter), 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 discussed further in the Introductory chapter.
When a vapor-impermeable membrane is used for the AB/WRB critical barrier (see the Water-Resistive Barrier (WRB) section of this chapter), the VR critical barrier is the AB/WRB membrane, and a separate VR membrane should not be used within this assembly.
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.
Exterior sheathing on this assembly is typically a gypsum-based product and should be a product resistant to organic growth and moisture. Fiberglass-faced products should be used; paper face products should be avoided.
The anchored masonry veneer 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 rainscreen cavity and exits the rainscreen system where cross-cavity flashings are provided.
In this assembly, a 2-inch-deep rainscreen cavity between the anchored masonry veneer and exterior insulation should be provided to encourage drainage and ventilation. At minimum, a 1-inch gap may be provided and is the minimum code-allowable depth. However, the risk that mortar droppings will reduce the drainage and ventilation within the rainscreen cavity is increased with smaller cavities. A 1-inch cavity should only be provided where a strict quality control program is implemented to ensure mortar droppings do not block the cavity. Fig. 2-15 demonstrates a typical rainscreen cavity for this assembly.
Where the rainscreen cavity is reduced, such as at window rough openings with return brick, a compressible free-draining filler is recommended; semi-rigid mineral fiber insulation may be used. In any case, mortar should not be packed within these cavities.
The rainscreen cavity is ventilated through vents located at the top and bottom coursing of each wall section. Top vents typically occur just below parapet blocking and below intermittent bearing elements such as floor line shelf-angles. Bottom vents also serve as weeps to assist with drainage of the rainscreen cavity. These vents/weeps are typically located just above bearing elements such as loose lintels, floor line shelf angles, or foundation walls.
Vents and weeps should be spaced a maximum of 24 inches on-center (e.g., every 2 to 3 masonry units) and filled with a cellular or mesh product that fills the head joint of a standard brick unit. It is important that weep fillers extend into the bed joint of the course to facilitate drainage. Weep tubes should not be used at vent/weep locations because they provide far less ventilation and are blocked easily with debris.
Mortar collection nets are recommended at all veneer-bearing locations to prevent mortar from blocking the rainscreen cavity and vents/weeps. It is best practice to use a trapezoidal-shaped open-weave, moisture-tolerant net.
Sheet-metal components for this assembly are reflected throughout the details located at the end of this chapter. Cross-cavity sheet-metal components are typically located at all bearing elements such as the head of a penetration (e.g., window head), floor line shelf angles, and the foundation. These flashings assist with draining the rainscreen cavity and also serve to protect fluid-applied or flexible flashing membranes that may exist beneath them. Counterflashing sheet-metal components assist only with watershedding and are typically located at window sill and parapet top conditions; they protect the cavity from water ingress while still allowing for cavity ventilation. An example of sheet-metal components is shown in Fig. 2-16.
Refer to the Introductory chapter for general recommendations on sheet-metal flashing products, including design considerations and materials.
For this assembly, anchored clay masonry will expand over time as a result of irreversible moisture gain, and the mortar joints will shrink slightly overtime. In the support system, the steel-framed members will experience little volume change. To avoid veneer damage, breaks must be provided in the veneer to compensate for differential movement between the cladding and support wall. Expansion joints also must be provided to allow for overall expansion of the clay masonry veneer; control joints must be provided for shrinkage where concrete masonry veneer units are used.
Differential movement between the wall structure and veneer is accommodated with a horizontal gap between the veneer and elements that are directly attached to the wall structure, such as shelf angles, parapet blocking, and windows. Locations where this gap should occur are indicated with an asterisk (*) in the details at the end of this chapter. At each horizontal gap, either a backer rod and sealant joint or cross-cavity sheet-metal flashing should be placed. The sizing and location of 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.
Expansion/shrinkage of the veneer or differential movement between the veneer, penetrations, and different cladding materials is accommodated with vertical joints in the veneer system. Vertical gaps minimize stresses between the veneer and other components and provide crack control for the masonry veneer. An example of expansion joint locations is shown in Fig. 2-17. All vertical gaps should be sealed with a backer rod and sealant. 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.
Refer to the Introductory chapter for more information on locating movement joints and sealant joint best practices.
Expansion joints (clay masonry veneer) or control joints (concrete masonry veneer) minimize stresses within the veneer and also between dissimilar materials such as at window jamb to veneer interfaces.
The steel frame walls and concrete floor slabs of this assembly provide the primary structure of this assembly. It is the responsibility of the Designer of Record to ensure that all structural elements are designed to meet project-specific loads and local governing building codes. Generic placement of the framing members and support elements are demonstrated within the details of this chapter and are provided for diagrammatic purposes only.
Masonry ties are used to connect the veneer to the metal stud–framed backup wall. They are designed to resist the out-of-plane loads applied to the wall, typically wind and seismic. At the same time, these ties must be flexible to allow the veneer to move in-plane relative to the metal stud framed wall.
Building codes provide prescriptive requirements for masonry ties secured to metal stud–framed walls, which include spacing, size, placement, and anchor type. The spacing requirements are summarized in Table 2-5 and are based on ACI-530 provisions. The use of these prescriptive requirements is limited to masonry veneer assemblies with a weight less than 40 psf, a cavity depth no more than 4.5 inches, and where the ASCE-7 wind velocity pressure (qz) is less than 55 psf (previously wind speed less than 130 mph). Wall assemblies that exceed these criteria require the designer of record to evaluate the building loads and materials and rationally design the anchorage system accordingly. The majority of masonry tie manufacturers have empirical testing data available to support the use of their anchorage systems when the cavity depth or loads exceed these criteria.
Prescriptive spacing requirements for anchored masonry veneers in Table 2-5 include special requirements for Seismic Design Categories D, E, and F and high-wind zones with velocity pressures (qz) between 40 and 55 psf. These higher seismicity and wind speed areas are common to some parts of the Northwest and are dependent on the geography and building occupancy category. Refer to local building code requirements to ensure seismicity and wind speed criteria are properly evaluated for the building occupancy and site conditions.
Typical tie types for reference are shown in Fig. 2-18. For steel stud–framed walls, the use of adjustable ties is required by the code. Based on local best practices, double eye and pintle type ties, whether a plate or screw type are preferred. Double eye and pintle ties are available from a number of manufacturers in a variety of sizes to meet project requirements in the Northwest.
Adjustable triangular wire ties are acceptable but may not be preferred by installers because the vertical tie orientation can complicate the exterior insulation installation process by requiring vertical orientation of insulation boards. Corrugate masonry ties and nonadjustable surface-mounted ties are not allowed by code for this assembly.
To prevent pull-out or push-through of the tie, embed each anchor a minimum of 1-1/2 inches into the veneer, with at least 5/8-inch mortar or grout cover at the outside face. The mortar bed thickness is to be at least twice the thickness of the anchor. To prevent excess movement between connecting parts of adjustable anchor systems, limit clearance between components to less than 1/16 of an inch. The vertical offset of adjustable pintle-type anchors may not exceed 1.25 inches.
Masonry ties should be fastened directly to the steel stud framing, through the exterior sheathing with minimum #10 self-tapping screws (0.190-inch shank diameter). They should not be fastened to the sheathing alone. While the code allows a horizontal anchor spacing up to 32 inches on-center, it is recommended that anchors be placed at 16 inches on-center horizontally to align with the typical stud spacing.
Ties should be hot-dipped galvanized carbon steel complying with ASTM A 153 Class B-2 or stainless steel complying with AISI Type 304 or 316. Fasteners used with these anchors shall be corrosion-resistant, either galvanized steel or stainless steel to match the anchor selection.
It is important to also recognize that recent energy code changes may require insulation thickness to exceed 3.5 inches. In such cases, the wall cavity will exceed 4.5 inches (when including a minimum 1-inch air space), which will trigger the code requirement for an engineered anchor system, rather than prescriptive compliance.
Anchored masonry veneers are supported vertically by the building’s foundation or other structural components. There are generally three methods of supporting masonry veneers:
While the function of each support method is different, they each must be designed to eliminate the possibility of cracking and deflection within the veneer. Selection of the appropriate support method should consider the design loads, material type, moisture control, movement provisions, and constructability.
For steel stud–framed wall assemblies, anchored masonry veneer must be supported by noncombustible construction, and any veneer that exceeds 30 feet in height must be supported at each story above 30 feet. Masonry below 30 feet in height must also be supported at each floor when used in Seismic Design Categories D, E, and F. Best practice for commercial construction is to support the lowest portion of the masonry cladding directly on the concrete foundation wall.
Intermediate support should be provided at every floor above 30 feet using galvanized steel shelf angles anchored directly to the floor slab. Fastening the shelf angle directly to the metal stud–framed wall should be avoided where possible due to the relative flexibility of the wall assembly and additional engineering design requirements. When fastening to the floor slab, shelf angles may be fastened directly against the floor slab, but it is recommended that they be anchored with discrete anchor to reduce thermal bridging through the insulation. The floor slab design should be sufficient to limit floor deflection to less than L/600 or 0.3 inches, whichever is less. Consideration must also be given to the vertical expansion and deflection between the shelf angle and the wall assemblies below. As noted in the Movement Joints sections of this chapter and the Introductory chapter, a joint should be provided beneath the angle and sealed with elastomeric silicone sealant.
Masonry cladding must also be supported at openings within the veneer, such as windows and doors. This may be done with shelf angles, as described above for larger openings, or with loose lintels at smaller openings. An example of lintel supports is shown in Fig. 2-19. Galvanized steel angles are typically used as lintels, except where architectural design dictates the use of reinforced masonry or precast concrete lintels for appearance. Steel angle lintels should span across the opening and bear a minimum 6 inches onto the adjacent masonry at the jambs of the opening.
Refer to the details at the end of this chapter for detailing of typical support elements.
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 veneer ties, vertical support ledgers and lintels, sheet-metal flashings, and fasteners.
Veneer ties should be hot-dipped galvanized carbon steel that complies with ASTM A 153 Class B-2 or stainless steel that complies with AISI Type 304 or 316 such as that shown in Fig. 2-18. Steel support angles such as ledger angles and lintels should be a minimum G185 hot-dipped galvanized. Sheet-metal flashing components should be manufactured of ASTM A167 Type 304 or 316 stainless steel, which is non-staining and resistant to the alkaline content of mortar materials.
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.
Fasteners used with all-metal components should be corrosion-resistant, either galvanized steel or stainless steel.
There are several types of anchored masonry veneer products that may be used with this assembly. Those most typical within the Northwest include facing brick made of clay or shale. Concrete facing brick and concrete masonry units are also used.
When using facing brick made from clay or shale, anchored veneer units should comply with ASTM C216 and be severe weather (SW) grade. When using concrete facing brick, anchored veneer units should comply with ASTM C1634. Hollow concrete masonry units used for veneer applications are typically 4-inches deep and should comply with ASTM C90.
Mortar designed for the anchored masonry veneer units should conform to ASTM C270, and the type selected should be appropriate for the veneer application; Type N mortar is acceptable for most anchored masonry veneer applications. When selecting mortar, the lowest compressive strength (softest) mortar that satisfies the project requirements should be used.
Appropriate product selection of masonry veneer unit and mortar materials is necessary to provide a durable and water-resistive cladding system. The masonry veneer units and mortar joints should also be installed in conformance with industry standard best practices and manufacturer requirements. The specifics of architectural characteristics and structural properties of the masonry veneer units, mortar, and reinforcing should be designed and reviewed by a qualified Designer of Record.
Various industry resources are available to assist with veneer design and installation methods and are listed in the References section.
A clear water repellent should be applied to the anchored masonry veneer of this assembly. Refer to the Introductory chapter for more information on selecting an appropriate clear water repellent and for best practice installation guidelines.
AB and WRB continuity is provided by the self-adhered sheet or fluid-applied AB/WRB field membrane, the AB/WRB rough opening head prestrip membrane, and the AB sealant transition to the window. Window strap anchors are bed in sealant during fastening to eliminate an AB discontinuity behind the strap anchor that would otherwise lead to both air leakage and water ingress.
A non-flanged window is used here. It facilitates future window repair and replacement without the need to remove the masonry veneer.
The hot-dipped galvanized steel loose lintel location allows the exterior insulation to be continuous up to the rough opening. Replace the upper portion of the two-piece sheet-metal flashing with a flexible self-adhered flashing membrane for a thermal improvement; confirm self-adhered flashing membrane compatibility with the AB/WRB manufacturer prior to installation. See Detail 2-D for an example of this detail approach.
AB and WRB continuity is provided by the fluid-applied AB/WRB field membrane, the fluid-applied AB/WRB jamb prestrip membrane, and the AB sealant transition to the window. Window strap anchors are bed in sealant during fastening to eliminate an AB discontinuity behind the strap anchor that would otherwise lead to both air leakage and water ingress.
This detail allows the exterior insulation to continue up to the rough opening. The sheet-metal attachment to the intermittent angle, exterior of the windows thermal break, improves thermal performance of the jamb condition. This detail is a thermally improved alternative to Detail 3-C.
The sheet-metal jamb trim is bed in continuous sealant against the anchored masonry veneer for WSS continuity.
Exterior insulation should be tightly installed around all penetrations including masonry ties.
A flexible self-adhered flashing membrane over the semi-rigid insulation promotes drainage of the rainscreen cavity at the floor line. It is a thermally improved alternative detail to a two-piece sheet-metal flashing as shown in Detail 2-A. When installing the self-adhered membrane ensure it is fit tightly to the substrate and sloped to drain.
The hot-dipped galvanized steel loose lintel reduces the amount of thermal bridging at the floor line shelf-angle when compared to a continuous shelf angle mounted tight to the concrete slab face.
Refer to the introductory chapter for alternative lip brick details that reduce the visibility of the backer rod and sealant movement joint. Note this joint is necessary for differential movement that will occur between the structure and anchored masonry veneer.
This chapter assembly is typically classified as a “metal-framed” (or “steel-framed”) above-grade wall for energy code compliance purposes. Prescriptive energy code compliance values for this assembly are summarized in Table 2-4 and describes:
For cavity insulation, steel-stud walls are typically constructed with 16-inch on-center stud spacing and can accommodate up to an R-15 batt insulation for 3-5/8 inch studs or R-21 batt insulation for 6-inch studs. Alternate insulation products may also be used to fill the cavity. Steel framing, because of its high thermal conductivity, can reduce the nominal thermal performance of the stud cavity insulation by approximately 40 to 60%. For this reason, continuous insulation is typically needed to meet prescriptive energy code compliance strategies.
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 2-4.
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 and tie selection to achieve a target thermal performance value. Options for thermally optimizing this assembly, as determined through the modeling results are also provided.
Masonry ties and floorline shelf angles penetrate the exterior insulation in this assembly and create areas of thermal bridging. Examples of the thermal bridging are described in Fig. 2-7 through Fig. 2-12. Where shown in Fig. 2-8, Fig. 2-10, and Fig. 2-12 the lighter blue thermal gradient color at the attachment describes a warmer temperature than the adjacent darker blue insulation face, which is an indicator of heat loss at the penetration through the insulation. Thermal bridging of the steel studs and concrete slabs are also depicted where the thermal gradient is orange (warmer) at studs and transitions to yellow (cooler) where cavity insulation occurs. Thermal bridging reduces the assembly’s actual thermal performance.
Three-dimensional thermal modeling demonstrates this assembly’s effective thermal performance with various insulation thicknesses, insulation R-values, masonry veneer ties, and standoff shelf angle options. 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—steel-framed wall with anchored masonry veneer:
The results of this modeling demonstrate the assembly effective R-value under various conditions and are shown in Table 2-2, Table 2-3, Fig. 2-13, and Fig. 2-14 (see page 2-13 and page 2-14); Fig. 2-13, and Fig. 2-14 are graphical representations of the results summarized in Table 2-2. Discussion of these results is provided below and key points for thermally optimizing this assembly are italicized in bold face.
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 which 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 on Table 2-6. 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.
Pricing is valued for the 2015–2016 calendar year.