Chapter 7: Single-Wythe CMU Systems

Table of Contents

Chapter 1: Introduction

Chapter 2: Masonry Systems

Chapter 3: The Building Enclosure

Chapter 5: Quality Control & Assurance

Chapter 7: Single-Wythe CMU Systems

Chapter 8: Thermal Performance & Energy Code Compliance

Appendix A:

Thermal Modeling

Glossary of Terms

Terms

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The Chapter 7 wall systems consist of a single-wythe concrete masonry unit (CMU) wall structure, often composed of split-face block. While many wall system variations may apply to single-wythe CMU wall systems, this chapter focuses on an interior-insulated CMU wall system that has demonstrated successful building enclosure performance within Colorado and southern Wyoming. This masonry wall system with interior insulation is most appropriate for low- to mid-rise commercial applications but may be used for residential applications in addition to some high-rise structures, where carefully considered.

Building Enclosure Control Layers

As noted in Chapter 3, an above-grade wall system provides control of liquid water, air, heat, and water vapor to serve as an effective and durable environmental separator. Control of these elements, specific to these systems, is provided by the following control layer systems and/or materials:

The water control layer, comprising the water-resistive barrier (WRB) system
The air control layer, comprising the air barrier system
The thermal control layer, comprising thermal insulation and other low-conductivity materials
The vapor control layer, comprising vapor-retarding materials

Table 7-1 illustrates the water-shedding surface and control layer locations for each system. The water-shedding surface and control layers are also shown on typical system details provided at the end of this chapter.

Water-Shedding Surface

The water-shedding surface reduces the water load on the enclosure.

The single-wythe CMU wall structure, including mortar joints, is the primary water-shedding surface of the wall system. Additional components include sheet-metal flashings and drip edges, sealant joints, and fenestration systems as shown on the details at the end of this chapter.

Water-repellent admixtures within the block and mortar and a surface-applied clear water repellent are recommended for single-wythe CMU wall systems. These repellents reduce moisture absorption and encourage water to shed off the face of the wall system. Additional measures encourage water shed, such as tooled “V” or concave shape (preferred) mortar joints and sufficient sheet-metal parapet cap design.

Joints around fenestrations and penetrations through the single-wythe CMU also serve as a water-shedding surface, thus they should be continuously sealed with a backer rod and sealant, except where drainage is necessary such as below a window rough opening.

Water Control Layer

The water control layer is a continuous layer designed and installed to act as the innermost boundary for water intrusion. The water control layer needs to be continuous across the wall face to serve as an effective control layer. The CMU block, mortar, and grout—inclusive of any integral water repellents and surface-applied water repellents—provide the water control layer in these systems. Additional water control layer materials include flashing membranes at parapet tops.

Penetrations within the wall also require continuity of the water control layer, thus penetrations such as fenestrations (e.g., windows and doors) and service penetrations are part of the water control layer. At fenestration rough openings, a flashing membrane (typically a fluid-applied flashing membrane) provides the function of a water control layer; it protects rough openings against water intrusion and can also minimize air leakage. This membrane is depicted in the details at the end of this chapter.

Additional measures can also improve this wall system’s water control:

The installation of continuous closed-cell spray foam (CCSPF) insulation inboard of the CMU structure. This insulation provides additional thermal, air, and vapor control as discussed throughout this chapter.
The installation of a fluid-applied WRB membrane on the inboard face of the CMU structure. This option is often used with interior continuous rigid board insulation products and can improve wall system airtightness.
The installation of a vapor-permeable elastomeric coating on the exterior face of the CMU wall. This coating bridges hairline cracks and increases the water-shedding potential of the CMU. This option is most often used with smooth-face block and may be used with either of the insulation options listed in Table 7-1. Elastomeric coatings are further discussed in Chapter 4.

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Air Control Layer

The air control layer comprises the air barrier system and is responsible for controlling the flow of air through the building enclosure, either inward or outward. Air flow is significant because it impacts heat flow (space conditioning), water vapor transport, and rain penetration control. Refer to Chapter 3 for a discussion regarding the air control layer and properties of the air barrier system.

For the single-wythe wall system shown in Table 7-1, the air barrier system is either:

The CCSPF interior of the CMU wall structure.
XPS insulation when fully taped and/or sealed at all joints, terminations, and penetrations.
The foil facer of board insulation products when the facer is fully taped and/or sealed at all joints, terminations, and penetrations.

Additional measures that can improve the wall systems air control include:

A fluid-applied air barrier and WRB membrane where used on the interior face of the CMU wall structure.
An elastomeric coating where used on the exterior face of the CMU wall structure.

These options assume the materials listed are installed at a thickness that provides a lesser air leakage rate than the allowable threshold discussed in Chapter 3.

To serve as an effective air barrier system, air control layer material(s) should be installed continuously across the wall face (behind all framing) and up to rough openings, penetrations, and roof and floor structures.

Although the fluid-applied air barrier and WRB membrane and elastomeric coating options listed function as the air control layer, additional sealing of board insulation products inboard of the wall system (where applicable) still require air sealing at joints, penetrations, and terminations. This air sealing minimizes the opportunity for warm, moisture-laden interior air to interface with the interior face of the CMU, where condensation or dampness could form in cooler seasons. Two planes of airtightness (one at the membrane/coating plane and one at the insulation face) may seem redundant; however, this redundancy may occur inherently if the membrane/coatings are also used to assist with water control purposes.

Trade activities including the installation of wall framing and services can sometimes damage the air control layer material, especially the foil facer of board insulation products; thus, this guide recommends additional quality control measures to ensure that any damage to joint, penetration, and termination sealing or damage to the insulation facer is repaired prior to closing up the wall.

When installing CCSPF, it is important to install the insulation in strict conformance with the manufacturer’s installation instructions. Improper installation could lead to premature cracking and delamination from the substrate, which can allow air to move between the insulation and substrate and increase condensation risk. Improper installation can also lead to increased risk of fire during installation. This guide recommends using only experienced applicators who are approved by the CCSPF product manufacturer.

Other considerations when using CCSPF insulation or other foam plastic insulation products include fire propagation with respect to code compliance. Make sure product selection, application, and use comply with local jurisdiction requirements. Volatile organic compounds may also need to be considered based on project material property requirements.

Vapor Control Layer

The vapor control layer retards or greatly reduces the flow of water vapor due to vapor pressure differences across enclosure systems. Unlike the other control layers presented in this guide, the vapor control layer is not always required to be continuous; the location of insulation and vapor permeance of the air barrier and WRB system can also impact the need for or necessary properties of the vapor control layer.

In this system the vapor control layer is:

Continuous closed-cell spray foam insulation at the interior face of the CMU along with any additional CCSPF that may be installed within the framed wall cavity. The vapor control layer exists throughout the depth of the CCSPF; a minimum 2 lb/ft3 density installed in a minimum of 2-inch lift is typically considered a Class II vapor retarder.
The face of the XPS or facer of board insulation products.

Thermal Control Layer

The thermal control layer controls heat flow and assists with controlling water vapor. For single-wythe CMU wall systems, the thermal control layer is either closed-cell spray foam insulation or board insulation.

At transition details, the thermal control layer includes insulation at the roof assembly, under-slab, and foundation elements. Windows and doors that penetrate the above-grade wall are also part of the thermal control layer.

The thermal envelope should be as continuous as possible across all assemblies and transitions to minimize heat loss, reduce condensation risk, and improve occupant thermal comfort. Continuity of interior insulation can be difficult to achieve at areas such as floor lines, slab edges, and some wall-to-roof transitions.

The insulation products most commonly used with this wall system are shown in Table 7-1, and additional discussion for each type of insulation is provided in Fig. 8-3 of Chapter 8. For this wall system, closed-cell spray foam insulation is often preferred for the following reasons:

To minimize the need to notch and fill gaps around penetrations, which is required with board insulation products.
To provide a fully adhered insulation layer, which reduces the risk that warm, moisture-laden interior air may contact the cooler CMU wall surface and condense. This risk can also be minimized with board insulation products by ensuring that all joints, penetrations, and terminations of the board insulation are fully taped and sealed for airtight installation.
To provide a supplemental water control layer function as discussed in previous sections of this chapter.

Vapor- and air-permeable insulation layers such as fiberglass and mineral fiber batt or unfaced semi-rigid mineral fiber insulation are typically avoided in this system. These products alone do not serve as air, water, and vapor control layers, and thus require additional materials or systems be installed to perform these control functions. When additional materials are implemented to serve as these control layers, the risk for condensation on the interior face of the CMU wall or any vapor-impermeable insulation layer should be carefully considered. Lack of a fully adhered membrane or insulation product at the interior or exterior face of this assembly may also reduce the water-resistivity of the assembly when compared to the CCSPF insulation strategy.

Alternative insulation products that are integral to the CMU block are also available, such as those shown in Fig. 7-3. These insulation options are often bridged by the web element of the CMU. Gaps may also exist between the insulation layers once installed. As a result, these options provide a lesser degree of thermal resistance than the continuous insulation options described in Table 7-1. Where these insulation options are used, additional air, water, and vapor control layers need to be considered.

Although masonry is defined as a noncombustible cladding material, the use of a combustible air barrier and WRB system product or foam plastic insulation products within a wall cavity can trigger fire propagation considerations and requirements. Depending on the local jurisdiction, IBC Section 1403.5¹ regarding vertical and lateral flame propagation as it relates to a combustible air and WRB system may require acceptance criteria for NFPA 285.² The use of foam plastic insulation within a wall cavity should also be addressed for IBC Chapter 26¹ provisions.

Structural Considerations

The CMU block wall of these wall systems provides the primary structure. It is the responsibility of the Designer of Record to ensure that all structural elements of the wall system are designed to meet project-specific loads and local governing building codes. Generic placement of the grout and reinforced elements shown within the details of this chapter are provided for diagrammatic purposes only.

Corrosion Resistance

For sheet-metal flashings integrated within this system (including through-wall flashings and sheet-metal drip flashings), it is best practice to provide components that are manufactured of ASTM A666³ Type 304 or 316 stainless steel and are non-staining and resistant to the alkaline content of mortar and grout materials. Where stainless-steel sheet-metal flashing components are not economically feasible or aesthetically desirable, consider using prefinished sheet metal. Where used, this guide recommends the base sheet metal is a minimum G90 hot-dipped galvanized coating in conformance with ASTM A653⁴ or minimum AZ50 galvalume coating in conformance with ASTM A792.⁵ This guide recommends coating the exposed top finish of the sheet metal with an architectural-grade coating conforming to AAMA 621.⁶

Accommodating Movement

CMU is a concrete-based product. It, along with the mortar, will shrink over time due to initial drying, temperature fluctuations, and carbonation. Not only will shrinkage movement need to be considered, but differential movement between the CMU structure and other structural elements due to deflection, settlement, and various design loads will need to be addressed.

To increase or maintain the water-resistivity of the single-wythe wall systems, crack control needs to be considered. Appropriate design of the material properties and reinforcing methods of the CMU wall will reduce cracking; however, implementing control joints within the CMU wall provides a plane of weakness to reduce shrinkage stresses and improve the long-term continuity of the water shedding surface.

Control joints in CMU can be constructed in several ways. Regardless of the method used, a continuous backer rod and sealant joint is installed at the joint as shown in Fig. 7-4 to assist with water shedding and to maintain a continuous water control layer. Refer to Chapter 4 for more discussion on locating and spacing movement joints and sealant joint best practices.

Concrete Masonry Unit (CMU)

The CMU in this system complies with ASTM C90.⁷ Mortar designed for the CMU conforms to ASTM C270⁸ or to ASTM C1714⁹ when specifying preblended mortar. The mortar type selected should be appropriate for the CMU application; type S is typically specified. Grout components should comply with ASTM C 476,¹⁰ while aggregate within the grout should comply with ASTM C 404.¹¹

Block and mortar are both specified with a water-repellent admixture as discussed in the Water Repellents section of Chapter 4. Additionally, refer to the National Concrete Masonry Association for additional resources on specifying block, mortar, and grout.¹²

The CMU and mortar joints of this system should be installed in conformance with industry-standard best practices and manufacturer requirements. The specifics of architectural characteristics and structural properties of the block, mortar, grout, and reinforcing should be designed and reviewed by a qualified Designer of Record.

Water Repellents

Both integral water-repellent admixtures and a surface-applied clear water repellent are used in this system and assist with reducing the water absorption of the CMU wall and encourage water shedding. Water-repellent admixtures should be used in both the CMU and the mortar. Admixture within block units should comply with NCMA TEK 19-7,¹³ while mortar admixture should comply with ASTM C1384.¹⁴ Both the CMU and mortar admixtures, as well as any surface-applied water repellent, should be compatible.

More discussion on surface-applied clear water repellents is provided in Chapter 4.

Quality Assurance and Control

High quality masonry wall system installations are the result of quality control and quality assurance measures that occur throughout the design and construction phases. The general design guidance provided throughout this guide provides the information necessary for understanding and designing the masonry wall system. More specifically, Chapter 5 provides a more in-depth discussion on the topic of quality assurance and quality control.

Chapter References

International Code Council. 2018 International Building Code (Country Club Hills, IL: International Code Council, Inc., 2017).
National Fire Protection Association. NFPA 285 – Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-Load-Bearing Wall Assemblies Containing Combustible Components (Quincy, MA: National Fire Protection Association, 2012).
ASTM International. ASTM A666-15 Standard Specification for Annealed or Cold-Worked Austenitic Stainless Steel Sheet, Strip, Plate, and Flat Bar (West Conshohocken, PA: ASTM International, 2015).
ASTM International. ASTM A653/A653M-15e1 Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the HotDip Process (West Conshohocken, PA: ASTM International, 2015).
ASTM International. ASTM A792/A792M-10(2015) Standard Specification for Steel Sheet, 55% Aluminum-Zinc Allow-Coated by the Hot-Dip Process (West Conshohocken, PA: ASTM International, 2015).
American Architectural Manufacturers Association. AAMA 621-02 Voluntary Specification for High Performance Organic Coatings in Coil Coated Architectural Hot Dipped Galvanized (HDG) and Zinc-Aluminum Coated Steel Substrates (Schaumburg, IL: American Architectural Manufacturers Association, 2002).
ASTM International. ASTM C90-16a Standard Specification for Loadbearing Concrete Masonry Units (West Conshohocken, PA: ASTM International, 2016).

ASTM International. ASTM C270-14a Standard Specification for Mortar for Unit Masonry (West Conshohocken, PA: ASTM International, 2014).
ASTM International. ASTM C1714/C1714M-16 Standard Specification for Preblended Dry Mortar Mix for Unit Masonry (West Conshohocken, PA: ASTM International, 2016).
ASTM International. ASTM C476-16 Standard Specification for Grout for Masonry (West Conshohocken, PA: ASTM International, 2016).
ASTM International. ASTM C404-11 Standard Specification for Aggregates for Masonry Grout (West Conshohocken, PA: ASTM International, 2011).
National Contract Management Association. “NCMA Home.” National Contract Management Association. Accessed March 27, 2018. https://www.ncmahq.org/.
National Concrete Masonry Association. TEK 19-7 Characteristics of Concrete Masonry Units with Integral Water Repellent (Herndon, VA: National Concrete Masonry Association, 2008).
ASTM International. ASTM C1384 Standard Specification for Admixtures for Masonry Mortars (West Conshohocken, PA: ASTM International, 2012).

[Download Detail 7-1 thru 7-7]

SINGLE-WYTHE CMU WALL: Window Head Detail

Legend

Typical Assembly:
– Interior gypsum board
– Steel stud-framed wall
– Closed-cell spray foam (CCSPF) insulation between studs (optional) and min. 2 inches continuous CCSPF
– Single-wythe CMU wall with water-repellent admixture at block and mortar
– Clear water-repellent
Sealant over backer rod
Fluid-applied air barrier and WRB flashing membrane
Continuous air barrier sealant tied to continuous seal at window perimeter
Storefront window
Preservative treated wood blocking

Detail Discussion

The flashing membrane extends from the interior framing to the CMU rough opening. The flashing membrane and the continuous air barrier sealant joint provide air and water control layer continuity from the window to the CMU wall.

Blocking at the window perimeter provides a low-conductivity solution for mechanically attaching the window as required by the window manufacturer.

Water-Shedding Surface & Control Layers

[Download Detail 7-1: Single-Wythe CMU Wall: Window Head Detail]

SINGLE-WYTHE CMU WALL: Window Head Detail

Legend

Typical Assembly:
– Interior gypsum board
– Steel stud-framed wall
– Closed-cell spray foam (CCSPF) insulation between studs (optional) and min. 2 inches continuous CCSPF
– Single-wythe CMU wall with water-repellent admixture at block and mortar
– Clear water-repellent
Storefront window on minimum 1⁄4-inch thick intermittent shims
Sealant joint over backer rod (weep at quarter points)
Sloped precast sill with chamfered drip edge, with sealant over backer rod at precast joints
Continuous air barrier sealant tied to continuous seal at window perimeter
Continuous back dam angle at rough opening perimeter, minimum 1-inch tall, with window fastened through the back dam angle per window manufacturer recommendations
Preservative treated wood blocking

Detail Discussion

The slope at the precast sill encourages water to drain away from the window rough opening. A chamfer is shown in the underside of the precast sill to form a drip. This encourages water to shed from the sill before reaching the masonry veneer below.

Attachment of the window is shown through a structural back dam angle in lieu of down through the sill membrane. This minimizes the risk for water intrusion into the wall cavity below should water exist within the window rough opening. Intermittent shims below the window encourage drainage of the rough opening. Water that may exist within the rough opening can exit through weeps in the exterior sealant joint.

Water-Shedding Surface & Control Layers

[Download Detail 7-2: Single-Wythe CMU Wall: Window Sill Detail]

SINGLE-WYTHE CMU WALL: Window Jamb Detail

Legend

Typical Assembly:
– Interior gypsum board
– Steel stud-framed wall
– Closed-cell spray foam (CCSPF) insulation between studs (optional) and min. 2 inches continuous CCSPF
– Single-wythe CMU wall with water-repellent admixture at block and mortar
– Clear water-repellent
Storefront window
Sealant joint over backer rod
Continuous air barrier sealant tied to continuous seal at window perimeter
Preservative treated wood blocking

Detail Discussion

The window is aligned with the rough opening blocking and insulation, rather than with the CMU wall, to provide better continuity of the thermal control layer. The continuous air barrier sealant joint, along with the flashing membrane, provide continuity of the air and water control layer.

Water-Shedding Surface & Control Layers

[Download Detail 7-3: Single-Wythe CMU Wall: Window Jamb Detail]

SINGLE-WYTHE CMU WALL: Base-of-Wall Detail

Legend

Typical Assembly:
– Interior gypsum board
– Steel stud-framed wall
– Closed-cell spray foam (CCSPF) insulation between studs (optional) and min. 2 inches continuous CCSPF
– Single-wythe CMU wall with water-repellent admixture at block and mortar
– Clear water-repellent
Rigid XPS insulation
Underslab vapor barrier
Rigid XPS underslab insulation
Hardscape sealant joint
Damp-proofing (optional)
Drainage composite or gravel backfill
Hardscape

Detail Discussion

The XPS insulation provides a thermal break between the concrete floor slab and the single-wythe CMU wall. This allows for thermal continuity between the underslab insulation and wall insulation.

Water-Shedding Surface & Control Layers

[Download Detail 7-4: Single-Wythe CMU Wall: Base-of-Wall Detail]

SINGLE-WYTHE CMU WALL: Roof Parapet Detail

Legend

Typical Assembly:
– Interior gypsum board
– Steel-framed wall
– Closed-cell spray foam (CCSPF) insulation between studs, min. 2 inches continuous CCSPF
– Single-wythe CMU wall with water-repellent admixture at block and mortar
– Clear water-repellent
Inverted roof membrane assembly
Typical Parapet Assembly:
– Inverted roof membrane
– Single-wythe CMU wall with water-repellent admixture at block and mortar
– Clear water repellent
Standing-seam sheet-metal coping with gasketed washer fasteners
Preservative-treated wood blocking
High-temperature self-adhered membrane

Detail Discussion

The sheet-metal coping with hemmed drip edge sheds water away from the wall top and CMU wall face below. It is recommended that the sheet-metal coping counterflash the top course of block by a minimum of 3 inches.

The CCSPF extends tight up to the underside of the deck and around roof structure and anchor elements. This reduces the opportunity for warm, moisture-laden interior air to contact the deck and CMU wall where it’s coldest.