WRB-AB sheathing streamlines wall assembly and cladding compatibility

by sadia_badhon | July 17, 2020 10:43 am

by John Chamberlin

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Abundance of choice is evident in today’s landscape of commercial cladding materials. Ranging from metal and brick to stone, exterior insulation finish system (EIFS), and fiber cement, architects and building owners can align their aesthetic visions in several ways to create beautiful buildings finished with a mix of cladding types. However, preserving the integrity of those mixed-media façades is largely about what goes on behind the walls. With the variety of cladding materials available today, and designers’ penchants to combine styles and textures, it is essential to specify building envelope controls that are compatible with multiple cladding types. This balancing act between aesthetic preferences and air and moisture management is where gypsum-integrated water-resistive barrier-air barrier (WRB-AB) sheathings shine.

These integrated materials streamline the entire construction process when different cladding selections are specified. Multi-component wall assemblies with separate WRB and AB layers call for advance work by designers who need to determine proper wall assembly attachments, as well as maintaining a single plane of insulation and weather protection for varying thicknesses of cladding. The separate layers of protection (i.e. WRB and AB) turn into additional levels of complexity for design and construction. Opting for an integrated sheathing system offers simplification and continuity of the air-moisture barrier.

Additionally, integrated gypsum offers more consistency in the WRB thickness and reduces field application issues. Designers need only concern themselves with joints and seams, rough openings, and penetration treatments.

Challenges of separate components

In conventional wall assembly design, separate products may be used for sheathing, WRB, and AB. Thermal control elements like continuous insulation (ci) add another layer of material-selection complexity. Exterior sheathing provides the essential support needed to make the barrier layers effective in multi-component cladding systems, and the individual materials must work symbiotically to achieve their overall objectives (consult “Considerations in the Design of Cladding Systems with Continuous Exterior Insulation” by Mar J. Klos).

However, assessing an individual material’s compatibility for thermal, vapor, air, and water/rain control layers can complicate the design. Gypsum-integrated options resolve the complexity more than non-integrated materials, when looking at the half-a-dozen criteria designers must consider, including weather conditions, fastener requirements, and exposure allowances (Figure 1). Further, an incompatible material can threaten the wall assembly’s performance.

Another item to consider is the interface between cladding materials and the WRB. Some claddings, like EIFS, require adhesive and chemical compatibility between their adhesive means of attachment and the WRB. Depending on the configuration, attachment method, and drainage details, the cladding may also influence the location and/or effectiveness of the drainage plane in an exterior wall assembly.

As a system, integrated sheathing starts with either wood or gypsum, modified through proprietary manufacturing processes, to produce sheathings that also act as WRBs and air barrier materials. Once the sheathing is installed, a complete and continuous system is created by sealing areas of discontinuity with tape or liquid flashing products. An integral WRB gypsum sheathing can act as a drainage plane without concerns for proper sequencing or shingle-lapping, which come with other technologies. However, the interface between the cladding and the WRB should still be reviewed.

Metal panels

Metal panels have become a popular cladding choice for accent or whole-building applications, but heat can build up behind them. As a result, when using liquid-applied or self-adhered WRBs and ABs in conjunction with metal, wall assembly designers need to have an understanding of the in-service temperature limitations of those barriers, which can be as high as 93 C (200 F), depending on the type of building and the kind, design, and location of the metal panel on the structure. Anecdotally, observations made when cladding choice causes a wall assembly to extend beyond the WRB’s temperature limitations indicate delamination, emulsification, or leaching of liquid materials in high-heat situations. Some liquid-applied or self-adhered WRBs and ABs are designed for higher temperatures. However, this sub-category of products underscores the complexity involved in choosing layers for multi-component systems.

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Stucco

Stucco and other commercial building finishes may require metal lath fasteners, so selecting the right sheathing is essential. If magnesium oxide (MgO) boards are used, chemical compatibility with fasteners could become an issue. A five-year study out of Denmark, where MgO boards had been widely used, found salts in the magnesium absorbed ambient humidity, causing fastener corrosion and moisture damage[3] to wood-framing materials. While MgO sheathings offer fire-resistance at a lower cost than gypsum, the potential corrosion risk could cause more expensive problems down the roads.

Masonry

Brick and cultured stone are popular options for building façades, but they are also highly absorbent. While building cladding is meant to be the first defense against bulk water intrusion, these material selections all but ensure water will penetrate the cladding. In these situations, designers must be confident their selected sheathing and WRB will drain and dry effectively.

When building wraps are selected as the WRB of choice, penetrations and perforations in the material become the issue. As building science expert Joseph Lstiburek explains, moisture intrusion[4] through fastener penetrations is fine, as long as the wall can handle (i.e. thoroughly dry) the amount of moisture that gets through. If papers tear because of the wind, insufficient folding around punched openings, or other issues, the amount of moisture infiltrating the wrap could overwhelm the sheathing’s drying capabilities.

Simplification in a single product

Gypsum-integrated WRB-AB sheathing systems combine water and air control into one system that will work behind almost all types of cladding, provided wall assembly designers have taken effective steps toward good water management. Direct-applied systems would be discouraged without the inclusion of a drainage cavity.

With extensive testing regarding permeability and wind-driven rain, manufacturers have calculated air- and water-barrier qualities that will work successfully with a range of cladding options, thereby eliminating the additional labor and supplies needed to install multiple layers. Third-party testing has been performed to evaluate a variety of performance characteristics of gypsum-integrated WRB-AB materials such as:

• adhesive compatibility, according to ASTM C297, Standard Test Method for Flatwise Tensile Strength of Sandwich Constructions;
• drainage capability, per ASTM E2273, Standard Test Method for Determining the Drainage Efficiency of Exterior Insulation and Finish Systems (EIFS) Clad Wall Assemblies, with adhesively attached claddings such as EIFS;
• effectiveness of integrated WRB sheathings when penetrated by mechanical means of cladding attachment, including multiple types of rainscreen bracket and clip systems, in wind-drive rain events, according to ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference; and
• fastener sealability of the WRB, and ability to hold out water under hydrostatic pressure when the barrier is mechanically penetrated, per ASTM D1970, Standard Specification for Self-adhering Polymer Modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection.

In all cases, integrated WRB sheathings have exceeded code requirements for performance.

Depending on the production process of the integrated materials, the protective WRB-AB layers may be incorporated into the gypsum while the slurry is poured, or applied as a coating to the finished gypsum panels. In both circumstances, the resulting single material eliminates the need for architects or engineers to consider the characteristics of multiple materials and their compatibility with the chosen cladding.

As it is with any building material, proper installation is essential for long-term performance. The gypsum-integrated WRB-AB sheathing panels should be butted tight with a 1.6-mm (¹/₁₆-in.) gap and screwed to the framing, according to the fastener schedule provided by the manufacturer. Installers will then need to seal the gaps, seams, and fastener locations using liquid-applied flashing, the type and thickness of which would be specified by the sheathing manufacturer. Depending on the situation, a backer rod might be recommended in some instances, but typically, the liquid flashing will suffice.

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In terms of code-compliance, the Air Barrier Association of America (ABAA) requires membranes factory-bonded to sheathing to display inherent self- or fastener sealability, as outlined in paragraph 7.9 of ASTM D1970. Integral gypsum WRB-AB sheathing boards meet this requirement on their own, so installers can be confident they will not have issues with excess moisture infiltrating fastener penetrations.

The step of sealing gaps and fasteners in a visible way with liquid-applied flashing also gives the sheathing an added element of quality control upon installation. Crew-members and building inspectors can easily see missed areas, unlike with building wraps where incorrect folds or laps might be difficult to spot, or liquid-applied moisture barriers that demand a specific thickness for proper performance. Ease of installation for integrated sheathing leaves installers and building owners confident that they are not creating new opportunities for air or water intrusion before the cladding even goes on.

Installers will also be pleased to hear gypsum-integrated sheathings are compatible with a wide range of climate and weather conditions. As climate zones impact the wall assembly as a whole, integrated sheathings can be used in any region, provided other wall components allow for it. Product limitations will vary by type and manufacturer, but generally the materials can be installed in any weather condition, and can be typically exposed for up to 12 months.

Case study

A North Carolina off-grid property known as Benoit Farms took advantage of gypsum-integrated WRB-AB sheathing to leverage its performance benefits and cladding compatibility. Atlanta-based architecture firm LG Squared was tasked with creating a building that would have reduced energy use while still managing the unpredictable weather of western North Carolina (Climate Zone 4). Dramatic shifts in moisture levels and temperature swings throughout the year, as well as potential forest fires in nearby mountain areas meant the firm needed a sheathing material that could maximize energy efficiency, provide a continuous WRB-AB layer, and protect the building, its occupants, and the surrounding landscape from fire risk.

The integrated sheathing addressed the majority of these concerns with one product and limited trips around the building to install it. The selected wall assembly incorporated a gypsum-integrated WRB-AB sheathing matched with a liquid flashing approved for damp conditions, which sealed the joints, seams, corners, and penetrations. Altogether, this installation created a hydrophobic, monolithic surface to block bulk water, and an airtight building enclosure to achieve energy-efficiency goals, all while allowing for vapor permeability and effective drying and providing fire resistance, as required by the National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Wall Assemblies Containing Combustible Components.

With barrier layers fully sealed, the construction team turned its attention to the exterior cladding and aesthetic of the building. The Benoit Farms project called for a layer of ci to achieve its energy-efficiency goals. On top of that, the project called for 20-gauge corrugated steel cladding on the residential building and one barn, and 14-gauge steel on a second barn. The cladding was attached to 20-gauge hat channels and secured through the ci and into 18-gauge metal framing with long roofing screws. Behind the scenes, the gypsum-integrated sheathing provided the air and water barrier, while serving as a rigid substrate for the insulation, and offering structural integrity in the way of shear strength for the wall assembly.

Climate and insulation considerations

Two schools of thought exist around keeping moisture out of buildings, recognizing cladding as the first line of defense. One approach favors wall assemblies—including cladding selection and installation techniques—designed to keep water out altogether. The other method acknowledges water infiltration in some form and quantity is inevitable, and effective moisture management is more important than keeping water out in the first place.

As discussed, many popular cladding options are susceptible to moisture, making the second approach a logical one for commercial builders. So, assuming some water is bound to get behind the cladding, gypsum sheathing is a suitable starting point. All gypsum sheathing is vapor permeable and has the capacity to retain its integrity even as it moves through wetting and drying cycles. In selecting a gypsum-integrated WRB-AB sheathing, designers benefit from this quality, as well as the consistency of a known permeability level that will not be changed by the addition of separate control layers. For example, an integrated sheathing rated at 25 perms will retain that rating on installation because it already accounts for the permeability of the WRB-AB. This is in contrast to a non-integrated sheathing whose permeability changes depending on the control layers applied.

However, the issue is complicated by the widespread use of continuous insulation code requirements on commercial buildings. Depending on the type of insulation and its installation—directly up against the WRB-AB layer, or within the cladding installation framing—the location of the drainage plane and drying capabilities of the wall assembly may change.

For example, in a cold climate, designers may choose an integrated WRB-AB sheathing, followed by mineral wool ci, and an air gap between the insulation and their cladding of choice. This wall assembly should dry well to the outside with the cladding spaced off insulation. However, if a foil-faced polyisocyanurate (ISO) ci is used, it would not dry well to the outside. Pressed up against the WRB-AB layer, the ISO would stop movement of moisture at its surface, and designers have to provide a drainage cavity.

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With the thermal control and moisture management requirements of wall assemblies varying by region, designers should remove as many complications as possible from the specification process. Gypsum-integrated WRB-AB sheathings achieve this uniform approach to moisture management, letting designers focus on the remaining building science requirements, regardless of the cladding.

Fire compliance achieved

The benefits of gypsum-integrated WRB-AB sheathing do not just stop at air and moisture protection and cladding compatibility. These materials also help buildings meet fire-safety standards.

WRBs, ABs, and ci are all intended to improve building performance. However, a renewed focus on exterior wall assemblies considers the propagation of fire due to the design and cladding materials. This issue was underscored by the 2017 Grenfell fire in London, United Kingdom. Specifically, authorities are enforcing code requirements of wall assemblies and compliance with NFPA 285 testing.

To meet the International Building Code (IBC) requirements for wall assembly fire performance, all WRBs must be in compliance with NFPA 285. Unlike systems comprising a primary WRB material and flashing accessories that are sometimes combustible, gypsum-integrated WRB-AB assemblies start with the fire-resistance of gypsum and typically incorporate noncombustible WRB sheathing with liquid flashing accessories. It is important designers who are creating wall assemblies with wood-based integrated sheathings, or integrated sheathings with a pre-cured liquid membrane on the surface, should recognize these are combustible materials and must find other ways to address fire-resistance requirements.

Some designers may wonder if all combinations of WRB/AB products and sheathing should be tested together to meet NFPA 285. Generally, if the exterior wall assembly includes a combustible component, then a test or engineering judgment is necessary. If all components of the specified assembly are noncombustible, then testing is unnecessary.

As of 2018, IBC updated its standard method for evaluating fire propagation characteristics for external wall assemblies. It specifies that NFPA 285 compliance testing criteria no longer considers WRB flashings or accessories to be part of the barrier, as they comprise such a small portion of the assembly. More to the point, this update separates multi-component assemblies into the primary WRB and accessories or flashing products. For example, a termination mastic on a self-adhered membrane system would be consider an accessory, while the membrane itself would be considered the primary WRB. With this in mind, sheathing made from fiberglass mat with a gypsum core—classified as a noncombustible WRB—is exempt from NFPA 285 assembly testing, and IBC 1402.5 requirements for combustible WRBs are inapplicable.

With that in mind, using an NFPA 285-approved WRB-AB sheathing simplifies the cladding process by bypassing the need for any additional testing specific to the barrier.

Give the design every advantage

As integrated WRB-AB sheathings increase in popularity, now is the time for architects and designers to give these materials a closer look. Why restrict cladding possibilities based on the parameters of inflexible multi-component barrier layer options? As an industry innovation, integral WRB-AB sheathing systems support cladding versatility and maintain fire and water-resistance compliance while simplifying wall assembly design and supporting efficient construction.

Broadening a world of design possibilities without the complication of individual component synergy eliminates the hassle of accounting for the differences across a variety of cladding and sheathing combinations. With gypsum-integrated WRB-AB sheathing, designers can accent the north-facing side of a building with brick, while styling the south in EIFS.

[7]John Chamberlin is a senior product manager at Georgia-Pacific. He is responsible for the DensElement Barrier System and the DensDefy line of products. Chamberlin has worked in the building materials industry for his entire career, focusing on new product development for disruptive technologies in the building envelope space. Chamberlin is on the board of the Air Barrier Association of America (ABAA). He graduated from the University of Tennessee with a bachelor’s degree in marketing and later received his MBA from Emory University. He can be reached at john.chamberlin@gapac.com[8].

Source URL: https://www.constructionspecifier.com/wrb-ab-sheathing-streamlines-wall-assembly-and-cladding-compatibility/

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