Who’s the Culprit in WRB-AB Leakage?

Overcoming issues with integrated sheathing
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Sponsored by Georgia-Pacific Gypsum
By Peter J. Arsenault, FAIA, NCARB, LEED AP

Learning Objectives:

  1. Explore how fasteners attached to sheathing become a point of vulnerability during extreme weather events, which can stress wall assemblies and cause water to enter the building envelope.
  2. Review the differences between standard test methods and the extreme conditions that were introduced by RDH Building Science in order to test several fastener options.
  3. Understand the differences between thin- and thick-mil fluid-applied barriers versus integrated WRB-AB sheathing solutions. Know which option performed best when exposed to high-wind and heavy-rain simulations.
  4. Discover best practices for cladding attachment options, depending on factors such as climate, building form, and architectural complexity, among others.


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This course can be self-reported to the AIBC, as per their CE Guidelines.
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Extreme weather events can stress wall assemblies, causing water to enter the building envelope. Fasteners attached through sheathing are a point of vulnerability requiring specific attention in order to avoid leaks and compromising building integrity. With some minor exceptions, the standardized test methods for water-resistive barriers (WRBs) and air barriers (ABs) do not consider the range of adhesive and mechanical cladding attachments. These standardized methods also do not include simulations of extreme wind and rain on the cladding attachment penetrations. In this course, we will examine parameters and outcomes of water penetration testing, specifically on cladding attachments using ASTM E331 methodology. The intent of this testing is to determine the ability to resist liquid water penetration of multiple attachment clips in three different WRB-AB configurations under varying conditions. The WRB-AB configurations include thin- and thick-mil fluid-applied barriers over glass mat gypsum sheathing and an integrated sheathing system, which integrates the WRB-AB into the fiberglass mat and core. Testing simulated extreme water and wind that pushed the tested WRB-ABs to the point of failure. The results demonstrate how the tested WRB-ABs performed with the main source of leakage occurring at the fasteners. Based on this information, best practices for addressing cladding attachment penetrations are offered under different scenarios and with specific climate and construction risk factors considered.

All images courtesy of Georgia-Pacific Gypsum

Exterior sheathing over conventional framed wall construction in commercial buildings needs to include a proven water-resistive barrier (WRB) and an air barrier (AB).

The Issues

Determining the relevant issues is the logical starting point when investigating any construction assembly, such as exterior walls in commercial buildings, which are the focus here. Specifically, we will look at known issues such as how to assure that WRBs and ABs remain continuous across changes in the construction. We will also address the importance of drying capability in a wall since it is reasonable to expect that water may penetrate in some form at some point in the life of the building. The remaining issue is focused on the variability of some common construction techniques that are used in exterior wall construction, including the impact of attaching cladding over a continuous WRB-AB. All of these are discussed further in the following sections.

Continuous Barriers

Framed exterior walls, using wood or metal studs, headers, etc., commonly use exterior sheathing secured to that framing to serve as the base or substrate for additional materials to be applied over it. Because of the need to be durable, consistent, resistant to fire, and easy to install, fiberglass mat-faced gypsum board has become one of the most popular choices for this type of sheathing on commercial buildings. Building and energy codes require that both a WRB and AB be present, and common practice today frequently sees these systems installed on the exterior face of the sheathing once it is in place. The WRB is to protect the rest of the construction and may also serve as a drainage plane to allow moisture or bulk water to drain away. The air barrier is to prevent uncontrolled air movement. Uncontrolled air movement can result in unnecessary energy consumption and also carry water vapor and/or pollutants through the wall assembly, which may have deleterious effects on the wall assembly and the building occupants.

Architects have choices in the ways that both the WRB and AB can be provided. We will look at four of the most common ones for commercial construction: liquid membranes that are rolled or sprayed onto the sheathing, self-adhered sheet membranes, foam plastic insulation, and integrated sheathing, which incorporates the WRB and AB directly into a sheathing product.

Liquid-applied barriers can be rolled or sprayed over exterior sheathing to create a continuous WRB-AB.

Liquid membranes: Continuous membranes that are liquid based are often used to provide either a WRB, an AB, or, in some cases, one liquid will provide both. They are installed at the job site either by spraying or rolling by hand, and to be effective, they must cover the gypsum board sheathing completely. The liquid nature of these membranes often makes them appealing because they can readily cover any irregular shapes or surfaces in the wall construction. However, since they are hand-applied, the quality of the installation is directly subject to the skills of the applicator. They also need the proper equipment, whether they are rolled on or sprayed on so that their application is uniform and consistent. It is important to note that each such product is tested for its effectiveness based on its final membrane thickness. Hence, some products need a measurably thicker and consistent application than others to achieve a full WRB-AB performance level.

At first glance, it is easy to assume that roll-on and spray-on liquid barriers are good choices because they cover over the entire surface, including all joints, seams, corners, and penetrations, and conform directly to any irregular surface. This creates a fairly uniform membrane as a result, with the continuity of the barriers being achieved by the liquid nature of the product that forms itself to the substrate. However, it is not necessarily the easiest to install given that weather conditions can hamper the timing and quality of its installation. It will also require some drying or curing time before anything else can be done, and the liquid membrane material used needs to be compatible with the substrate.

Self-adhered membranes: A different choice for WRB-AB systems can be found in self-adhered sheet membranes. Since these have been common for some time in the roofing and waterproofing industries, it is not surprising that there are similar products in use for providing barriers in wall assemblies too. In the case of this barrier type, a protective paper coating may be peeled off the sticky side of the membrane, which allows it to then be directly adhered to the sheathing. As you might expect, this type of product works best on smooth, flat, continuous surfaces. Skillful attention is needed to cut and fit the self-adhered membranes to places like building corners, discontinuous edges, openings for windows and doors, etc. It should be noted, too, that there are different types of self-adhered membranes made from different materials. This is important to be aware of since self-adhered membranes are commonly certified and used as WRBs, but they may or may not be certified as an air barrier as well. If that is the case, then a separate AB material will be needed. It is also important to note that the large number of seams along the edges of the strips of the membrane make it more of a challenge since continuity needs to be achieved along all of those edges and seams.

Foam plastic insulation: Certain foam plastic insulation, particularly if it is closed-cell rigid boards, may qualify as a WRB or an AB. This might be convenient since it is commonly installed over the face of exterior sheathing to act as a thermal barrier of continuous insulation on the outside of the building. However, keep in mind that just because it is present in the wall assembly doesn’t mean it has been tested and certified as either a WRB or an AB. In some cases, the material itself may be fine, but without a proven means to address the edges and perimeter of the individual rigid insulation boards, it does not qualify as a continuous assembly. In such cases, a separate WRB and AB are still needed.

Integrated sheathing: Some gypsum sheathing manufacturers have developed and now offer the latest innovation in barrier technology, namely, incorporating both a WRB and an AB directly into a fiberglass mat-faced gypsum sheathing. This allows one engineered product to provide multiple control layers with a single installation of the exterior gypsum sheathing. As such, integrated gypsum sheathing products allow a simpler, more streamlined installation process with less labor required compared to adding separate layers for the WRB and AB. This simplicity of the system makes it ideal for use in all types of buildings, helping to assure quality and control costs.

Integrated sheathing incorporates the WRB and AB directly into the sheathing in a single product, eliminating steps and reducing construction time.

Integrated sheathing systems address joints and seams in an equally simple and effective manner. Liquid flashing is applied and spread just in the areas where it is needed over fasteners, along joints and seams, and around openings. This is far simpler and much less time consuming than covering the entire face of the sheathing with a liquid membrane. Further, the combination of the integrated sheathing and the liquid sealant has been tested and certified to provide a truly continuous AB and WRB. In essence, the whole system uses the best of all the other choices—the effectiveness of proven barriers in the flat, smooth areas of a building, and the flexibility to fully address irregular shapes, surfaces, corners, edges, and other conditions.

Based on this brief review of common WRB-AB choices, it is clear that the manner of addressing continuity is different for each of them. This is an important issue because it is often that continuity that makes the difference between an effective barrier and one that will leak. The best choice will likely depend on the details of a particular building, the availability of skilled installers (or lack thereof), and the project budget.


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Originally published in Architectural Record
Originally published in June 2019