Sheathing Membranes – Important Characteristics and Codes

In Part 1 of this two-part blog series, we looked at what sheathing membranes are and how they are used in exterior wall assemblies. To recap, these membranes typically consist of thin sheet materials placed on the exterior side of the sheathing board in exterior wall assemblies. The function of the membranes is to resist the transfer of water and sometimes air (depending on the properties and need) while also allowing for water vapor diffusion (i.e., outward drying) in most scenarios. A number of materials may serve these functions and selection can be complicated.

To make matters more complex, building code requirements for air- and/or water-resistive membranes can be unclear. This blog article will explore the nuances of North American building codes, including the most current I-codes and the National Building Code of Canada, as they relate to the water-resistive barrier function of these wall membranes. Code requirements related to air barriers will be explored in a separate blog article. In addition, outlining code requirements, current sheathing testing standards and desirable material properties will be introduced in the blog to follow.

North American Building Code Requirements

In the United States, the most predominant model codes are the I-codes; the I-codes most relevant to wall membranes are the International Building Code (IBC) and International Residential Code (IRC).  Several local jurisdictions also have their own model building codes, so local requirements for sheathing membranes should be investigated. In Canada, the National Building Code of Canada (NBC) is used by the majority of the country to define building requirements. Some Canadian provinces and a few Municipalities have their own building codes which are based heavily on the NBC and therefore have similar requirements. The IRC, IBC and the NBC all require that a water-resistive barrier (WRB) be used in exterior wall assemblies with specific requirements outlined in Table 1.

One notable complexity is the different WRB requirements for built-up traditional stucco vs. non-stucco cladding systems in the I-codes. The root cause of this difference is the codes’ development process. The International Code Council (ICC), the organization responsible for the I-codes, is a union of three separate organizations: The Building Officials and Code Administrators (BOCA), the International Conference of Building Officials (ICBO), and the Southern Building Code Congress (SBBCI). The I-codes include elements from all three organizations’ previous model building codes. For example, kraft paper’s connection with stucco in the IRC stems from the ICBO model code, which was primarily used in the western United States, where the use of both stucco and kraft paper is common.

In addition, a traditional built-up stucco cladding often creates situations where the WRB experiences greater water exposure because of moisture drive through the stucco. Consequently, there is potential for cracking and a potential for minimal drainage on the surface of the WRB as the stucco often bonds directly to it.

To compensate for this, the IRC prescribes two layers of protection or an enhanced water-resistant membrane (e.g., Type II). The 2018 IBC has similar requirements for stucco cladding, but refers to the specification standard ASTM E2556/E2556M rather than the material-specific standard for kraft paper.

This difference touches on the second complexity of the I-codes: their reference to material-specific standards and the inclusion of language such as “equal to”, “equivalent”, and “other approved materials”. Material-specific standards for specification cannot be used to assess different types of materials and sometimes do not even test important material properties. The ASTM E2556/E2556M  attempts to bridge this gap by allowing for different property tests and setting equivalency levels between different materials. One fault with this standard is that it is only intended for mechanically fastened membranes and cannot be used to assess important properties of self-adhered sheathing membranes. In relation to the “approved materials”, the approval process is not defined within the codes and is generally at the discretion of the authority having jurisdiction (AHJ). However, the ICC Evaluation Services (ICC-ES) standard AC38, “Water-Resistive Barriers” and corresponding ICC-ES Evaluation Reports (ESR) for sheet membranes help code officials decide on acceptable materials.

Canadian codes are in some ways more straightforward, but this simplification reduces versatility and creates other problems. For example, the NCB 2015 requires that sheathing membranes meet the CAN/CGSB-51.32 standard, which was created in 1977 to evaluate asphalt paper and is no longer applicable to modern sheathing membranes, as demonstrated by its withdrawal from service. The standard does not even test one of the primary functions of these materials, liquid water transmission resistance!

A number of sheathing membrane products (typically of polymeric sheet type) can’t meet the performance requirements of this outdated standard because their dry-cup water vapor permeance exceeds the maximum limit it sets (approximately 24 US Perms). The NBC recognizes this limitation and allows for ‘alternative solutions’, which are typically developed on a project-by-project basis by a design professional and must be approved by the AHJ. However, this process can be highly limiting to widespread adoption of materials and may lead to varied and untested solutions.

Canadian Construction Materials Centre (CCMC) Product Evaluations can be used to help demonstrate that sheathing membranes meet the performance requirements of the NBC. This is a similar process to the ICC-ES evaluation of sheet membranes with the AC38 specification standard. However, use of a non-CGSB tested membrane within a project is still dictated by the local AHJ.

Related Material Specifications

Having knowledge of material standards and specifications can help unravel some of the complexity in North American building codes and provide insight into important WRB material properties.

As can be seen, a number of critical sheet material properties are not tested by some of the material-specific standards; for instance, the ASTM D226 standard for felt paper does not include vapor permeance or water resistance testing. Instead, it relies heavily on physical material properties, including density, and saturation weight, to prove adequate performance. The kraft paper standard UU-B-790a (basis for the UBC Standard 14-1), includes more performance property testing (e.g., permeance and water resistance), and seems to have set the industry standard for strength requirements. However, some of the prescribed testing cannot be directly applied to polymeric membranes—particularly the water-resistant testing. Similarly, the Canadian CAN/CGSB-51.32-M77 standard cannot be applied to most modern polymeric materials, due to permeance limits and original intent for asphalt-based products, and it does not test for water resistance. In addition to not being applicable to different materials, the older standards do not include up-to-date accelerated aging and UV exposure conditioning regimes.

As demonstrated, there are drastic differences between the material-specific specification standards, which leads to complexity in determining performance equivalency. The newer and more inclusive sheet specification standards (ICC-ES AC38, ASTM E2556/E2556M, and CCMC sheet evaluation) attempt to resolve this complexity and include up-to date and relevant conditioning regimes. The American standards, ICC-ES AC38 and ASTM E2556/E2556M, are an accumulation of historical testing procedures and direct the user to which tests are appropriate for what material. In contrast, the Canadian CCMC specification has standardized testing and only states testing results and whether or not a membrane is approved.

Additional Factors to Consider

Although these specifications are used to determine code compliance and demonstrate a minimal level of performance, they do not address all of the relevant properties for sheet membranes. For instance, the water resistance of polymeric membranes may be affected by surfactants, which can leech from some wood claddings. Other considerations include the cost of the material and whether the manufacturer is able to provide local project support. Table 3 lists a number of important properties that should be considered when selecting a sheet membrane, including properties that are not sufficiently addressed (or addressed at all) by current prescriptive standards.

With the increase in high-performance buildings, the inclusion of innovative products is creating new questions and challenges. For example, little direction is provided on the requirements for self-adhered sheet membranes within codes, standards, and specifications. The ICC-ES AC 38 standard does include provisions for self-adhered membranes, however, it references a self-adhered flashing specific specification to define adhesion requirements. Developing relevant material-specific specifications can help assess these important material specific properties such as adhesion. Another topic of research is correlating UV conditioning to field performance. Even though modern testing includes specified conditioning, it is difficult to predict how materials will react in-service due to the numerous variables that interact with UV degradation (i.e. heat, moisture, location, air quality etc.).

The industry will continue to evolve newer materials and jurisdictional requirements will continue to lag, but perhaps in the future North American codes will include more flexibility towards “accepted” or engineered solutions. What is critical, though, is that these materials are properly assessed for relevant properties to ensure they can perform their intended function throughout the life cycle of the building.