Water Resistive Barrier (WRB) and Drainage

With an appropriate structure in place, the building codes—both the IBC and the IRC—are concerned with the long term durability of that structure. As such, they include requirements related to protection from weather and, specifically, water that can penetrate an assembly and cause deterioration or damage to the wood. In this regard, the codes offer some specific definitions of items that need to be understood: (IBC 202, IRC R202)

• Water-Resistive Barrier (WRB): A material behind an exterior wall covering that is intended to resist liquid water that has penetrated behind the exterior covering from further intruding into the exterior wall assembly.
• Exterior Wall Covering: A material or assembly of materials applied on the exterior side of exterior walls for the purpose of providing a weather-resisting barrier, insulation, or for aesthetics, including, but not limited to, veneers, siding, exterior insulation and finish systems, architectural trim, and embellishments such as cornices, soffits, and fascias.
• Exterior Wall Envelope: A system or assembly of exterior wall components, including exterior wall finish materials, that provides protection of the building structural members, including framing and sheathing materials, and conditioned interior space, from the detrimental effects of the exterior environment.

Chapter 14 of the IBC describes the particulars for exterior walls, including mandatory performance requirements (IBC 1403). Chapter 7 of the IRC has similar requirements (IRC R703.1.1). In clear and certain terms, the codes state that “[e]xterior walls shall provide the building with a weather-resistant exterior wall envelope.” It goes on to require that “[t]he exterior wall envelope shall include flashing...(and) shall be designed and constructed in such a manner as to prevent the accumulation of water within the wall assembly by providing a water-resistive barrier behind the exterior veneer...and a means for draining water that enters the assembly to the exterior.” Keeping in mind that this is all about the general integrity of the exterior wall, the clear intent is to avoid damage to the wall from water and weather by requiring the use of flashing, a water-resistive barrier, and drainage for any water that does penetrate.

The IBC and IRC identify two options for WRB materials, first stating: “A minimum of one layer of No.15 asphalt felt, complying with ASTM D 226 for Type 1 felt...” (IBC 1404.2, IRC R703.2). Asphalt felt has certainly been a traditional material for many thermal envelope installations, including walls and roofs; however, it is seen as a rather obsolete material compared to other modern alternatives. Fortunately, the second IBC and IRC option is to allow “other approved materials,” which refers to “materials that are acceptable to the building official or authority having jurisdiction” rather than a long list of alternatives.

Section 1703 of the IBC provides the building official with the means to request independent information on other materials in order to issue such an approval.

One of the most common means to identify a material that can readily be approved is for a manufacturer to apply for an Evaluation Service Report (ESR) prepared by ICC Evaluation Services (ICC-ES), a nonprofit, third-party certification subsidiary of the ICC. Data submitted for specific products must come from an accredited testing agency or laboratory to be considered. ICC-ES then conducts thorough evaluations of that independent data to determine whether a product complies with the appropriate code provisions or safety measures as claimed. If successful, an ESR is written that lets code officials know that a product meets the applicable building code and is safe for installation in construction. The distinction between this process and simply supplying testing data to a code official lies in the fact that ICC-ES provides a continuing process. They perform ongoing audits to ensure that products meet quality-control standards and continue to comply with the findings of the original evaluation. The whole process is accredited by the American National Standards Institute (ANSI), and is set up to truly and completely comply with Section 1703 of the IBC, which the code official is relying upon.

Other approved materials used to meet the WRB requirements currently include a variety of forms. Some are self-adhered sheet membranes (sometimes called “peel-and-stick” materials) that have traditionally been used in roofing applications and were also popular on below-grade walls. There are also spray-on systems that are formulated to be applied in thin or thick applications depending on the product and its makeup. Additionally, there are other types of sheet goods and systems that each have their own requirements for installation. Note that there are a variety of manufacturers and products, such that performance can vary considerably based not only on the product, but also on the quality control of installing them in the field. Seams and joints are particularly important in this regard and can be a significant source of problems later on if not treated correctly—compromising the reason for the WRB in the first place.

In designing a wall assembly, it is important to be aware that there are materials available that offer an integrated product approach, such that a sheathing product can have a WRB surface already adhered to it. This eliminates the need to identify a separate product and saves a step during installation, lowering labor costs. Such products offering an all-in-one systems approach often use a continuous tape as part of their system to seal all joints and corners and keep water from penetrating anywhere into the rest of the wall assembly. They also use a coordinated flashing system to seal any penetrations and allow water to drain away from the structure.

Photo used with permission of Huber Engineered Woods LLC ©2012

The IBC and IRC codes require a water-resistive barrier behind the exterior veneer and a means for draining water that enters the assembly to the exterior.

The second half of the requirement of a WRB is to provide “a means for draining water that enters the assembly to the exterior.” This means that any water that penetrates the outer cladding must be able to drain away and avoid potential damage of the wall. The means to do this is typically an irregular surface, channels, weep holes, etc. that create enough of a gap between the WRB and the cladding to allow for drainage. The industry standard for the effectiveness of such code-required drainage is currently ASTM 2273: Standard Test Method for Determining the Drainage Efficiency of Exterior Insulation and Finish Systems (EIFS) Clad Wall Assemblies. While there is a focus on EIFS, the testing procedure and outcomes have been accepted for other clad wall assemblies as well. A WRB material or surface that can also demonstrate efficiency according to this ASTM standard is generally considered to comply with the code requirements of the code as well.