Tag Archives: WRB

WRB: Water (or Weather?)-resistive Barrier

slaton patterson sutterlinFAILURES
Deborah Slaton, David S. Patterson, AIA, and Jeffrey N. Sutterlin, PE

Using the acronym ‘WRB’ is common, but the intended meaning is often misunderstood, as it can refer to either a ‘water’ or ‘weather’-resistive barrier, with the two having different performance expectations.

The 2012 International Building Code (IBC), Section 1403.2−Weather Protection requires exterior walls “provide a building with a weather-resistant exterior wall envelope…designed and constructed… to prevent the accumulation of water within the wall assembly by providing a water-resistive barrier behind the exterior veneer.” Water-resistive barriers are defined in Section 1404.2 as “a minimum of one layer of No. 15 asphalt felt, complying with ASTM D226, Standard Specification for Asphalt-saturated Organic Felt Used in Roofing and Waterproofing, for Type 1 felt or other approved materials…to provide a continuous water-resistive barrier behind the exterior wall veneer.”

For adhered stone veneers, Section 1405.10.1.1 requires water-resistive barriers to be installed per section 2510.6, which in turn directs the reader back to Section 1404.2, but adds the requirement the water-resistive barrier be vapor-permeable with the equivalent performance of two layers of Grade D paper when applied over wood-based sheathing. (Section 2510.6 of the 2015 IBC has been revised to require a vapor-permeable barrier with performance at least equivalent to two layers of a water-resistive barrier complying with Type I of ASTM E2556, Standard Specification for Vapor Permeable Flexible Sheet Water-resistive Barriers Intended for Mechanical Attachment, which includes polymer-based barriers.) It should be noted IBC’s Chapter 14−Exterior Walls does not reference weather-resistive barriers.

The American Architectural Manufacturers Association (AAMA) defines weather-resistant (not ‘resistive’) barriers as a surface or a wall responsible for preventing air and water infiltration to the building interior. Manufacturers of polymer-based barriers (i.e. building wraps) also distinguish between water-resistive and weather-resistive barriers, with the latter providing the added benefit of also serving as an air barrier for the vertical building enclosure.

While not as potentially destructive as bulk water leakage, moisture transport via air infiltration can contribute to moisture-related problems in the building enclosure. Weather-resistive barriers are often more robust as compared to water-resistive barriers and require taping of all laps and terminations to resist not only water penetration, but also air infiltration.

Code-mandated water-resistive barriers are typically limited to residential and low-rise structures, while weather-resistive barriers are commonly specified for commercial buildings or projects where a higher level of performance is desired of the vertical building enclosure and control of interior environmental conditions is critical.

It is important the design professional and installer understand the intended purpose of the specified WRB—to resist water or to resist air and water—as the installation varies between the two types of barriers. However, regardless of whether the WRB is intended to function as a water- or weather-resistive barrier, the WRB must be properly installed so as to maintain continuity of the barrier. This requires the WRB to be properly integrated with flashings, wall openings, and all adjacent enclosure assemblies, and to be sufficiently overlapped, correctly shingled, and properly sealed or taped at exposed laps (horizontal and/or vertical, depending on its intended purpose), as barrier discontinuities result in potential entry points for water (and air) to migrate into the building.

The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of the CSI or The Construction Specifier.

Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates, Inc. (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at dslaton@wje.com.
David S. Patterson, AIA, is an architect and senior principal with WJE’s Princeton, New Jersey, office, specializing in investigation and repair of the building envelope. He can be e-mailed at dpatterson@wje.com.
Jeffrey N. Sutterlin is an architectural engineer and senior associate with WJE’s Princeton office, specializing in investigation and repair of the building envelope. He can be contacted via e-mail at jsutterlin@wje.com.

 

Clarification on wall systems article

The April 2013 issue of The Construction Specifier included a technical feature by J.W. Mollohan, CSI, CCPR, CEP, LEED GA, entitled, “Exterior Wall Assemblies: Are You Getting What You Specified?”  We received the following letter from Cliff Black, a CSI member and a building envelope product manager for Firestone Building Products.

I am writing in regard to the article on exterior wall assemblies. I agree with the author the issue is certainly a challenging one for the design and specifying community. I would like to cite the bracketed statement at the top of page 57, which states, “buildings of two stories or more.” This appears to be taken in the context of the design of 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, addressing multi-story fire propagation.

However, the International Building Code (IBC) 2603.5 states NFPA 285 is required for buildings of any height for Types I through IV construction incorporating combustible plastic insulation in the exterior wall assembly. IBC Chapter 14 (“Exterior Walls”) calls for differing requirements for water-resistant barriers (WRBs) and various combustible claddings, qualified by height.

In this case, I believe the statement should read “buildings of any height,” rather than “buildings of two stories or more.”

 

Mr. Mollohan replied to Mr. Black, and has allowed us to share it with other readers of the magazine:

 

Good catch, Clint! You are absolutely correct that one must be familiar with multiple chapters of the IBC to determine whether an NFPA 285 test is required. My error, and your correction, illustrates the difficulty of this provision. I am attaching an adaptation of a flow chart originally created by Barbara Horwitz-Bennett of DuPont Building Innovations for guidance to interested readers:

Diagram_edited-1