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

Medium-Risk Climate and Construction Factors

RDH goes on to identify a different combination of conditions as “medium risk” for leakage. This would apply to buildings located in a mixed climate with a combination of wet and dry conditions. The building itself may be a mid-rise structure in an urban setting; otherwise it would be a low-rise building in a rural setting, on a hilltop, or in a coastal location. The building form would be of medium size with a mix of simple and complex assemblies. The architectural complexity would be of a medium level with features that might include smaller overhangs and only a few bumpouts or balconies. This medium-risk condition is also characterized by cladding that is directly applied to the sheathing with no open joints. The cladding attachment design might include things like knife edge or bayonet fasteners or load-bearing clips, but in all cases, the critical drain space would need to be present.

Recommended best practices for sealing against air and water leakage will differ depending on the climate and construction factors being designated low, medium, or high risk.

For medium-risk conditions the best-practice recommendations build on the low-risk recommendations in that manufacturer instructions should still be followed and all screw heads, girts, and any attachment plates still need to be screwed tight to the sheathing. However, to guard against possible leakage at this level of exposure, RDH further recommends using a wet-set sealant around all cladding attachment fasteners. That would include installing any clips or fasteners through a dab of liquid flashing or liquid-applied membrane. This is a fairly easy thing to do but does require a bit of diligence to see that it is in fact carried out at all fastener locations.

High-Risk Climate and Construction Factors

Finally, RDH identifies a combination of conditions that its has determined to be “high risk” for leakage. This would include buildings located primarily in a coastal climate where wind and rain conditions are known to be more continuous or intense. The building itself would be a high-rise structure in an urban setting or a mid-rise project in a rural setting. It could also include any building on a hilltop or located immediately on a coast or shore. The building form would be a large, complex assembly of box shapes, meaning many outer and inner corners that could compromise the integrity of the WRB-AB. The architectural features could include frequent bumpouts, balconies, or an articulated facade. The high-risk condition is also characterized by cladding that is open jointed as in a rainscreen type of facade. Cladding attachment would be done using continuous, full-depth girts that are installed either horizontally or vertically but with no drain gap behind the girts (held tight to the sheathing).

For high-risk conditions, RDH intends that everything recommended for low- and medium-risk conditions is followed during installation (i.e., everything screwed tightly, follow manufacturer’s recommendations, seal cladding fasteners). In addition, specifically when Z-girts or spacer clips are used to support the cladding, the top edges of those girts or spacers should be sealed continuously with a liquid sealant or liquid flashing. Further, bottom drainage holes should be provided in each girt so water does not collect and pool on the girts and seep into the wall assembly.

Note that all of these best-practice recommendations relate to a variety of differing factors, so they all need to be taken into account individually and holistically when deciding whether or not to implement additional steps. When in doubt, particularly if a given building overlaps these three different risk levels in certain characteristics, the conservative approach would be to do more, not less, to protect against leakage.


The principles of creating high-performance exterior framed walls center on maintaining the continuity of WRBs and ABs over exterior sheathing. The testing procedures and results presented in this course are meant to inform architects and other design professionals on determining appropriate measures to take to help ensure continuity and avoid leakage under differing conditions of risk. Some of the primary conclusions include the following.

Conclusion No. 1: Fasteners in Cladding Support Systems Can Leak in Heavy Weather Events

Testing has identified the real culprits in leakage is through fasteners that support cladding systems. While this was observed primarily in simulated weather events, those leaks have the potential to occur anytime.

Conclusion No. 2: Follow Best Practices Based on Building and Climate Conditions

Incorporating the best practices learned from this testing and known building science will help assure better performance of the WRB-AB in exterior walls. Remember the level of risk of leakage is based on different climate and construction factors as discussed, so discerning the conditions is the first step in determining the best practice strategies to follow.

Conclusion No. 3: Employ WRB and AB Strategies that Provide Drainable and Dry-able Wall Assemblies

In addition to purely considering WRB and AB membranes, provide construction techniques that address drying potential and vapor diffusion/permeability in the walls. This includes selecting a permeable sheathing/WRB-AB solution that will allow for proper drying of the wall if water does penetrate into the assembly for any reason.

Incorporating these conclusions into the design and construction of buildings will help improve the performance of exterior wall systems that use conventional framing and exterior sheathing with an integrated or liquid-applied WRB-AB.

Peter J. Arsenault, FAIA, NCARB, LEED AP, is a nationally known architect, consultant, continuing education presenter, and prolific author advancing building performance through better design. www.pjaarch.com, www.linkedin.com/in/pjaarch


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Originally published in Architectural Record