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

The Test Results

After all of the test panels were set up the testing was administered as described, technicians recorded the time, conditions, and other observations throughout the test period. What follows is a summary of their findings from these tests. They answer the basic question of how each of the different panels and configurations perform under the stress of the different test conditions.

Weather Conditions

The first finding was that under normal weather conditions (i.e., the code-required level of water spray and wind/ air pressure), water did not penetrate into the wall cavity through the cladding attachment fasteners in any of the assemblies. This is fairly good news and suggests that any of the approaches described in the test panels meet code minimum requirements and need not use water leakage as a deterrent for their selection under normal conditions. However, as the testing continued and the degree of water and wind were increased beyond normal conditions, all of the test panels ultimately leaked (i.e., reached failure) as intended due to the increasing intensity of the water and air pressure conditions created. This suggests that for buildings located in areas where higher wind and water conditions are a concern, the specifics of the sheathing, WRB-AB, and cladding attachment system need to be looked at more closely.

Type of WRB

Under the controlled laboratory conditions, the WRBs were all installed properly over a small area so they would be expected to perform as intended without any complications. And indeed, the RDH team determined that the type of WRB used did not appear to make a measurable difference in the results during the testing. However, the testing showed that, in extreme weather conditions, leakage can occur when cladding attachment systems use fasteners that penetrate the WRB-AB regardless of the WRB-AB solution. RDH noted that once water found a penetration point, it followed that penetration into the assembly whether thin-mil fluid-applied WRB, thick-mil fluid-applied WRB, or integrated sheathing was the subject of the test panel.

Means of Attachment

This was observed to have the most bearing on why the assemblies leaked when subjected to the test conditions. In general, if the cladding attachment (Z-girts, spacers) and fasteners were installed tight to the sheathing or fluid-applied WRB surface, water did not penetrate around the fasteners, and assemblies did not leak until extreme conditions were reached. Likewise, if the cladding fastener was installed so it was adequately spaced away from the sheathing surface, allowing the water to drain behind the cladding attachment, the assemblies did not leak until extreme conditions were reached. Most of the leaks happened when the space between the WRB surface and cladding attachment was just enough for water to enter behind the attachment but not wide enough for the water to effectively drain.

Drainage and Drying

Since the test results determined that leakage potential exists for any type of WRB system under certain, albeit somewhat extreme conditions, there is a demonstrated need to adequately address drainage and drying potential in the exterior walls. That means the gap and drainage capability of the exterior wall remains critically important. It also means that vapor diffusion and drying potential needs to be considered and taken into account in exterior wall design, particularly in areas prone to severe weather.

Overall, the testing revealed that the real culprit in leakage through WRBs-ABs is not a failure in the WRB-AB technology itself but in the attachment systems and fasteners used to attach the cladding to the wall. In all of the conditions tested, leakage occurred when water and air at elevated levels were able to reach and travel along the attachment fasteners, through the barriers, and into the wall assembly.

Best Practices

RDH went a step further beyond the testing it performed by combining it with its general knowledge and experience on exterior wall designs to identify some best practices in the creation of high-performing exterior walls. The focus of these best practices remains on framed walls with exterior gypsum sheathing with an integrated or liquid-applied WRB-AB. Essentially, RDH bases these best-practice recommendations first on identifying a variety of risk factors and then on the appropriate responses to the particular combination of those risk factors.

Risk Factors

The risk factors identified by RDH fall into essentially three categories: climate, design, and construction factors. Climate risk factors include the specific climate zone where the building is located. Simply put, the risk of water leakage in rainy Seattle is not the same as in dry Phoenix. For purposes of their classification, the eight climate zones are broken into three types: dry (like Phoenix), coastal (like Seattle or other places on oceans or significant bodies of water), and mixed (sometimes wet, sometimes dry across the year).

In the interest of determining best practices, RDH identified different climate and construction risk factors and defined low-, medium-, and high-risk conditions.

Design risk factors relate directly to the final design of the building. The building exposure may be predetermined by the site in terms of urban, rural, or hilltop settings, but the building height may be controllable by design—the taller the design, typically the more exposure to wind and weather if there is nothing around it to act as a buffer and protect the walls. The building form is another design factor that can influence risk. Essentially, the more complex the building form, the more likely it is that the WRB-AB and cladding systems can be difficult to install and achieve true continuity. Beyond form, other aspects of architectural complexity can influence the performance of exterior walls. These include the presence (or absence) of things like overhangs, balconies, bumpouts, and other facade articulations.

Construction risk factors are linked to the design and installation techniques of the cladding. Some cladding is intended to be closed or continuous along all of the joints (such as masonry), some is open around joints and sections (as in a rainscreen design), and some may be directly applied to the sheathing (as in siding). The method of attaching the cladding to the building is similarly a construction risk factor. The presence or absence of a drainage gap is clearly a significant factor, but so is the choice of cladding support (i.e., clips or girts).

With each of these risk factors identified, the next step is to categorize appropriate combinations of climate, design, and construction risks into a ranking order, which RDH has done and described as follows.

Low-Risk Climate and Construction Factors

RDH identifies a combination of conditions as “low risk” for leakage if all of the climate, design, and construction risk factors are low. This would apply to buildings that are first located in a dry climate. The building itself would be a low-rise structure and located in an urban setting (i.e., protected by other buildings). The building form would be small and simple with architectural features that include overhangs for wall protection but no balconies that can interrupt those wall systems. The low-risk condition is also characterized by cladding that uses closed joints (i.e., not open rainscreen type). The cladding attachment design is non-load bearing, tied back to the sheathing, and provides a good drain/vent space.

In low-risk conditions, the recommended best practice is based on simply following the instructions of the WRB-AB manufacturer. It is reasoned that this should suffice since most of these recommendations are based on meeting code requirements that are indicative of typical or low-risk wind and rain conditions. WRB-AB systems and cladding attachments were shown to perform well under such normal conditions during testing, provided the girts or any attachment plates or were screwed tight to the sheathing. Therefore, tightly screwing everything down is presumed as an obvious best practice.


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Originally published in Architectural Record