More can be less: Cumulative coatings

August 1, 2021

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

In the construction industry, the term coating refers to a material applied onto (or impregnated into) a substrate for aesthetic, protective, and/or functional purposes. For building enclosures, an exterior coating is often used to provide protection against moisture infiltration. Selection of an appropriate coating for a specific wall assembly requires proper understanding of the characteristics, advantages, and disadvantages of each system, as well as of the intended coating substrate and its condition, anticipated environmental exposure, and service life and maintenance requirements.

Example of failed section of coating where at least three coating layers have been applied to a concrete masonry unit (CMU) wall. Photo by Jeffrey N. Sutterlin, WJE

One physical characteristic critical to the long-term performance of the coating system is water vapor permeability. Understanding there is a high likelihood for moisture to reach the underlying coating substrate (even for systems intended to function as a barrier), coatings with high water vapor permeance (i.e. breathable) are important to in-service performance. Use of breathable coatings avoids trapping water behind the coating, which can lead to blistering and delamination of the coating, plus potential freeze-thaw damage to underlying substrate.

At a retail complex in the northeastern United States, a recently applied coating system was exhibiting widespread blistering and delamination. Based on discussions with the project architect, an elastomeric coating product had been specified to be topically applied over the existing single-wythe concrete masonry unit (CMU) exterior wall assembly to address recurring water infiltration. The specification recognized the importance of substrate preparation and coating permeability; it required all unsound existing ‘breathable’ coating be removed prior to application of the vapor-permeable (greater than 10 perm) coating product specified for the project.

However, the design did not consider the cumulative effect an additional coating layer would have on the overall permeance of the coating systems on the wall. In general, the overall permeance of the coating system can be determined by totaling the water vapor resistance of each layer, and then taking the inverse of the resistance. For example, three layers of a 10-perm coating would equate to a system that has a rating of 3.33 perm, which would be characterized by the International Building Code (IBC) as a Class III vapor retarder, or ‘semi-permeable.’

Overall permeance should not only be considered when applying a new coating over an existing system, but also when evaluating the long-term maintenance of a coating during the service life of a building. For example, if a coating is anticipated to be reapplied every five years, there will be four layers of the coating present after 15 years, unless the existing coating is removed prior to reapplication. In addition to the effect of multiple coating layers on the ‘breathability’ of the entire coating system, multiple existing layers of a coating may reduce the ability of the overall coating system to adhere to the substrate, as the weight of the overall system may overcome the bond strength of the original coating.

Even when the characteristics of a coating appear to be appropriate for a particular application, consideration needs to be given to the wall system and substrate conditions, existing coatings, and long-term coating performance under the assumed maintenance plan for the system.

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