‘Breathable’ coatings for masonry walls

FAILURES
Deborah Slaton and David S. Patterson, AIA

Masonry in sound condition typically does not require a coating (or penetrating surface sealer) to perform properly.* Coatings used with brick masonry should be breathable, especially in colder climates—they must allow moisture entering the wall system to escape. The degree to which water or water vapor can transport through a building assembly is defined as the material’s vapor permeability, and is measured in perms. Materials with a perm rating greater than 1 are considered ‘breathable’, while coatings with ratings of 1 perm or less are considered vapor retarders (The 2017 American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) Handbook–Fundamentals lists three vapor retarder classes, citing the supplement to the International Code Council (ICC) 2007, ranging from less to more vapor permeable. Class I is 0.1 perm or less, Class II is more than 0.1 perm but less than or equal to 1, and Class III is more than 1 perm but less than or equal to 10.). The permeance of surface sealers and coatings can vary widely with the specific composition and proportions of the product, as well as with applied thickness and continuity (The presence of multiple coating layers can reduce a coating system’s breathability.). For example, a pigmented 100 percent acrylic latex coating used on the exterior of stucco or masonry with a dry film thickness (DFT) of 76.2 μm (3 mils) is noted to have a rating of 19 perms by ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials. Products with very high perm ratings are intended to allow moisture entering the substrate to escape as water vapor through the coating to the atmosphere.

In the example shown here, several coating layers were applied over time to the inside face of brick parapets on a building constructed in the 1930s. Initial coating layers included asphaltic membranes, which were commonly installed to make the inside face of brick parapets more resistant to water penetration. Over the years, several layers of additional coatings were applied over the asphaltic membranes. Although the later coatings were to some extent breathable, the presence of previously applied non-breathable layers of asphalt caused the masonry wall assembly to be permeable only in one direction—through the front (uncoated) face of the wall.

Lack of maintenance of joints in the coping and wall, and deterioration of the masonry, allowed bulk water to freely enter the wall, resulting in areas where the brick within the wall was near saturation due to absorption, and rendering the masonry more vulnerable to water-related damage. As the coatings deteriorated over time with age and exposure, additional water was able to enter the brick masonry through cracks and breaches in the membrane. Moisture was trapped by the non-breathable coating, resulting in spalling and deterioration of the brick. A thin layer of brick and/or mortar present on the back of the coating in some areas of failed membrane indicates the plane of failure occurred at or near the substrate’s surface, where moisture was trapped.

Failure of brick that has been covered by a non-breathable coating underscores the importance of selecting appropriate repairs when needed. If a coating does need to be applied, the masonry must first be repaired as needed, with all joints between brick units properly pointed.

^*Jeffrey N. Sutterlin, PE, of the Princeton office of Wiss, Janney, Elstner contributed to this article.

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

Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates (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 office in Princeton, New Jersey. He specializes in investigation and repair of the building envelope. He can be reached at dpatterson@wje.com.

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