Q.: Stucco panels on one of our structures were treated with a silicone water repellent, flooded onto the surface by brush. Then an asbestos-containing coating was applied. After about 2 years the coating started peeling off in large pieces from about 6 square inches to about 1 foot square. We enclose some samples of the peelings. The architect insists that the stucco finish was not installed properly because some of it is adhering to the coating that peeled off. We believe that the stucco was painted too soon and that when the moisture could not penetrate the coating, it caused the coating to peel. What can you tell us?
A.: We do not have facilities to analyze or examine the peeled coating. We can say that peeling usually results where moisture has built up below the coating and has no opportunity to escape. If the coating does not have enough breathability for the conditions, the water vapor pressure will push it off. We suspect that is what happened on this job. The fact that a thin layer of stucco was pulled off with the paint probably means only that the strength of bond between the coating and the stucco was higher than the tensile strength of the stucco. The motive force was most likely an excessive buildup of vapor pressure below the coating. Exactly what caused the vapor pressure to build up so high is something we cannot say; it could have been too early an application of the coating or it could have been high relative humidity within the building.