Selected Answers

From Zenith Czora of Durotech Industries, Inc. on May 28, 2019:
Water vapor permeability is a desirable property f ...read more Water vapor permeability is a desirable property for an exterior coating to allow water vapor to escape from the CMU walls or allow the CMU walls to breathe. But, if the permeability is too high, carbon dioxide can diffuse from the atmosphere into the wall structure and cause detrimental effects to the wall structures.The pH of the cementitious walls will slowly turn acidic and will destroy the passive layer of reinforcing steel bars. Once the passive layer is destroyed, rusting of steel bars begins. On the other hand, low permeable or less breathable coating could also cause some issues. When the diffusion of water vapor chills to its dew point, it condenses to liquid water and become trapped underneath the coating. The entrapped moisture can cause potential deterioration of substrates from cyclic freezing and thawing, possible biological growth, or blistering of coatings that will eventually lead to delamination.  Hence, there is a need for coatings with moderate vapor permeability which can prevent CO2 from diffusion into the wall and also allow moisture to escape. For many applications on breathable substrates like CMUs, it is important for the coating to resist or repel liquid water and yet be permeable enough to water vapor. Exterior coatings should be able to withstand penetration by wind-driven rain, both to prevent water from entering the substrate and to prevent any contaminants such as airborne particulates and acids from being carried into the substrate with water.