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24 ROOF COATINGS 2017 COATINGSPROMAG.COM styrene-ethylene/butylene-styrene (SEBS) coating is needed. ese are all vapor retarders, having the capacity to stop or greatly impede moisture transmission. At typical application rates, these products will have permeance values of 0.05 to as low 0.001 U.S. perms. In contrast, silicone, acrylic, and most urethane coating systems breathe, and even in higher builds, they typically have perme- ance levels of over 3.0 U.S. perms. Breathers are the default in maintenance coating applica- tions because they do not change the way a building functions with respect to moisture transmission. If the building already had a vapor retarder, such as an unvented metal deck or a built-up roof (BUR) system, adding a breather on the surface will not change the movement of moisture in that roof. If it didn't have a vapor retarder, it's very likely it was never intended to have one. In both cases, a breathable coating is safe to use. One exception is when a building relied on a vapor retarder coating initially and that coating has weathered away; that building's design demands a vapor retarder coating, such as an SEBS. Also some buildings may have a vapor drive that goes inward and may benefit from a vapor barrier coating. A good example of this is a cold storage building. SEBS coatings, which can be black or white, are the most common vapor barrier coatings today. ey resist ponding water very well, and they cure in humid and cold conditions much like a silicone. VOC restrictions are the main limitation to their use. Adhesion is another fit-for-use consideration in any appli- cation, and one that cuts across all coating types. Coatings are compounded products, blended of many materials and in different proportions, meaning the resulting properties of any one type are not universal. is fact is most apparent with adhesion, where specific additives may be used to enhance adhesion to materials, such as SPF, ethylene propylene diene monomer (EPDM), metal, or thermoplastic polyolefin (TPO) roofs. It is not correct to say all acrylics stick to wood and all silicone coatings stick to SPF; it depends on invisible cross- linkers and adhesion promoters that the user will not be privy to. Manufacturers often publish adhesion in accordance with ASTM coating standards using the ASTM D903 peel adhesion test that shows adhesion under ponding water conditions. Always confirm that a coating is tested for adhesion specifically to the surface you need to protect, or do a test area. Weaknesses and Downsides As with any roofing material, there are some long-stand- ing challenges in working with any specific type of coating. Acrylics do not fare well under ponding water, and their cure is greatly affected by wet/cold weather. Silicones are impos- sible to recoat with unlike materials. Repairs to silicones, even when made with the same product, often exhibit poor adhesion. Shelf-life is typically short for silicones. Urethanes are generally the most toxic of the three main roof coating chemistries, and they are often the least fire resistant. Urethanes will often solidify if they are not stored in a condi- tioned space. One of these particular weaknesses often drives the final choice. To compensate for these general shortcomings, a few important sub-types are important to recognize. e short list is high-solids silicone, aliphatic polyurethane, hybrid polyureas, and premium acrylics. ese coatings offer improve- ments in VOCs, film builds, cure rates, durability, reflectance, and color selection. ese are beyond that of the standard products, but often at a higher cost. High-solids silicone coatings offer a very low VOC, as low or lower than most water- borne products. ese high-solids silicones are an important option to consider wherever there are tight VOC regulations. So Many Coatings Products may need to be considered for ultraviolet (UV) resistance or stability, elasticity, VOC levels, and hail resistance. When considering a coating for a specific project, be sure to consider all of the necessary characteristics, such as performance, weather and region, and safety. One size does not fit all!

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