Friday, December 2, 2011

An Overview of White Roof Coatings: Part 3

Many white roof coatings (like those from Conklin) are waterproof, but some are not. The property of permeability (perm rating) to liquid water, water vapor, and gases varies greatly, depending on the type of coating.
  • Acrylic roof coatings are breathable, which means they have a high moisture vapor transmission rate or permeability.
  • Silicone roof coatings are also classified as breathable types.
  • Butyl rubbers, hypalons, and neoprenes have a very low permeability, i.e., they are highly resistant to moisture transmission.

Keep in mind that the perm rating should not be confused with weatherability, or resistance to weathering. A roof coating with low permeability still may require a protective topcoat, to ensure satisfactory weathering resistance.

Cleanliness & Aging
Roofs that have white coatings in arid, dusty regions, or in places where farm plowing or construction exposes the earth to wind, are likely to accumulate dirt more than in areas with greenery or where occasional rainfall washes away the dirt. The frequency and intensity of precipitation and the slope of the roof also affect the cleanliness of the coatings over time.  As with other white surfaces, white coatings discolor and darken slightly after several years of service.

Two values of reflectivity are often quoted to represent the performance of new coatings and three-year-old weathered white coatings, respectively. Generally, a small decrease in reflectivity occurs over time, depending on several factors. Wind-blown dirt and dust can decrease the reflectivity of white coatings, depending on the age of the coating and regional climate characteristics.

UV radiation tends to be blocked rather than reflected by a white coating. A white roof coating still protects against UV radiation even when foreign particles reduce the reflectivity.  To maintain their reflectivity, roofs may be periodically refreshed with a new topcoat, typically for less than the cost of the initial coating.

The maintenance schedule depends on the type of coating, type of roof, the purpose of the coating, and regional climate differences. Typically, white roof coatings should be refreshed every three to seven years.

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