White roof coatings contain transparent polymeric materials, such as acrylic, and a white pigment, such as titanium dioxide (rutile), to make them opaque and reflective. Other white pigments sometimes used are the anatase form of titanium dioxide, and zinc oxide. These coatings typically reflect 70-80% of the sun's energy. Despite the white appearance, these pigments only absorb 5% or so of the sun's energy which falls in the ultraviolet. Thus, the pigments help protect the polymer material and the substrate underneath from UV damage. The addition of ceramic beads and fire retardants are also available upon request.
These coatings are applied in thicknesses considerably greater than typical white paints, ranging up to about 1 mm and reflectance increases with thickness. The substrate must be clean and compatible with the coating system. The achievement of the very highest reflectance values requires sufficient pigment and a smooth substrate. If the substrate is already light in color, it can be made highly reflective with less pigment (fewer coats).
To help maintain the high reflectance of the freshly applied white coating, several issues are important. A completely horizontal roof, with ponding water after rain, is likely to become quickly soiled, with a corresponding loss of reflectance. Of course it is also very likely to fail by leaking! A mildewcide additive can retard biological growth with its resulting stains. There is some variation in how tightly dirt adheres to coatings.
Heat Reflectivity is a major component to the performance of Duracool coatings. It is important to understand three terms:
EMISSIVITY
is the percentage of absorbed energy a material can radiate away from itself.
ALBEDO
is the measure of reflectivity to the full spectrum of the sun's energy. This includes non-visible ultra-violet or infrared areas of the spectrum.
CONDUCTIVE HEAT TRANSFER
is a direct function of the temperature difference between a roof system's surface and the interior air.
Most roof systems [especially metal, BUR with or without metallic coating, or black EPDM] have very low emissivity and very low albedo which results in high surface temperatures and elevated conductive heat transfer. This is a major cause of heat gain. Bright, white Duracool provides a very high emissivity and albedo. This reduces surface temperatures to near ambient, essentially stopping conductive heat transfer.
