Cool roofs fall into one of three categories: roofs made from inherently cool roofing materials, roofs made of materials that have been coated with a solar reflective coating, or green planted roofs. While green roofs are in many ways the best, they are also the most difficult to construct and maintain. Inherently cool roofs like vinyl are relatively easy to install, while the easiest and in this respect the best cool roof is a coated roof.
Inherently cool roofs
White vinyl roofs, which are inherently reflective, achieve some of the
highest reflectance and emittance measurements of which roofing materials are
capable. A roof made of thermoplastic white vinyl, for example, can
reflect 80 percent or more of the sun’s rays and emit at least 70% of the solar
radiation that the building absorbs. An asphalt roof only reflects between 6 and
26% of solar radiation, resulting in greater heat transfer to the building
interior and greater demand for air conditioning.
An existing (or new) roof can be made reflective by applying a solar
reflective coating to its surface. The reflectivity and emissivity ratings for
over 1000 reflective roof products can be found in the CRRC (Cool Roofs Rating Council) website.
Green roofs provide a thermal mass layer which helps reducing the flow of
heat into a building. The solar reflectance of green roofs varies depending on
the plant types (generally 0.3-0.5).Because of the lower solar reflectance, green roofs reflect less sunlight and absorb more solar heat than white roofs. The absorbed heat in the green roofs is trapped by the greenhouse effect and then cooled by evapotranspiration.
© 2012, Richard Matthews. All rights reserved.
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