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Cool roofs speaks to what are cool roofs and why use cool roofs.
They are required as roofs are exposed to solar radiation on a daily basis and as that radiation is absorbed and converted to heat, the temperature of the roof covering rises.
Depending on the intensity of the radiation and the portion of it retained by the covering, roof surfaces may routinely reach temperatures of 150 F (65C) or higher. High roof temperatures can lead to overheating of interior spaces, reduced comfort for building occupants, increased building energy consumption, the need for larger, more expensive cooling equipment and shortened lifespan of roofing materials.
It also increases the contribution to the urban heat island effect through elevation of the surrounding air temperature. Selecting a cool roof covering that minimizes such heating can significantly reduce these effects.
Solar heating of roofs is principally affected by two properties of the of the roofing material:
Solar Reflectance or Albedo
This is a measure of a roof’s tendency to reflect solar radiation rather than absorb it. Solar reflectance is measured on a unitless scale from 0 to 1, where 1 represents a material that reflects all solar radiation and 0 also represents one that absorbs all solar radiation. A higher solar reflectance corresponds to a cooler roof.
This is a measure of a material’s capacity to radiate infrared heat energy and cool itself as its temperature rises. Like solar reflectance, thermal emittance is measured on a scale of 0 to1 and a highr thermal emittance implies a cooler roof.
Cool roof criteria differ among energy conservation standards and green building programs. Requirements for the US Environmental Protection Agency (EPA) Energy Star program are based solely on a roof coverings solar reflectance, measured both when when the covering is new and after it has been weathered.
Requirements for the US Green Building Council’s LEED for New Buildings are based on a roof coverings solar reflective index (SRI). SRI is a measure of solar heating potential, derived according to ASTM E 1980, that accounts for a material’s reflective and emittive properties, as well as for its ability to lose heat through thermal conductance to the surrounding air.
Two roofing materials with the same SRI are expected to achieve the same surface temperature under comparable exposures. Higher SRI values correspond to cooler roof coverings with an SRI value of 0 corresponding to a standard reference black surface and a value of 100 corresponding to a standard reference white surface.
Comparative solar heating properties for common roofing materials are available online. Product specific data can be obtained from the manufacturer’s product literature or from the Cool Roof Rating Council, an independent organization that maintains a roof material rating program and publishes the properties of such. In comparison to traditional dark coloured EPDM or bitumous membranes, highly reflective cool membranes on low slope roofs can reduce roof surface temperature by as much as 50 to 75 degrees Fahrenheit (25-40C) and cut building cooling costs by an estimated 15 to 25 percent.
These roof materials on steep roofs have the potential to save an estimated 5 to 10 percent of building cooling costs.
Cool Cover Roof Coverings
Cool cover roofing materials are nonwhite in colour but nevertheless reflect a significant portion of the sun’s radiation. Cool colours are formulated with pigments that are selectively reflective to different portions of the solar spectrum.
They are highly reflective of near infrared (NIR) radiation, an invisible component of solar radiation that accounts for over half of the total heat energy radiated by the sun, while they remain absorptive in the visible light spectrum which accounts for their apparent colour.
Cool colour pigments can be applied to aggregate granules used to coat asphalt shingles, as well to sheet metal, clay or concrete tile, fiber cement shingles and other roofing materials to produce products meeting cool roof standards for steep roofs. As the formation of cool colour pigments continues to evolve, smooth surface roofing materials with reflectance values as high as 0.45 and granule surfaced materials with values as high as 0.30 are anticipated.