What Makes a Cool Roof Cool
Cool Roof, Thermal Emittance, Infrared Reflectance July 18, 2008
Today’s tip is about the properties that govern how cool a cool roof is. Now that summer is upon us, and facilities are enduring the long, hot days that require lots of cooling energy, a cool roof can be a key part of facility executive’s energy reduction plan.
The two principles at work on a cool roof are infrared reflectance and thermal emittance. Infrared reflectance means the roof’s ability to reflect a portion of the sun’s infrared radiation – the part of the solar spectrum most responsible for heat gain. Some pure white cool roofs have infrared reflectances as high as 80 percent. That’s very good. But the key here is that the roof must be clean. Especially in big cities, where cool roofs have the most benefit because of the urban heat island effect, the smog and other city-related crud can collect on a roof and darken it. To get the maximum benefit of a roof, it should be kept as clean and white as possible.
The other value is thermal emittance, or a roof’s ability to reradiate energy that has been absorbed back into the atmosphere. In hot climates and during hot summer a months, roofs with high thermal emittance are good because they also prevent
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