Defining a Cool Roof

Cool roof performance is a function of two radiative properties: Solar Reflectance and Infrared Emittance.  These two properties define how “cool” a roof product is. 

Solar reflectance is defined by the Cool Roof Rating Council (CRRC) as, “The ratio of the reflected flux to the incident flux”.  In other words, the number reported for solar reflectance for a roof product is a decimal number less than one that represents the fraction of light reflected off the roof.  For example: “high” reflectance materials, such as white painted metal roofing, have values of around 0.70 (a.k.a. 70 percent).  That is, only 30 percent of the light from the sun is retained by the roof. 

Infrared emittance is defined by CRRC as, “The ratio of the radiant heat flux emitted by a sample to that emitted by a black body radiator at the same temperature”.  In plain terms, emittance is a decimal number less than one that represents the fraction of heat that is re-radiated from a material to its surroundings.  For example, low emittance products such as unpainted Galvalume™, have values around 0.10 (a.k.a. 10 percent).  Emittance is the a critical measurement for municipalities that are concerned with the phenomenon known as the “Urban Heat Island” effect, a topic discussed later in this article. 

Perhaps the most prevalent misconception regarding cool roofing is that specifying the highest possible Solar Reflectance and Infrared Emittance properties makes sense for every project. This is simply not true. There are different climate zones throughout the United States and not all will benefit from high reflectance and emittance – in some climate zones high emittance may actually be detrimental to overall energy performance. It is important to evaulate the specific needs of the building and location with a roofing expert before specifying the exact material for your roof.