What IS a Cool Roof? And How to Rank Them

A cool roof can be defined in many ways by people or by different municipal codes. At the end of the day, however, a cool roof reflects and emits the sun’s heat back skyward without allowing it to pass into the building or home. The more sun it reflects and emits, the cooler the roof.

Rating a cool roof is done with two properties — solar reflectance and thermal emittance. Both properties are measured on a scale from 0 to 1, where 0.90 is the same as 90%. Higher values of solar reflectance & thermal emittance are considered to be cooler.

Solar reflectance is the key property for any roofing product. The higher the solar reflective value the more efficient the product is in reflecting sunlight and heat away from the building and thereby reducing roof temperature. Higher solar reflection will dramatically reduce energy bills for air conditioning in the hotter months. This can be very important in large metropolitan areas where peak loads are a concern.

Solar emittance of a material plays a lesser role and refers to its ability to release heat that is stored in the material itself. With the exception of metals, most construction materials have large emittance values of 0.85 or more. Studies have shown that high emissivity will help to reduce cooling load in warmer, sunnier climates. There is a possible connection between low emissivity and heat retention in colder climates, but more studies are needed here.

What is cool, and what is not

The Cool Roof Rating Council (CRRC) was created in 1998 to develop accurate and credible methods for evaluating & labeling solar reflectance and thermal emittance (radiative properties) of roofing products. The CRRC has an online searchable products database which is available for homeowners, contractors, service providers and other interested parties.

All CRRC testing is conducted by accredited testing laboratories. For aged ratings, product samples are exposed for three years at CRRC approved test farms. Product ratings are verified periodically through the CRRC’s Random Testing Program, which basically ensures that most products will be re-evaluated again at some point in the future.

A few words on Energy Star roofing products. Products bearing the Energy Star seal must meet a minimum initial and aged solar reflectance value. Emissivity is not currently a requirement for Energy Star qualification. The EPA is entertaining the idea of of adding an emissivity component to the Energy Star specification at some future date.

Currently Energy Star qualifications for low slope roofs must have an initial solar reflectance of at least 0.65. After 3 years, the solar reflectance must be no less than 0.50. Steep slope roofs must be 0.25 and 0.15, respectively. Both are set quite low, once you look at it.

Energy Star also requires that each company’s warranty for reflective roof products must be equal in all material respects to the product warranty offered by the same company for comparable non-reflective roof products. A company that sells only reflective roof products must offer a warranty that is equal in all material respects to the standard industry warranty for comparable non-reflective roof products.

What to look for in a cool rating

Ideally, you would want to use a roofing material that has the highest solar reflectiveness with the least depreciation of reflectivity over a 3-year period. Decreases in reflectivity will incur decreases in energy savings.

You can usually dismiss solar emittance values, unless it is deemed useful for reducing cooling load in warmer, sunnier climates. In most cases, however, there is not much difference in solar emittance between comparable product lines. Plus a majority of products tend to increase in emissivity as they age.

Energy Star qualified products, while a good place to start, is a very easy goal for manufacturers to achieve. You should research the CRRC database or individual product spec sheets to seek out higher initial & aged reflective ratings.

Research product warranties. Compare higher reflective, shorter lifespan materials against lower reflective, longer lived materials. Balance that with the cost of installing the cool roof and the energy savings you will realize over the life of the roof. You can use the Energy Star roof comparison calculator to get a rough idea of what those savings might be.

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