Making your new roof work for you

Replacing a low-slope roofing system is a costly investment, however there can be an upside to it as well. Aside from keeping your building, it’s contents, and occupants protected from the elements,a critical part of the decision making process should involve the color of the material used in your new roof. In warmer climates, a light colored or reflective surface is optimal in reducing cooling costs while colder environments can take advantage of darker colored materials to help with heating costs. Regardless of the outside environment, a substantial layer or layers of insulation is needed to maximize the return on your investment.

Three important roof membrane characteristics that should be considered are the Reflectivity, Emissivity, and Solar Reflective Index. The higher the Reflectivity the better the membrane reflects the sun’s radiation. Usually the lighter color the membrane the better the reflectivity, such as a white thermoplastic roof membrane will have a much higher reflectivity than a black rubber or tar roof. Some of the highest reflective roof membranes have an initial Reflectivity greater than 87%. Mirrors are great reflectors, while cloth velvet is a very poor reflector.

Emissivity is a measure of the efficiency in which a surface emits thermal energy. A black membrane roof will most likely have higher emissivity than a white membrane roof. For example, on a sunny warm day in the mid-afternoon you place your hand on a white or lighter colored car’s hood…most likely you’ll feel a warmth that you are able to withstand and keep you hand on the hood. Conversely if you place your hand on a black or darker color car’s hood….you probably can’t leave you hand placed on the hood very long because of the extreme heat that the darker color has absorbed is emitting this heat to your hand.

Solar reflectance index (SRI), SRI combines Reflectivity and Emittance to measure a roof’s overall ability to reject solar heat. Standard black (reflectivity 5%, emittance 90%) has an index of 0, and standard white (reflectivity 80%, emittance 90%) has an index of 100. Very hot materials can actually have negative values and very cool materials can have values greater than 100. Below is a table to illustrate some roofing type SRIs.

Here’s a comparison of other roofing systems as measured by Lawrence Berkeley Labs and the Florida Solar Energy Center:

Roofing System SRI Reflectivity Emittance
White EPDM 84 69% 87%
Kynar Coated Metal, White 82 67% 85%
New, Bare Galvanized Steel 46 61% 4%
Light Gravel on BUR 37 34% 90%
White Granular Surface Bitumen 28 26% 92%
Dark Gravel on BUR 9 12% 90%
Black EPDM -1 6% 86%

It is important to note that Duro-Last Roofing membrane has one of the highest SRIs in the roofing industry with a SRI of 108. Generally when designing a roof for optimal thermal performance, the higher the SRI the better.

JBK Roofing can help you gather information about roof membranes and roof components that will help you decide which roof system best fits your next roof project.

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