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reflectance and emittance

A high solar reflectance - or albedo - is the most important characteristic of a cool roof as it helps to reflect sunlight and heat away from a building, reducing roof temperatures.

-- EPA Reducing Urban Heat Islands: Compendium of Strategies


A high thermal emittance also plays a role, particularly in climates that are warm and sunny. Together, these properties help roofs to absorb less heat and stay up to 50-60°F (28-33°C) cooler than conventional materials during peak summer weather.1

Building owners and roofing contractors have used cool roofing products for more than 20 years on commercial, industrial, and residential buildings. They may be installed on low-slope roofs (such as the flat or gently sloping roofs typically found on commercial, industrial, and office buildings) or the steep-sloped roofs used in many residences and retail buildings.

Through the ENERGY STAR program, EPA and the Department of Energy (DOE) help consumers and other purchasers identify the most energy efficient roofing products.

Roofing materials with the ENERGY STAR label have met the minimum solar reflectance and reliability criteria. Based on 2006 data from more than 150 ENERGY STAR partners, shipments of cool roof products have grown to represent more than 25 percent of these manufacturers' commercial roof products and roughly 10 percent of their residential roof products.

Source: EPA's Reducing Urban Heat Islands: Compendium of Strategies


Terms to Know

Thermal Emissivity (emittance) is a measure of solar radiation (sun's rays) that is first absorbed and then radiated from a roof, expressed as a number between zero and one. The higher the value, the better the roof radiates heat. Any solar radiation that is not reflected (rejected outright, as with a mirror) is absorbed; this absorbed energy is then either radiated (i.e., emitted as infrared) or retained (adiabatic heat). In theory, the surface temperature of a material with 100% thermal emissivity would be equal to the ambient temperature (i.e., that of the surrounding air). Heat that is not released via the relatively efficient mode of infrared emittance is retained by the roof, which causes a buildup of heat. That heat is then released to the surrounding atmosphere via the less efficient (and slower) mode of conduction and convection.

Solar Reflectance Index (SRI) indicates the roof's ability to reject solar heat, and is essentially the combined value of reflectivity and emittance. It is defined such that a standard black is zero (reflectance 0.05, emittance 0.90) and a standard white is 1.00 (reflectance 0.80, emittance 0.90). Because of the way SRI is defined, very hot materials can have slightly negative SRI values, and very cool materials can have SRI values exceeding 1.00.