Cool Roofs Reject Heat, Lower Cooling Loads

OTHER PARTS OF THIS ARTICLEPt. 1: This PagePt. 2: Lower Your Energy Bills with Cool RoofsPt. 3: Cool Roofs Generate Lower Surface TemperaturesPt. 4: Cool Roofs: Develop a Maintenance PlanPt. 5: Cool Roofs: Title 24, LEED Provide Standards and Guidelines

The availability of viable and energy-efficient cool roofing products has expanded greatly in recent years. Building owners and managers, specifiers, and architects increasingly use these products because of their capacity for energy savings.

Managers in institutional and commercial facilities also can achieve several other benefits, such as lower interior temperatures, greater occupant comfort, and smaller cooling loads on HVAC systems. A growing number of building codes also mandate the use of cool roofs, and more voluntary programs include them as a key element of sustainable design.

By understanding some of the science behind cool roofs and the questions to ask in specifying these systems, managers can more effectively match product options to the organizations’ needs.

How Is It Cool?

Specifying a cool roof for an institutional or commercial facility can be a smart decision, but not everyone is clear on what exactly cool entails. Cool roofs have been around for quite some time and are increasingly popular options in many areas of the United States. Still, managers often do not understand the performance details of cool roofs.

A cool roof is highly reflective and can easily emit the small amounts of heat it absorbs. Reflecting energy from the sun back to the atmosphere allows the surface of the roof to remain cooler, and, compared to a traditional roof, it transfers less heat into the building.

Managers can achieve cool roofs by using a white or light-colored single-ply membrane, using a cool coating over a black or dark membrane, or using a factory-painted or coated metal system. Cool capsheets, shingles and tiles also are available. Most cool-roof applications have a smooth, light-colored surface.

Two radiative properties define a cool roof. Solar reflectance describes the fraction of solar energy the roof reflects. Thermal emittance describes the relative ability of the roof’s surface to radiate absorbed heat.

Both of these properties are measured from 0 to 1, and the higher the number, the cooler the roof. There is no universal definition for a cool roof, so different codes and programs have developed their own requirements for minimum radiative properties that constitute a cool roof.

Here is where part of managers’ confusion comes in. Many elements of a roof’s structure, such as insulation, can affect a building’s interior temperature and it performance. These elements are obviously important, but “cool roof” refers specifically to the roof’s radiative surface properties.

Radiative properties also can cause confusion because solar reflectance and thermal emittance are not common concepts. Consider another way of looking at the issue: The roof surface will stay cooler if it reflects the sun’s energy and quickly releases energy it absorbs than it will if it absorbs the energy. Reflectance and emittance quantify these two properties.

In stands to reason that light-colored surfaces have high reflectance, and many early rebate programs for cool roofs used this characteristic as the only requirement. Uncoated shiny metal surfaces reflect the sun’s energy efficiently, but they have very low emittance and, as a result, become very hot during the day. A roof that demonstrates both high solar reflectance and high thermal emittance is essential for managers seeking to improve a facility’s energy performance.

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  posted on 4/1/2009   Article Use Policy

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