In the United States buildings are responsible for almost two-thirds of national electricity consumption and more than one-third of total primary energy use. As a result, the current energy crunch has made conservation measures within these structures more important than ever.
Heating and cooling costs are generally the major expenses associated with the operation of a building. Therefore, any reduction in these costs through the use of energy-efficient building envelope components makes sense. Unfortunately, the roof can be the least energy-efficient component of a building envelope. Is it any wonder, then, that cool roofing has become such a hot topic?
Cool roofing is gaining in popularity due to its ability to reduce cooling and heating energy usage. Utility companies are also interested in cool roofing because it can help reduce the peak demand in electricity during the afternoon hours of summer months, preventing power disruptions. And, from an environmental point of view, cool roofing can also help to mitigate a phenomenon known as the heat island effect.
The concept of cool roofing
Cool roofing is based on the premise of minimizing heat gain through the roof surface. To understand how this happens one must look at the solar energy spectrum as shown in Figure 1. Ultraviolet energy in lower wavelengths amounts to only three percent of the total energy striking the earth's surface. Visible light energy is 40 percent and infrared energy in the longer wavelengths accounts for the largest percentage of the spectrum. When IR energy strikes the earth's surface, we feel it as heat.
The Solar Energy Spectrum
Cool roofing is described by two main terms: solar reflectance and thermal emittance. Total solar reflectance is the percentage of all solar radiation that is immediately reflected from a surface. Any energy that is not reflected from a surface is absorbed by the material. Some of this is transferred to heat that can be removed by convective transfer from air flow over the surface. Some of the heat can be conducted through the surface. More importantly, some of the heat can be re-emitted to the night sky in the form of infrared wavelength energy. The latter phenomenon is known as thermal emittance. The combination of solar reflectance and thermal emittance properties of a material determine the surface temperature of a roof and its ability to act cool.
Some typical radiative properties of common roofing materials are shown in [Table 1]. Metal roofing has a wide range of solar reflectance and thermal emittance values. In the unpainted condition, a metallic surface has a very low TE but a relatively high TSR. When a paint system is applied to the surface, the TE is very high regardless of the color. However, the TSR can vary depending on the color and/or pigmentation used.
Solar Reflectance | Thermal Emittance | |
* depending on color | Source: ORNL and LBNL | |
Metal (unpainted) | 0.60-0.80 | 0.04-0.10 |
Metal (painted) | 0.10-0.75* | 0.80 + |
Comp Asphalt Shingles | 0.05-0.25 | 0.90 |
Single Ply Membranes | 0.70-0.80 | 0.85+ |
Built Up Roofing | 0.05-0.80 | 0.90 |
Modified Bitumen | 0.05-0.25 | 0.90 |
Concrete/Clay Tile | 0.20-0.70* | 0.90 Most PopularWebinars & Whitepapers Find us on 
 
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