Cool Roofing Samples (Emittance)

The Heat Island Group at Lawrence Berkeley National Laboratory, Berkeley, CA have measured solar reflectance of roofing samples with an UV-VIS-NIR Spectrometer with an integrating sphere and they measured the spectral emittance of the samples with a FTIR Spectral Emissometer. The following writeup and graphs are from their webpageat eetd.lbl.gov/HeatIsland/CoolRoofs/Samples.html


"Below are examples of complete reflectance and emittance data for several metal roofing samples made of cool roofing materials. These measurements show examples of complete laboratory information needed to determine radiative heat exchange by a roof which, in turn, can be used to estimate peak roof temperatures.

"The spectral solar reflectance is the total reflectance (diffuse and specular) as a function of wavelength, across the solar spectrum (wavelengths of 0.3 to 2.5 µm). It is used to compute the overall solar reflectance, using a standard solar spectrum as a weighting function. It also contains the information in the visual range (0.4 to 0.7 µm) which is sufficient to compute the color coordinates for color matching with other materials.

"The spectral thermal emittance (the graphs on the right) contains the information for computing the overall thermal emittance, using a blackbody curve as the weighting function. The spectral range is about 5 to 40 µm. If the spectral thermal emittance is approximately a horizontal line (a "gray" body), then the overall emittance is adequate for computing longwave radiative radiative exchange between the roof and the atmosphere. If the spectral thermal emittance deviates markedly from a horizonal line, then the details of the spectral emittance and the atmospheric emittance are necessary for a complete computation.

Note that the hunter green sample (middle graph) looks green to the eye because of the reflectance "bump" at 0.5 µm. The average solar reflectance, at 0.086, is almost as low as black (zero).""The burgundy sample (bottom graph) looks red due to the increase in reflectance near 0.7 µm. The visible reflectance is only about 0.1, but the relatively high reflectance in the near infrared (0.7 to 2.5 µm) yields an overall solar reflectance of 0.226."The emittance for all these samples is roughly 0.9, with an abrupt fall-off near 6 µm. Link to: Roof Heat Transfer > Emittance"

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Galvalume (top graph), due to the inclusion of aluminum metal in the zinc anti-corrosion coating, is more reflective to sunlight than traditional galvanized steel which has a solar reflectance around 0.5.A further coating, with a clean acrylic material (low graph), can be used to raise the infrared emittance without significantly changing the solar reflectance.