A Database for Cool Materials

P. Berdahl, H. Akbari, L. Gartland, C.K. Smith, F. Yang

To design buildings that can provide comfort with less air-conditioning energy use, we need information about materials that remain cool in the sun. Cool materials that reflect incident solar energy back out to space can also be used to cool entire cities. While it is certainly well known that white materials are useful (and black materials are poor) for these purposes, reliable and accurate information on the "coolness" of construction materials is often lacking. Particularly important are roofing materials. To satisfy the need for better information, a Cool Materials Database is being constructed.

Most common roofing materials absorb solar radiation, reflecting only a small portion of the incident energy. Dark roofs reach peak temperatures of 82°C (180°F) on hot, calm sunny days, Such high temperatures lead to significant heat conduction into the building through the roof insulation and into air-conditioning ducts. Roof temperatures are determined primarily by the heat balance at the outer roof surface. High solar reflectance is obviously desirable. In addition, the building designer wishes to enhance heat transfer from the hot roof to the environment, which occurs by thermal emission of infrared radiation and by heat convection.

Solar reflectance values are found in the literature for a few building materials, but the available information is quite limited. Manufacturers, if they measure reflectance at all, often measure the visible reflectance. However, at least half of the energy content of sunlight is in the invisible infrared and ultraviolet portions of the spectrum. The best procedure for measuring solar reflectance in the laboratory is the use of a spectrometer to measure the monochromatic reflectance across the solar spectrum, as is shown for some white roof coatings in the Figure. Then a standard solar spectrum is used as a weighting function to compute the overall fraction of solar energy reflected under typical atmospheric conditions. In our laboratory, we perform these measurements with a Perkin-Elmer Lambda 19 US-VIS-NIR spectrometer fitted with a Labsphere integrating sphere.

Infrared emittance values for building materials have not been widely measured. A common strategy for non-metals and metals with opaque coatings is to assume the emittance is independent of wavelength and equal to 0.9, as is often the case. However, measurements are needed to see how generally this rule of thumb is applicable. Bare metal surfaces and aluminum-pigmented coatings are known to have lower emittances, which also vary with wavelength in the thermal infrared range. We have recently completed the setup of a new instrument, a Spectral Emissonmeter, which permits the determination of spectral emittance from 5 to 40 micrometers wavelength. It consists of a Bruker Instruments IFS 28 Fourier Transform InfraRed (FTIR) spectrometer, fitted with an external port and an external chamber for housing the heated sample under test.

In the next year, an initial version of the Cool Materials Database will be published in paper and electronic forms. It will include tabulations of solar reflectance and infrared emittance of building materials and lists of manufacturers. It will be accompanied by analyses that interpret the data and provide advice for its use. In-house measurements of solar reflectance and infrared emittance will be used to provide data not otherwise available and to allow us to evaluate the data we are able to obtain from other sources.

Figure. Spectral reflectance of seven white roof coatings. Also shown is the shape of the solar spectrum (lower curve), which indicates how the solar energy is distributed over wavelength. The overall solar reflectances of these materials are Toughkote, 0.85; Acryshield, 0.83; TriLastic, 0.83; Guardkote, 0.74; Koolseal, 0.81; MCI, 0.80; and Triangle No. 7, 0.84. These values are typical for high-quality white roof coatings.

Reference

Berdahl P, Bretz S. Preliminary survey of the solar reflectance of cool roofing surfaces. Energy and Buildings, Special Issue on Urban Heat Islands and Cool Communities, 1995 (in press).

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