 Diagram of heat flows into and out of a roof. The heat flowing through a roof is a function of the difference in temperatures on either side of the roof. |
Roof Heat Flows
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Diagram of heat flows into and out of a roof. The heat flowing through a roof is a function of the difference in temperatures on either side of the roof. |
The surface temperature of a roof is mainly determined by the vigorous heat flows at the outside surface. Of these external energy flows, convective cooling is the least precisely known. The solar and infrared radiative cooling can be readily calculated if the solar reflectance and infrared emittance is known. Link to: Emittance. Once the roof temperature is known, the heat flow leaking into the interior is readily computed. Decreasing a roof's emittance may lead to an increase energy use. Link to: Emittance Impacts Energy Use.
Spectral Distribution of Radiant Roof Heating
 The spectral distribution of radiant roof heating. Equal areas represent equal heat flows. |
The above figure illustrates the 24-hour average radiant heat flows to and from a black and emissive roof, as a function of the wavelength. Equal areas represent equal heat flows. The solar spectrum ranges from 0.3 µm in the ultraviolet (UV), through the visible, to the near infrared (0.7 to 2.5 µm). The cooling effect, due to the emission of thermal infrared radiation (4 to 40 µm), depends on the roof temperature. A roof at air temperature has a net cooling between 8 and 13 µm (the atmospheric "window"), while a hot roof has a broader net emission spectrum.
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Outdoor measurements on the 12 samples in the graph above show how the temperature rise in full sun is inversely correlated with the solar reflectance values measured with our instruments in the laboratory. Link to: Coof Roofs Instrumentation. Materials with emittance of approximately 0.9 fall near the straight line. Materials with lower emittance, particularly galvanized steel, fall above the line, due to their limited ability to emit thermal radiation. (Source: P. Berdahl and S. Bretz, Energy and Buildings 25, 149 (1997)). There are, unfortunately, many problems in measuring emittance. Link to: Emittance. We also collaborate with manufacturers to find ways to make roofing materials cooler. Link to: Collaboration with Industry Sample data for some examples of cool roofing materials are available. Link to: Cool Roofing Samples