-----Original Message-----
From: Brian Whatcott
Sent: Wednesday, May 27, 2009 5:47 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] paint your roof?
We have only heard that the reflective roofing material is colder than the ambient night air, and this should be true for the best reflectors AND the
worst, surely?
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Suppose you have a metal reflector. It will be highly reflective in the visual and the IR. So it will have very low IR emissivity, so it will not efficiently cool at night.
If you have an opaque material that has low reflectance in the IR, then its IR emissivity is high, so it will efficiently cool at night.
The best cool roofs have high solar reflectance due to the diffuse white color and high thermal emittance associated with the polymer that is transparent at solar wavelengths and absorbing (and therefore emitting) at the long IR wavelengths.
The spectral efficiency of cool roofs and cool paints is determined by both the solar reflectance (0.25-2.5 microns) as well as the IR (~5-25 micron) emittance.