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Remember that emissivity and absorptivity are generally equal and often
vary with wavelength. And for opaque materials, emissivity generally
equals (1- reflectivity) at a given wavelength.<br>
<br>
So it would be possible to have a spectrally selective roof which
reflected the sun's visible and short wave infrared energy well (high
reflectance, low emittance) but also radiated energy well at the longer
infrared wavelengths emitted at its temperature rather than the sun's
(low reflectance, high emittance). This would be the ideal cool roof
material.<br>
<br>
This roofing material probably exists.<br>
<br>
Dave Bryan<br>
Third Level Design<br>
<br>
<br>
<br>
Brad Painting wrote:
<blockquote
cite="mid:34791680911291257h59f9b6f2r487f021cc6d6ddfd@mail.gmail.com"
type="cite">It seems to me that a roof that emits more radiation will
have a greater warming effect on the building. Some houses in warm
climates have radiant barriers because the aluminum has a <i>low</i>
emissivity, thus blocking the infrared radiation. But both LEED and
Energy Star suggest high emissivity for warm climates. Does this make
sense?<br>
<br>
>From LEED NC Reference Guide v. 2.2:<br>
<br>
"To maximize energy savings and minimize heat island effects, materials
must exhibit a high reflectivity and a high emissivity over the life of
the product."<br>
<br>
>From Energy Star website:<br>
<br>
"In warm and sunny climates highly emissive roof products can help
reduce the cooling load on the building by releasing the remaining heat
absorbed from the sun."<br>
<br>
Thanks,<br>
<br>
Brad<br>
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