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Correction -<br>
<br>
There a quite a few materials in that report that reflect both solar
visible and infrared but there's none that have reflectance in the near
infrared and low in the visible. I think that means they didn't find
dark colors that could be very effective cool roofs.<br>
<a class="moz-txt-link-freetext"
href="http://www.azcoolroof.com/downloads/Resources/Article%20Laboratory%20Testing%20of%20the%20Reflectance%20Roofing%20Materi.pdf"></a><br>
Dave Bryan<br>
Third Level Design<br>
<br>
<br>
Brad Painting wrote:
<blockquote
cite="mid:34791680911291921j2184a7e1h509a4085ac8a57a@mail.gmail.com"
type="cite">Thanks for the replies,<br>
<br>
I get it now. The "emittance" would take
the form of infrared radiation, which cannot pass through opaque
objects. So like Alex said, the the roof material would reject energy
to the air but not "downwards" through the roof lining.<br>
<br>
What was really throwing me was reading that the most emissive
material is purely black, while the least is purely reflective (<a
moz-do-not-send="true" href="http://en.wikipedia.org/wiki/Emissivity">http://en.wikipedia.org/wiki/Emissivity</a>).
So it seemed that if you went with a high solar reflectance it would
have to have low emissivity. But Dave, are you saying that these
properties can be split somewhere along the electromagnetic spectrum?
If a material reflects a certain wavelength, can it not emit that
wavelength?<br>
<br>
We skipped over the section on radiation in Heat Transfer :(<br>
<br>
Brad<br>
<br>
<br>
<div class="gmail_quote">On Sun, Nov 29, 2009 at 9:34 PM, David Bryan
<span dir="ltr"><<a moz-do-not-send="true"
href="mailto:dbryan@amerindian.com">dbryan@amerindian.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div bgcolor="#ffffff" text="#000000">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 type="cite">
<div>
<div class="h5">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>
</div>
</div>
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