[Bldg-sim] Why should roofs have high emissivity?

[David Bryan]

Correction -

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.

Dave Bryan
Third Level Design


Brad Painting wrote:
> Thanks for the replies,
>
> 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.
>
> What was really throwing me was reading that the most emissive 
> material is purely black, while the least is purely reflective 
> (http://en.wikipedia.org/wiki/Emissivity). 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?
>
> We skipped over the section on radiation in Heat Transfer :(
>
> Brad
>
>
> On Sun, Nov 29, 2009 at 9:34 PM, David Bryan <dbryan at amerindian.com 
> <mailto:dbryan at amerindian.com>> wrote:
>
>     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.
>
>     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.
>
>     This roofing material probably exists.
>
>     Dave Bryan
>     Third Level Design
>
>
>
>     Brad Painting wrote:
>>     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 /low/
>>     emissivity, thus blocking the infrared radiation. But both LEED
>>     and Energy Star suggest high emissivity for warm climates. Does
>>     this make sense?
>>
>>     >From LEED NC Reference Guide v. 2.2:
>>
>>     "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."
>>
>>     >From Energy Star website:
>>
>>     "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."
>>
>>     Thanks,
>>
>>     Brad
>>     ------------------------------------------------------------------------
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>

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