[Bldg-sim] Why should roofs have high emissivity?
David Bryan
dbryan at amerindian.com
Mon Nov 30 08:19:43 PST 2009
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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