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

[Vishal Garg]

Please see this site for cool colors: http://coolcolors.lbl.gov/

Vishal
Head, Center for IT in Building Science | IIIT-H | Research University |
Gachibowli | Hyderabad 500 032
Website: cbs.iiit.ac.in | Tel#+91 40 6653 1125| Fax: #+91 40 6653 1413|
Mobile # +919949990900| Email: vishal at iiit.ac.in


On Mon, Nov 30, 2009 at 9:49 PM, David Bryan <dbryan at amerindian.com> wrote:

>  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.
>
> <http://www.azcoolroof.com/downloads/Resources/Article%20Laboratory%20Testing%20of%20the%20Reflectance%20Roofing%20Materi.pdf>
> 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>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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