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

Vishal Garg vishal at iiit.net
Mon Nov 30 09:49:48 PST 2009


Here is the latest article:

http://www.lbl.gov/Tech-Transfer/success_stories/articles/coolroof.html


Q&A on cool roofs:

http://coolcolors.lbl.gov/assets/docs/fact-sheets/Cool-roof-Q%2BA.pdf


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 11:09 PM, Vishal Garg <vishal at iiit.net> wrote:

> 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
>>>
>>> ------------------------------
>>>
>>> _______________________________________________
>>> Bldg-sim mailing listhttp://lists.onebuilding.org/listinfo.cgi/bldg-sim-onebuilding.org
>>> To unsubscribe from this mailing list send  a blank message to BLDG-SIM-UNSUBSCRIBE at ONEBUILDING.ORG
>>>
>>>
>>>
>>
>>
>> _______________________________________________
>> Bldg-sim mailing list
>> http://lists.onebuilding.org/listinfo.cgi/bldg-sim-onebuilding.org
>> To unsubscribe from this mailing list send  a blank message to
>> BLDG-SIM-UNSUBSCRIBE at ONEBUILDING.ORG
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/bldg-sim-onebuilding.org/attachments/20091130/bd22f767/attachment-0002.htm>


More information about the Bldg-sim mailing list