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

[David Bryan]

Brad -

Good discussion. To answer your last question:
Yes, a material that is a good reflector at a given wavelength will be a 
poor emitter at that wavelength. But the wavelength, or band of 
wavelengths, that a material emits at is inversely proportional to its 
temperature.  So if a material is a good reflector of solar radiation, 
i.e. visible light and near infrared, it will not need to emit radiation 
at these frequencies to stay cool because roofs do not get as hot as the 
sun. It will want to emit at a longer wavelengths, at which it may have 
a higher emissivity (and be a poor reflector). 

As per Charlie Curjica's comments, white paint is a good candidate 
because it reflects most of the visible radiation and has a high 
emittance at roof temperatures.  It would be a better cool roof material 
if it also reflected the solar near infrared - most of which it absorbs. 
The attached link is to an interesting study of the properties of roof 
materials. Essentially it says that for the materials tested, none was 
found that had a high reflectivity to both solar near infrared and 
visible energy, and if you have to choose, it's more important to 
reflect the visible spectrum than the near infrared.

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 
> <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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