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

David Bryan dbryan at amerindian.com
Sun Nov 29 18:34:25 PST 2009


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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