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

[Abaza Hussein]

A key issue is the difference in values between the reflectance and the emissivity of a surface. For example, some white paint has higher emissivity than reflectance which makes it better than aluminum foil.
Hussein Abaza
----- Original Message -----
From: "Larry Degelman" <ldegelman at suddenlink.net>
To: "Brad Painting" <bradpainting at gmail.com>, bldg-sim at lists.onebuilding.org
Sent: Sunday, November 29, 2009 8:43:05 PM GMT -05:00 US/Canada Eastern
Subject: Re: [Bldg-sim] Why should roofs have high emissivity?

Yes, it makes sense.  High emissivity allows a roof to radiate its stored heat to the sky, thus aiding the cooling effect, so, yes, good for hot climates!

Note also that ASHRAE 90.1 gives credit for (i.e., relaxes) the U-Factor requirement for such roofs in climate zones 1, 2 and 3 (the warmest zones).  The ASHRAE Standard uses the term "High Albedo" for roofs with high reflectance AND high emissivity -- this is addressed in Section 5.5.3.1.1 and Table 5.5.3.1 in the 2007 version.

Regards,

===========================
Larry O. Degelman, P.E.
Professor Emeritus of Architecture
Texas A&M University
College Station, TX 77845
ldegelman at suddenlink.net
===========================


From: Brad Painting 
Sent: Sunday, November 29, 2009 2:57 PM
To: bldg-sim at lists.onebuilding.org 
Subject: [Bldg-sim] Why should roofs have high emissivity?


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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