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

[Jeff Haberl]

Brad.

This would be the best for hot climates, and there are many materials that do this. In fact, most materials have a high emissivity, which is a surface property.

A surface with a low emissivity traps heat, for example for use on a flat plate collector (i.e., high absorptance, low emissivity).

Jeff
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Jeff S. Haberl, Ph.D., P.E., FASHRAE...................jhaberl at esl.tamu.edu

Professor......................................................Office Ph: 979-845-6507

Department of Architecture.......................Lab Ph: 979-845-6065

Energy Systems Laboratory.......................FAX: 979-862-2457

Texas A&M University..............................77843-3581

College Station, Texas, USA.......................URL: www-esl.tamu.edu

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From: bldg-sim-bounces at lists.onebuilding.org <bldg-sim-bounces at lists.onebuilding.org>
To: bldg-sim at lists.onebuilding.org <bldg-sim at lists.onebuilding.org>
Sent: Sun Nov 29 14:57:22 2009
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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