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

Jeff Haberl jhaberl at tamu.edu
Sun Nov 29 19:18:39 PST 2009


Probably not.

Low emissivity is only accomplished painfully.

For example, black chrome on flat plate solar collectors is a plating process...that must be kept clean, or else the surface reverts to an emissivity of 0.8 - 0.9...of common materials....however, I'm not aware of it application to metal roofs.

Another example is where one makes minute record groves in the surface, but this too must be kept super clean...no dust...which is impossible for a roof. I never saw this applied to a solar collector.

Another material is minute ceramic beads embedded in epoxy...a special material used at the IAH airport...very durable...but very expensive...and still must be kept very, very clean.

One final example is Martin Black...a patented material made by the Martin Marietta company (the forerunner to Lockheed-Martin). This material also had to be kept clean.

Jeff

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Jeff S. Haberl, Ph.D.,P.E., FASHRAE..............jhaberl at 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, 77843..................URL:www.esl.tamu.edu

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________________________________
From: bldg-sim-bounces at lists.onebuilding.org [bldg-sim-bounces at lists.onebuilding.org] On Behalf Of David Bryan [dbryan at amerindian.com]
Sent: Sunday, November 29, 2009 8:34 PM
To: Brad Painting
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Why should roofs have high emissivity?

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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