[Bldg-sim] "Horses mouth" question.

Robin Mitchell rdmitchell at lbl.gov
Mon Oct 11 14:10:32 PDT 2010

Hello -


I forwarded this email to colleagues working in LBNL's Cool Roof group
(Jonathan Slack (JLSlack at lbl.gov) and Haley Gilbert (hegilbert at lbl.gov )),
and got this response back from them:




"By reducing the emissivity, he's reducing the ability of the roofing
material to emit thermal IR and thereby drop it's temperature. Hence, the
roofing material is increasing in temperature, thus increasing heat flow
into the structure, thus increasing demand on the cooling system. The most
effective cool roofs are both high in reflectivity and high in thermal
emittance. Haley and I have all sort of nice graphics on this if your
collaborator would be interested in seeing them.

Bottom line for his model - increase reflectance, increase emissivity.
Luckily, all the real world cool roof materials actually do have high
emissivities. An example of a poor cool roof material would be polished
copper - even though it's reflectance is high, it's low emissivity dooms it
to poor performance in a hot climate."



Robin Mitchell

Windows & Daylighting Group

Lawrence Berkeley National Laboratory


From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of John Aulbach
Sent: Monday, October 11, 2010 12:01 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] "Horses mouth" question.



Question on Cool Roof and the way to simulate them in eQuest.


My boss came to me with a cool roof and asked me to do a quick eQuest on it.
I did a 25,000 sf single story office in Los Angeles, saw the roof outside
emissivity was 0.9.  I changed it to 0.5 and the electric usage went UP. I
change it again to 0.12 and the electric usage went up AGAIN. The HVAC is a
standard single zone pacakge unit with furnace heat.


Am I seeing reality or what am I missing?





John R. Aulbach, PE, CEM

Senior Energy Engineer


Partner Energy

1990 E. Grand Avenue, El Segundo, CA 90245
W: 888-826-1216, X254| D: 310-765-7295 | F: 310-817-2745

 <http://www.ptrenergy.com/> www.ptrenergy.com
<mailto:%7C%20jaulbach at ptrenergy.com> | jaulbach at ptrenergy.com




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