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<p class=MsoNormal><span style='color:#1F497D'>Hi Ed,<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>I would account for emissivity
by specifying the absorptance property of the surface in the detailed edit mode
(spreadsheet view is fastest way to edit multiple roof constructions). For
the purposes of defining roof constructions to figure out their thermal
behavior*, Emissivity = Absorptance. <o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>The solar reflective index (SRI)
is another way of defining the same thing, based on a different scale between some
grade of white and black paint rather than absolutes (zero-to-1.0). There
are probably charts out there that could translate SRI into absorptance/emissivity
if you have only the SRI to go off of…<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>Right-click in eQuest one of the
cells for entering your construction absorptance, and you’ll be led to a
DOE help file that has a handy reference chart of absorptances of various
exterior finishes.<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>-----<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>* Since I know we’re
a group of learners, I figured you and others might be interested to hear (correct?)
my logic behind “emissivity = absorptance” for modeling roof
constructions in eQuest and other energy modeling ventures... <o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>First, let me relate my
understanding of emissivity through glass: Light energy hitting glass
does one of three things – (A) reflect off the surface, (B) transmit
through the glass, (C) absorb into the glass as heat energy. Absorbed
light energy (option C) is simultaneously radiated (aka “emitted”
ala “emissivity) from both sides of the glass (into and outside of your
building). “Low-e” glass is glass that has a low emissivity,
and thus doesn’t get really hot and heat up your building via heat
radiation as much in the summer when hit by daylight.<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>Where emissivity is a <u>fraction
of radiation emitted</u> from a surface, absorptance is a measure of the </span><u>fraction
of radiation absorbed</u>. One is energy out, the other is energy in.<span
style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>To ground my explanation a bit, I
googled up something from my thermodynamics courses that had gone a bit fuzzy:
<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><a
href="http://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation">Kirchoff’s
law</a>: “</span>At thermal equilibrium, the <a
href="http://en.wikipedia.org/wiki/Emissivity" title=Emissivity>emissivity</a>
of a body (or surface) equals its <a
href="http://en.wikipedia.org/wiki/Absorptivity" title=Absorptivity>absorptivity</a>.”<span
style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>If we assume, for the purposes of
thermal energy modeling, that within each modeled hour the roof construction is
in thermal equilibrium (not in the process of heating up or cooling down), we
can say “radiation in = radiation out,” and apply Mr. K’s law
above, concluding Emissivity = Absorptance. <o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>The potential inaccuracy of this
assumption lies in the reality of how a “cold” roof, for example, in
the morning hours will absorb more heat into its construction than it emits,
until it reaches a thermal equilibrium point later in the day. In the
evening hours, the reverse will be true and the roof will radiate heat at a higher
fraction than its absorptivity. <o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>That said, I am <b><i>semi-confident</i></b>
that eQuest and other programs using a “construction by layers”
methodology will compensate for such an inaccuracy by approximating the actual absorption/radiation
effects of thermal massing over time. If I’m wrong on that point, I
will have to lick my wounds and hope someone on this list can set me straight
before I drive this eQuest train into a wall! <o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>In other words, I’d love
to be corrected if it’s in order =). Otherwise, hope this helps!<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>~Nick<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<div>
<p class=MsoNormal><span style='color:#1F497D'><img border=0 width=119
height=37 id="Picture_x0020_1" src="cid:image001.jpg@01CA4058.DEB3B760"
alt="cid:489575314@22072009-0ABB"></span><b><span style='font-family:"Stylus BT","sans-serif";
color:#2D4D5E'><o:p></o:p></span></b></p>
<p class=MsoNormal><b><span style='font-family:"Stylus BT","sans-serif";
color:#2D4D5E'><o:p> </o:p></span></b></p>
<p class=MsoNormal><b><span style='font-family:"Stylus BT","sans-serif";
color:#2D4D5E'>NICK CATON, E.I.T.</span></b><b><span style='font-size:12.0pt;
font-family:"Stylus BT","sans-serif";color:#2D4D5E'><o:p></o:p></span></b></p>
<p class=MsoNormal><span style='font-size:7.5pt;color:#CC9900'>PROJECT ENGINEER</span><span
style='font-size:7.5pt;font-family:"Times New Roman","serif";color:#CC9900'><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;color:#2D4D5E'>25501 west
valley parkway<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;color:#2D4D5E'>olathe ks 66061<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;color:#2D4D5E'>direct 913
344.0036<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;color:#2D4D5E'>fax 913
345.0617<o:p></o:p></span></p>
<p class=MsoNormal><i><span style='font-size:10.0pt;color:#2D4D5E'>Check out
our new web-site @ </span></i><span style='color:#1F497D'><a
href="www.smithboucher.com" title="blocked::www.smithboucher.com"><span
style='font-size:10.0pt'>www.smithboucher.com</span></a></span><u><span
style='font-size:10.0pt;color:blue'> </span></u><span style='color:#1F497D'><o:p></o:p></span></p>
</div>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
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<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'>
<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
equest-users-bounces@lists.onebuilding.org
[mailto:equest-users-bounces@lists.onebuilding.org] <b>On Behalf Of </b>Ed
Garcia<br>
<b>Sent:</b> Monday, September 21, 2009 9:44 AM<br>
<b>To:</b> <b>Subject:</b> [Equest-users] Cool Roofs<o:p></o:p></span></p>
</div>
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<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I a new user to eQuest and need to model cool roofs. I
see the user specified roof properties, but I do not see how to add the SRI or
emissivity of the roof. Would appreciate some guidance.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><span style='font-family:"Arial","sans-serif"'>Regards,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial","sans-serif"'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial","sans-serif"'>Ed<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial","sans-serif"'><o:p> </o:p></span></p>
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