<br><font size=2 face="sans-serif">Good Job, Charlie,</font>
<br>
<br><font size=2 face="sans-serif">The distinction between Solar and Thermal
Bands and the variation of the properties of the material along the wavelengths
are the key to settle this aperient contradiction. </font>
<br>
<br><font size=2 face="sans-serif">Thank you, Charlie.</font>
<br>
<br><font size=2 face="sans-serif">Shawn Lee</font>
<br><font size=2 face="sans-serif">PhD, PE</font>
<br><font size=2 face="sans-serif"> </font>
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<td><font size=1 face="sans-serif"><b>"D. Charlie Curcija" <curcija@designbuildersoftware.com></b></font>
<br><font size=1 face="sans-serif">Sent by: bldg-sim-bounces@lists.onebuilding.org</font>
<br><font size=1 face="sans-serif">11/30/2009 07:35 AM</font>
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<div align=right><font size=1 face="sans-serif">To</font></div>
<td><font size=1 face="sans-serif"><muehleisen@iit.edu>, "'Brad
Painting'" <bradpainting@gmail.com></font>
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<div align=right><font size=1 face="sans-serif">cc</font></div>
<td><font size=1 face="sans-serif">bldg-sim@lists.onebuilding.org</font>
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<div align=right><font size=1 face="sans-serif">Subject</font></div>
<td><font size=1 face="sans-serif">Re: [Bldg-sim] Why should roofs have
high emissivity?</font>
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<br><font size=2><tt>Whoa, just a moment! This discussion is starting
to deteriorate into half<br>
truths and inaccuracies. First of all, most common (and inexpensive)
paints<br>
satisfy criteria of having high solar reflectance and high thermal<br>
emissivity. Ordinary white paint meets this criteria and for that
reason is<br>
very applicable for cool roofs. The key here is that there are two
major<br>
bands of radiation, Solar (encompassing roughly UV, visible and near<br>
infra-red wavelengths - from around 0.38 microns to 2.5 microns) and far<br>
infra-red (also known as thermal) band (from about 2.5 microns to 50<br>
microns). Sun, with "surface" temperature of around 9300
F emits radiation<br>
in, well Solar band, hence the name of the band. Absorbed radiation
in a<br>
roof surface emits radiation energy in thermal band, because its temperature<br>
is 100-200 F, which is much less than 9300 F. Thus, surface properties
need<br>
to be evaluated at these two major bands.<br>
<br>
Kirchoff's Law states that emissivity and absortivity of a surface is same<br>
at the same wavelength, so if we slightly extrapolate this principle to
the<br>
entire band (not quite true but good enough for understanding the problem),<br>
than one can say emissivity and absorptivity at solar band are equal and<br>
emissivity and absorptivity at thermal band are equal. While color
of<br>
paints affects solar absorptivity (thus white paint is reflective while<br>
black paint is absorptive), all of these colors have same or nearly the
same<br>
emissivity at the thermal band (0.9). This gives simple and inexpensive<br>
solution to cool roof problem. Paint the roof with white paint or
other<br>
light color, keep it relatively clean (because dirt will increase solar<br>
absorptivity of the surface) and you got the cool roof.<br>
<br>
Observation from several posters that high emissivity of the surface helps<br>
in cooling climates is correct, meaning that high thermal emissivity of
the<br>
roof surface will help dissipate absorbed solar energy, since the<br>
temperature to which roof is radiating in thermal band (sky, surrounding<br>
objects) is quite a bit lower than the roof temperature.<br>
<br>
Charlie Curcija<br>
DesignBuilder Software<br>
<br>
-----Original Message-----<br>
From: bldg-sim-bounces@lists.onebuilding.org<br>
[mailto:bldg-sim-bounces@lists.onebuilding.org] On Behalf Of Ralph<br>
Muehleisen<br>
Sent: Sunday, November 29, 2009 10:34 PM<br>
To: Brad Painting<br>
Cc: bldg-sim@lists.onebuilding.org<br>
Subject: Re: [Bldg-sim] Why should roofs have high emissivity?<br>
<br>
Brad,<br>
<br>
The emissivity and absorptivity of a passive material at any given<br>
wavelength are equal, as stipulated by Kirchhoff over 100 years ago.<br>
The only materials that violate this law are phase change materials<br>
(which only violate it in a time average sense) or materials that have<br>
some funky quantum mechanical mechanisms for heat removal.<br>
<br>
Most common building materials have a similar absorpitivity (and<br>
emissivity) for both short wavelengths (visible light i.e. solar<br>
energy) and long wavelengths (infrared). It is quite possible however<br>
to design materials with a different absorptivity=emissivity for short<br>
wavelengths and long wavelengths. This is what makes a so-called<br>
"cool" roof.<br>
<br>
If you write a simple spreadsheet to compute the temperature on a roof<br>
using an energy balance you can show that for a standard roof (i.e.<br>
absorptivity=emissivity about the same at all wavelengths) the lowest<br>
roof temperature and therefore lowest heat gain to the building will<br>
be at a roof for a low absorptivity=low emissivity.<br>
<br>
As important (or perhaps more important) in the overall roof design is<br>
controlling the roof surface temperatures. High temperatures mean<br>
that the rooftop will undergo large temperature swings which means<br>
large expansion/contraction and more thermal stress on the roof. High<br>
roof temperatures also mean locally high air temperatures which means<br>
any rooftop condensers will have decreased efficiency.<br>
<br>
A high absorptivity black roof can truly get hot enough to boil water<br>
or fry eggs (210+ degrees F) on a hot summer day without wind<br>
throughout much of the US. An aluminum roof on the other hand might<br>
get up to 160 F.<br>
<br>
I've got an excel spreadsheet that I use in my building enclosure<br>
design class that does the heat balance calculation and will show you<br>
the heat transfer from solar insolation, long wavelength radiation<br>
exchange to the sky, convection to the sky and conduction to the<br>
interior.<br>
Anyone who wants a copy can email me and I'll email it back to them.<br>
<br>
Ralph<br>
<br>
Ralph Muehleisen, Ph.D., P.E., LEED AP, FASA<br>
Assistant Professor and Director of the Miller Acoustics Lab<br>
Civil, Architectural and Environmental Engineering<br>
Illinois Institute of Technology<br>
Chicago, IL 60616<br>
muehleisen@iit.edu<br>
tel: 312-567-3545 fax:312-567-3519<br>
<br>
<br>
<br>
On Sun, Nov 29, 2009 at 2:57 PM, Brad Painting <bradpainting@gmail.com><br>
wrote:<br>
> It seems to me that a roof that emits more radiation will have a greater<br>
> warming effect on the building. Some houses in warm climates have
radiant<br>
> barriers because the aluminum has a low emissivity, thus blocking
the<br>
> infrared radiation. But both LEED and Energy Star suggest high emissivity<br>
> for warm climates. Does this make sense?<br>
><br>
> From LEED NC Reference Guide v. 2.2:<br>
><br>
> "To maximize energy savings and minimize heat island effects,
materials<br>
must<br>
> exhibit a high reflectivity and a high emissivity over the life of
the<br>
> product."<br>
><br>
> >From Energy Star website:<br>
><br>
> "In warm and sunny climates highly emissive roof products can
help reduce<br>
> the cooling load on the building by releasing the remaining heat absorbed<br>
> from the sun."<br>
><br>
> Thanks,<br>
><br>
> Brad<br>
><br>
> _______________________________________________<br>
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