[Equest-users] Cool Roofs

[Nick Caton]

Hi Ed,

 

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.  

 

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...

 

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.

 

-----

 

*   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...  

 

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.

 

Where emissivity is a fraction of radiation emitted from a surface,
absorptance is a measure of the fraction of radiation absorbed.  One is
energy out, the other is energy in.

 

To ground my explanation a bit, I googled up something from my
thermodynamics courses that had gone a bit fuzzy:  

Kirchoff's law
<http://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation> :
"At thermal equilibrium, the emissivity
<http://en.wikipedia.org/wiki/Emissivity>  of a body (or surface) equals
its absorptivity <http://en.wikipedia.org/wiki/Absorptivity> ."

 

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.  

 

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.  

 

That said, I am semi-confident 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!  

 

In other words, I'd love to be corrected if it's in order =).
Otherwise, hope this helps!

 

~Nick

 

 

 

 

 

NICK CATON, E.I.T.

PROJECT ENGINEER

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

Check out our new web-site @ www.smithboucher.com 

 

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Ed
Garcia
Sent: Monday, September 21, 2009 9:44 AM
To: Subject: [Equest-users] Cool Roofs

 

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.

 

 

Regards,

 

Ed

 

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