[Equest-users] Cool Roofs

[Chris Wark]

I'm afraid Nick's explanation assumes that a cool roof exposed to sunlight behaves as a black body with infinite insulation underneath. Instead, most roofing materials  have a thermal emissivity of something like 0.9, give-or-take 0.08, regardless of solar reflectivity which can range from 0.05 to 0.95.
In this case, the heat emitted is only long-wave infrared radiation rather than the full solar spectrum, so the emissivity value is the ratio of roof heat being emitted relative to a black body at the same temperature and is not really related to the broadband solar radiation. In fact, texture or surface roughness has a bigger effect on infrared emissivity than visible color - a polished black surface will usually have a lower thermal emissivity than a rough white surface, depending on the material. Yes, the absorbed solar radiation, regardless of wavelength, has to heat up the roof but the emissivity number describes a material characteristic that is not dependent on how the material got heated in the first place (like from the inside of the building).
The material that the SRI is based on has both solar reflectivity and thermal emissivity of around 0.9 (as I recall). Since thermal emissivity is similar for so many roofing materials, the SRI number is usually a reasonable estimate for reflectivity. When taking a ballpark guess at roof surface inputs during concept phase, I use an SRI number for reflectivity and 0.9 for emissivity.
Christopher Wark
Project Engineer
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From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
Sent: Monday, September 28, 2009 3:49 PM
To: Ed Garcia
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Cool Roofs

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



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NICK CATON, E.I.T.
PROJECT ENGINEER
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direct 913 344.0036
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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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