Why should roofs have high emissivity?

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It seems to me that a roof that emits more radiation will have a greater
warming effect on the building. Some houses in warm climates have radiant
barriers because the aluminum has a *low* emissivity, thus blocking the
infrared radiation. But both LEED and Energy Star suggest high emissivity
for warm climates. Does this make sense?

"To maximize energy savings and minimize heat island effects, materials must
exhibit a high reflectivity and a high emissivity over the life of the
product."

"In warm and sunny climates highly emissive roof products can help reduce
the cooling load on the building by releasing the remaining heat absorbed
from the sun."

Thanks,

Brad

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Brad.

This would be the best for hot climates, and there are many materials that do this. In fact, most materials have a high emissivity, which is a surface property.

A surface with a low emissivity traps heat, for example for use on a flat plate collector (i.e., high absorptance, low emissivity).

Jeff S. Haberl, Ph.D., P.E., FASHRAE

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Yes, it makes sense. High emissivity allows a roof to radiate its stored heat to the sky, thus aiding the cooling effect, so, yes, good for hot climates!

Note also that ASHRAE 90.1 gives credit for (i.e., relaxes) the U-Factor requirement for such roofs in climate zones 1, 2 and 3 (the warmest zones). The ASHRAE Standard uses the term "High Albedo" for roofs with high reflectance AND high emissivity -- this is addressed in Section 5.5.3.1.1 and Table 5.5.3.1 in the 2007 version.

Regards,

Larry O. Degelman, P.E.

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Remember that emissivity and absorptivity are generally equal and often
vary with wavelength. And for opaque materials, emissivity generally
equals (1- reflectivity) at a given wavelength.

So it would be possible to have a spectrally selective roof which
reflected the sun's visible and short wave infrared energy well (high
reflectance, low emittance) but also radiated energy well at the longer
infrared wavelengths emitted at its temperature rather than the sun's
(low reflectance, high emittance). This would be the ideal cool roof
material.

This roofing material probably exists.

Dave Bryan

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Probably not.

Low emissivity is only accomplished painfully.

For example, black chrome on flat plate solar collectors is a plating process...that must be kept clean, or else the surface reverts to an emissivity of 0.8 - 0.9...of common materials....however, I'm not aware of it application to metal roofs.

Another example is where one makes minute record groves in the surface, but this too must be kept super clean...no dust...which is impossible for a roof. I never saw this applied to a solar collector.

Another material is minute ceramic beads embedded in epoxy...a special material used at the IAH airport...very durable...but very expensive...and still must be kept very, very clean.

One final example is Martin Black...a patented material made by the Martin Marietta company (the forerunner to Lockheed-Martin). This material also had to be kept clean.

Jeff S. Haberl, Ph.D.,P.E., FASHRAE

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Thanks for the replies,

I get it now. The "emittance" would take the form of infrared radiation,
which cannot pass through opaque objects. So like Alex said, the the roof
material would reject energy to the air but not "downwards" through the roof
lining.

What was really throwing me was reading that the most emissive material is
purely black, while the least is purely reflective (
http://en.wikipedia.org/wiki/Emissivity). So it seemed that if you went with
a high solar reflectance it would have to have low emissivity. But Dave, are
you saying that these properties can be split somewhere along the
electromagnetic spectrum? If a material reflects a certain wavelength, can
it not emit that wavelength?

We skipped over the section on radiation in Heat Transfer :(

Brad

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Brad,

The emissivity and absorptivity of a passive material at any given
wavelength are equal, as stipulated by Kirchhoff over 100 years ago.
The only materials that violate this law are phase change materials
(which only violate it in a time average sense) or materials that have
some funky quantum mechanical mechanisms for heat removal.

Most common building materials have a similar absorpitivity (and
emissivity) for both short wavelengths (visible light i.e. solar
energy) and long wavelengths (infrared). It is quite possible however
to design materials with a different absorptivity=emissivity for short
wavelengths and long wavelengths. This is what makes a so-called
"cool" roof.

If you write a simple spreadsheet to compute the temperature on a roof
using an energy balance you can show that for a standard roof (i.e.
absorptivity=emissivity about the same at all wavelengths) the lowest
roof temperature and therefore lowest heat gain to the building will
be at a roof for a low absorptivity=low emissivity.

As important (or perhaps more important) in the overall roof design is
controlling the roof surface temperatures. High temperatures mean
that the rooftop will undergo large temperature swings which means
large expansion/contraction and more thermal stress on the roof. High
roof temperatures also mean locally high air temperatures which means
any rooftop condensers will have decreased efficiency.

A high absorptivity black roof can truly get hot enough to boil water
or fry eggs (210+ degrees F) on a hot summer day without wind
throughout much of the US. An aluminum roof on the other hand might
get up to 160 F.

I've got an excel spreadsheet that I use in my building enclosure
design class that does the heat balance calculation and will show you
the heat transfer from solar insolation, long wavelength radiation
exchange to the sky, convection to the sky and conduction to the
interior.
Anyone who wants a copy can email me and I'll email it back to them.

Ralph Muehleisen, Ph.D., P.E., LEED AP, FASA

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Whoa, just a moment! This discussion is starting to deteriorate into half
truths and inaccuracies. First of all, most common (and inexpensive) paints
satisfy criteria of having high solar reflectance and high thermal
emissivity. Ordinary white paint meets this criteria and for that reason is
very applicable for cool roofs. The key here is that there are two major
bands of radiation, Solar (encompassing roughly UV, visible and near
infra-red wavelengths - from around 0.38 microns to 2.5 microns) and far
infra-red (also known as thermal) band (from about 2.5 microns to 50
microns). Sun, with "surface" temperature of around 9300 F emits radiation
in, well Solar band, hence the name of the band. Absorbed radiation in a
roof surface emits radiation energy in thermal band, because its temperature
is 100-200 F, which is much less than 9300 F. Thus, surface properties need
to be evaluated at these two major bands.

Kirchoff's Law states that emissivity and absortivity of a surface is same
at the same wavelength, so if we slightly extrapolate this principle to the
entire band (not quite true but good enough for understanding the problem),
than one can say emissivity and absorptivity at solar band are equal and
emissivity and absorptivity at thermal band are equal. While color of
paints affects solar absorptivity (thus white paint is reflective while
black paint is absorptive), all of these colors have same or nearly the same
emissivity at the thermal band (0.9). This gives simple and inexpensive
solution to cool roof problem. Paint the roof with white paint or other
light color, keep it relatively clean (because dirt will increase solar
absorptivity of the surface) and you got the cool roof.

Observation from several posters that high emissivity of the surface helps
in cooling climates is correct, meaning that high thermal emissivity of the
roof surface will help dissipate absorbed solar energy, since the
temperature to which roof is radiating in thermal band (sky, surrounding
objects) is quite a bit lower than the roof temperature.

Charlie Curcija

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Good Job, Charlie,

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.

Thank you, Charlie.

Shawn Lee
PhD, PE

"D. Charlie Curcija" Sent by: bldg-sim-bounces at lists.onebuilding.org
11/30/2009 07:35 AM

To
, "'Brad Painting'" cc
bldg-sim at lists.onebuilding.org
Subject
Re: [Bldg-sim] Why should roofs have high emissivity?

Whoa, just a moment! This discussion is starting to deteriorate into half
truths and inaccuracies. First of all, most common (and inexpensive)
paints
satisfy criteria of having high solar reflectance and high thermal
emissivity. Ordinary white paint meets this criteria and for that reason
is
very applicable for cool roofs. The key here is that there are two major
bands of radiation, Solar (encompassing roughly UV, visible and near
infra-red wavelengths - from around 0.38 microns to 2.5 microns) and far
infra-red (also known as thermal) band (from about 2.5 microns to 50
microns). Sun, with "surface" temperature of around 9300 F emits
radiation
in, well Solar band, hence the name of the band. Absorbed radiation in a
roof surface emits radiation energy in thermal band, because its
temperature
is 100-200 F, which is much less than 9300 F. Thus, surface properties
need
to be evaluated at these two major bands.

Kirchoff's Law states that emissivity and absortivity of a surface is same
at the same wavelength, so if we slightly extrapolate this principle to
the
entire band (not quite true but good enough for understanding the
problem),
than one can say emissivity and absorptivity at solar band are equal and
emissivity and absorptivity at thermal band are equal. While color of
paints affects solar absorptivity (thus white paint is reflective while
black paint is absorptive), all of these colors have same or nearly the
same
emissivity at the thermal band (0.9). This gives simple and inexpensive
solution to cool roof problem. Paint the roof with white paint or other
light color, keep it relatively clean (because dirt will increase solar
absorptivity of the surface) and you got the cool roof.

Observation from several posters that high emissivity of the surface helps
in cooling climates is correct, meaning that high thermal emissivity of
the
roof surface will help dissipate absorbed solar energy, since the
temperature to which roof is radiating in thermal band (sky, surrounding
objects) is quite a bit lower than the roof temperature.

Charlie Curcija

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Brad -

Good discussion. To answer your last question:
Yes, a material that is a good reflector at a given wavelength will be a
poor emitter at that wavelength. But the wavelength, or band of
wavelengths, that a material emits at is inversely proportional to its
temperature. So if a material is a good reflector of solar radiation,
i.e. visible light and near infrared, it will not need to emit radiation
at these frequencies to stay cool because roofs do not get as hot as the
sun. It will want to emit at a longer wavelengths, at which it may have
a higher emissivity (and be a poor reflector).

As per Charlie Curjica's comments, white paint is a good candidate
because it reflects most of the visible radiation and has a high
emittance at roof temperatures. It would be a better cool roof material
if it also reflected the solar near infrared - most of which it absorbs.
The attached link is to an interesting study of the properties of roof
materials. Essentially it says that for the materials tested, none was
found that had a high reflectivity to both solar near infrared and
visible energy, and if you have to choose, it's more important to
reflect the visible spectrum than the near infrared.

http://www.azcoolroof.com/downloads/Resources/Article%20Laboratory%20Testing%20of%20the%20Reflectance%20Roofing%20Materi.pdf

Dave Bryan

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Correction -

There a quite a few materials in that report that reflect both solar
visible and infrared but there's none that have reflectance in the near
infrared and low in the visible. I think that means they didn't find
dark colors that could be very effective cool roofs.

Dave Bryan

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Thanks for all of your thorough explanations. The issue is much clearer now.
I'll work my way through the resources and see if any other questions pop
up. Thanks again.

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Please see this site for cool colors: http://coolcolors.lbl.gov/

Vishal

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Here is the latest article:

http://www.lbl.gov/Tech-Transfer/success_stories/articles/coolroof.html

Q&A on cool roofs:

http://coolcolors.lbl.gov/assets/docs/fact-sheets/Cool-roof-Q%2BA.pdf

Vishal

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A key issue is the difference in values between the reflectance and the emissivity of a surface. For example, some white paint has higher emissivity than reflectance which makes it better than aluminum foil.
Hussein Abaza

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This has been an interesting thread. What I am wondering is do you plan on getting a LEED point for a cool roof? If so and you are not going to use a "pre-approved" product how do you plan to show compliance with whatever emissivity or reflectance values are required? If pretty sure if you just painted your roof white you would need to show some test data per the correct procedures that your roof has the correct values. Anyone have experience with this?

Brad in Boise

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The point related to decrease in reflectivity due to dust deposition (as
indicated by Charlie) is quite significant (especially in dusty areas,
places with high SPM in air, and having less rain) which may become one of
the major causes for deviation of the actual energy saving from simulation
results. One may have to exert little more while attempting to calibrate the
model with actual results if the dust is not cleaned periodically.

Dr.-Ing. Jyotirmay Mathur

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I was actually just writing an article but I wanted to know for any future
designs. It's my understanding that if the coating is opaque, the solar
reflectance and emittance only depend on the coating itself. The LEED
Reference Guide v. 2.2 lists coolroofs.org as a resource, which seems to
have a pretty comprehensive database of ratings (SR and emittance) for
field-applied paints/coatings. I'm not sure what to do if you can't find
data on your specific coating.

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