Why should roofs have high emissivity?

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In some cases it may be counter productive to use a high emissivity
roof. I have worked on uncooled warehouses where the team used an
approved roofing product to get that point but the heating energy
increased enough to lower the savings enough to lose an EAc1 point.

Chris Jones

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Chris makes a good point to consider with cool roofs.

A cool roof will indeed reduce solar heat gain to the roof which reduces its
temp (good in summer) and reduces cooling requirements in summer but will
increase heating requirements in the winter.

Until someone develops a material where the emissivity changes with
temperature (and not just wavelength) a cool roof that is good in summer
will be bad in winter.

So, in colder climates, a cool roof can indeed increase the overall energy
use of a building.

Even so, some northern cities like Chicago, mandate cool roofs in building
code. Why? Because the cool roof will reduce the urban heat island effect
where the city has increased temperatures compared to the surrounding
areas.

The thought is that the overall benefits of the reduction in urban heat
island effect in summer is more important than the increased energy use that
comes from increased winter cooling load.

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

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Of course, if your roof is well insulated, it will be covered with snow. I
doubt very much that your ?warm? roof will really help much. Go with the
cool roof.

Chris Schaffner, PE, LEED AP

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I'll add some general info to the discussion and do a little summing up.

LBL has studied emissivity and reflectance and has annual energy impact
results for hot and cold climates:
http://eetd.lbl.gov/HeatIsland/CoolRoofs/HeatTransfer/Heating.html

The Heat Island Group has a lot of information available pertinent to
the emissivity discussion:
http://eetd.lbl.gov/HeatIsland/

We've been focusing on roofs, but window design also has to consider the
same effects. For glazing, we add the property of transmissivity
(radiation passing through the surface) and we have to get more specific
about the types of radiation (solar, visible, UV, infrared) and the
direction (outside vs. inside, as well as angle of incidence).
http://www.commercialwindows.umn.edu/issues_energy1.php

For opaque building surfaces, such as roofs, walls and window frames, it
is understood that unless stated otherwise:
Reflectivity and absorptivity values are for solar radiation.
Emissivity values are for thermal/far-infrared radiation.

1 - reflectivity = absorptivity

So when ASHRAE 90.1 Table G3.1 requires you to model a roof reflectivity
of 0.30, you can use an absorptance of 0.70 (such as when modeling in
eQUEST).

While there are technical distinctions, the following terms are
generally used interchangeably:
Reflectance/reflectivity
Absorptance/absorptivity
Emittance/emissivity

William Bishop, EIT, LEED(r) AP

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Another point to consider is that ambient temperature affects HVAC system
performance. If you?ve got rooftop mounted condensers, they?ll operate much
better on a ?cool? roof.

Not sure how you?d model that effect.

Chris Schaffner, PE, LEED AP

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True indeed.

Except when your roofs are not covered with snow.

Here in Chicago, at least the past few years, snow has come in bigger
chunks with more warmi and cold periods in between. And rooftop snow has
been melting during the warm periods so rooftops have been bare for more of
the winter.

It would be an interesting study (maybe I can find an undergrad or
interested masters student) to actually look at a typical flat roof small
commercial building and see if the increased summer efficiency of cooling
equipment from a cool roof offsets the increased winter heating load.

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

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I'm starting a new "thread" as this is a definite tangent from the
emissivity discussion of the past few days.

A question occurred to me on reading Ralph / Chris's comments below...
does ANY energy modeling program/engine today use the snow cover data
present in TMY2 files? There's obvious implications for things like
roof surface thermal behavior and hourly photovoltaic array performance
when you consider the reality of significant snow cover presence through
the day...

Of note... I'm no expert, but for others' reference I found a nice
resource this afternoon clarifying the what/why/how of TMY files here:
http://rredc.nrel.gov/solar/pubs/tmy2/

NICK CATON, E.I.T.

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Yes, EnergyPlus uses the albedo in the TMY3 or snow cover in other files
to adjust the diffuse radiation.

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Thanks for the response Drury!

So EnergyPlus will essentially adjust the reflectivity of a roof surface
hourly based on the albedo TMY data - that's promising! Can we affect
the modeled behavior to account for means of preventing accumulation,
such as snow-melt cable or roof geometries?

Also, can EnergyPlus (and/or, again, any other energy modeling
software/engine) take account for the variable insulative effects of
snow incident on the roof surface? I realize accurately/realistically
modeling this behavior could be tricky/impossible with the removal of
snow depth information in TMY3 from TMY2 (from:
http://www.doe2.com/Download/Weather/TMY3/Users_Manual_for_TMY3_Data_Set
s.pdf
).

I'm bringing up and pressing this point because I think we as community
of energy modelers might want to begin modeling the thermal effects of
hourly roof snow cover (if we aren't already), both as a means of
influencing building design for energy-conscious decision making and as
a means of more accurately modeling real-life thermal behavior. I did
some quick fact checking - snow has a fairly significant insulative
property (approximately R-1 per inch), and I am concerned that any
energy modeling performed for buildings in northerly climates might be
significantly off-base without accounting for the realities of
snow-cover on buildings.

PS: I had to read up on the term albedo myself. Recommended reading
for others at: http://en.wikipedia.org/wiki/Albedo

PPS: It appears snow cover in energy modeling is a pretty deep topic in
other circles... I found further recommended reading (powerpoint) for
those interested at:

http://www.google.com/url?sa=t&source=web&ct=res&cd=1&ved=0CAcQFjAA&url=
http%3A%2F%2Fwww.nws.noaa.gov%2Fiao%2FInternationalHydrologyCourseCD1%2F
1031%2Fhfcourse_snow.ppt&ei=SvEXS5aYLoL0ManeyN8C&usg=AFQjCNEoDwnXD8Cynrr
ptTs5TPgYJ8kbHw&sig2=yXdMijCKnJOu8J-gKEGsxg

NICK CATON, E.I.T.

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JRR wrote:

How soon will Energy Plus software support .... PURCHASED SNOW ???
It's great insulation !!! and it changes the albedo of the roof....
You could even
capture rain / snow melt for re-use.... and you could even Snow every
night and
melt it with the Sun to be caught the next day... avoiding peaking
charges.....

Enough viewing into my bag of patent tricks for this evening.........
If you are intrigued by these suggestions you have my email above.

John R Ross III PE

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