Emissivity is not the same as absorptance/absorptivity. Not
even close. Emissivity is measured in Far IR range (thermal radiation),
while asbortptivity is measured in Solar (UV, VIS, Near IR) range. Big
difference, since solar radiation is at approximately 5600K, while thermal
radiation is at approximately room temperature (plus/minus 40K).
BTW, this is a common misconception and very often these
quantities are confused, leading some to believe that white painted surface has
low emissivity, which is not true. Unless the paint is specifically designed
to be low-emissivity (not easy), all paints have emissivity of 0.9 regardless
of color or appearance. However, shiny white paint will have very low
absorptivity, while dull black paint will have very high absorptivity. This
behavior is utilized in the design of cool roofs, where low solar absorptivity is
sought.
Since all surfaces in buildings are painted, it is safe to
assume that emissivity is always 0.9, which is probably the reason that there
is no field to enter it, however this should change as we see more and more
specially designed low-e paints, even in window frames.
D. Charlie Curcija
DesignBuilder Software
16 Bridge St.
Millers Falls, MA 01349
Tel: (413) 256-4647
Fax: (413) 256-4823
cell: (413) 575-3487
email: curcija@xxxxxxxxxxxxxxxxxxxxxxxxx
web: http://www.designbuildersoftware.com
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From:
EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On
Behalf Of Linda Lawrie
Sent: Wednesday, June 02, 2010 10:22 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] calculating emissivity of materials
At 03:39 AM 6/2/2010, ifigeneiat wrote:
I would like to know how E+ is calculating the emissivity of materials (not the glazing ones). I am guessing it is an indirect proceedure, as there is no specific input field for this subject.
I believe the input absorptance/absorptivity for materials is the same as
emissivity.
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