Emissivity of materials
Posted by Richard Raustad, Last modified by Michael J. Witte on 28 November 2017 07:15 PM
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
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