[EnergyPlus_Support] Re: calculating emissivity of materials

["ifigeneiat" <ifigeneia@xxxxxxxxxxxxxxxx>]

Dear Charlie, 

Thank you very much for your detailed and welldocumented answer. I do agree with you, however I must underline the fact that many buildings are not painted and furthermore many of them could be using cool materials plastering on vertical surfaces as well as on horizontals, such as roofs. As regards to the engineering reference not all materials are gray and opaque. 

It would be really helpful if this was an input choice in E+. 

Regards

I.

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "D. Charlie Curcija" <curcija@...> wrote:
>
> 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:  <mailto:curcija@...>
> curcija@... 
> web:  <http://www.designbuildersoftware.com/>
> 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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