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WindowEquivalentLayer - SHGC vs Solar Transmittance at Normal Incidence

asked 2018-11-27 05:49:04 -0500

updated 2020-01-20 10:16:38 -0500

Hi all,

I'm working with the window equivalent layer model and there is something that I don't really understand regarding the reporting of SHGC and Solar Transmittance at Normal Incidence of the windows. From the output details document, I can see that the Solar Transmittance takes into account the layers of shading, while the SHGC doesn't.

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I understand that my construction with curtains (Drape material) - WINDOWEQLAYERCURT - has a different SHGC than Solar Trans. However, I don´t get what is going on with the construction without them - WINDOWEQLAYER - which is only composed of two layers of glazing with Argon Gas inbetween them.


Is the SHGC only related to one of the Glazing layers? Even if that is the case, my glazing specs don´t really specify anything like a SHGC of 0.556. These are the detailed constructions and materials:

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Thanks! Rafael

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answered 2018-11-27 14:01:38 -0500

"Center-of-glass" means that it accounts for only the heat flow from outside to inside through each layer of the window assembly, and does not account for any heat flow through the frame and other edge effects. You can read more background on the concept here.

The SHGC values you see reported are for the combination of all fenestration layers (glazing, gas, shades, etc.). The SHGC value of your window assembly without curtains (0.556) is the ratio of how much heat enters the building through a window divided by the amount of incident solar radiation that hits that same window. This falls in line with typical values for double-pane assemblies.

For future reference, if you can't determine from the Input-Output Reference documentation how EnergyPlus uses input parameters for specific calculations, you want to review the Engineering Reference documentation. Here is the section for the equivalent layer fenestration model that you are using.

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Many thanks Aaron from the detailed answer. That makes sense but if I have no frame nor divider asigned to the window, why do the SHGC and Solar Transmittance differ?

Moreover, when I report the hourly Surface Window System Solar Transmittance, values are somewhere around 0.37, far form the SHGC of 0.556. I understand that there are quite a few calculations happening under-the-hood, bur then, which value should I compare to the SHGC given by the glazing manufacturer, the reported SHGC (0.556) or the lower Solar Transmittance (0.374)?

rafael.alonso's avatar rafael.alonso  ( 2018-11-27 17:02:08 -0500 )edit

I found all the answers to my questions below. Thanks!

rafael.alonso's avatar rafael.alonso  ( 2018-11-27 17:36:34 -0500 )edit

answered 2018-11-27 17:33:22 -0500

updated 2018-11-27 17:35:17 -0500

Actually, after digging a bit more into it and in case someone is interested, all the answers and differences between the Solar Transmittance of the glazing and its SHGC are in this report of the SimpleGlazingSystem model used inside E+.

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Asked: 2018-11-27 05:49:04 -0500

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Last updated: Nov 27 '18