Window SHGC is different between Windows 6 and Energyplus
Posted by Richard Raustad on 31 August 2011 08:29 PM
When I export from Window, I get different answers between Window, EnergyPlus with Full Spectrum, EnergyPlus with Average Spectrum. All three should be the same performance values.

Window v6.3.9.0
Conductance {W/m2-K} 1.093
SHGC 0.203
Visible Transmittance at Normal Incidence 0.389

EnergyPlus V6 with Full Spectrum
Conductance {W/m2-K} 1.103
SHGC 0.22
Visible Transmittance at Normal Incidence 0.388

EnergyPlus V6 with Average Spectrum
Conductance {W/m2-K} 1.103
SHGC 0.183
Visible Transmittance at Normal Incidence 0.39

The SHGC calculations in EnergyPlus for window layers input using full
spectral data use a spectral weighting data set (derived from Optics5 data
file ISO-9845GlobalNorm.std) that is different from the WINDOW 5 default
spectral weighting data set (W5_NFRC_2003.std). This difference accounts for
most of the variation in SHGC values reported by EnergyPlus and WINDOW 5/6 for
full spectral data window layer input. This variation is more pronounced for
window constructions of three glass layers or more.

By defaults, EnergyPlus and Window 6.3 are using different standards files
to calculate the optical properties. EnergyPlus IORef describes which
file/data was used. In Window 6.3, you can change the standards file at
File|Preferences|Optical Data|Standards File. If same standards file were
used, the calculated SHGC should match (not exactly but very close).
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