diffuse transmittance in OS

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EnergyPlus only supports diffusing windows in a binary way, at least from the perspective of EnergyPlus' standard materials; they are either 100% specular transmitting (i.e. clear glass) or 100% diffusing. To this extent, these are supported in OpenStudio, both on the Radiance side as well as the EnergyPlus side. You need to make a material that has the "Solar Diffusing" property set to "yes". The Radiance translator will make a "trans" material for those windows, at 100% diffuse transmittance (of the transmitted component).

Unfortunately we don't directly support materials that exhibit a mix of specular and diffuse transmittance in OpenStudio at this time.

Support for diffusing materials in Radiance was added after the 1.6.0 release, in one of the "iteration releases". I recommend you grab the next major release (1.7.0) when it is released at the end of this month, as the support for diffuse materials in Radiance has been tested more thoroughly for this release. I will reiterate the caveat I mention in my previous answer, as well: the Radiance diffuse material support is all-or-nothing; if you have a material in the construction that is SolarDiffusing = YES, the construction will transmit 100% of the transmitted component of visible light in a Lambertian manner.