Partially underground floor with Open studio + Sketchup plugin.

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It sounds like you are referring to a walkout basement. The best model in EnergyPlus for this foundation type is Kiva. In fact, if you look at its inputs, it specifically describes how to model a walkout basement using multiple Kiva foundation objects with stepped wall surfaces.

Unfortunately, the OpenStudio Application does not support Kiva right now, so you would need to:

1. Add the Kiva inputs directly to the EnergyPlus IDF input file, or
2. Use the OpenStudio SDK to programmatically add the Kiva inputs