Modeling the flow of heat and moisture through the earth defies some of the typical simplifying assumptions made in EnergyPlus's (EP) surface heat balance. Specifically, to approximate the impact of ground coupling, you can’t just model the ground as a thermal zone, adjacent to building zones due to the following two complications:

• The thermal zone surface conduction calculations model heat transfer in 1-dimension, which thus assumes the entire surface plane is isothermal (temperature is constant). But ground heat transfer needs to be modeled in 2 or even 3 dimensions (since it’s a large, wet, thermal mass that dynamically changes along the footprint of the entire building at different depths).


• Swings in ground temperatures operate on the timescale of months, and even years, whereas EnergyPlus thermal zones (which represents the ground in that example) operates on an hourly timescale.

The most accurate way to do this in EnergyPlus is with Kiva[1], which models dynamic, multidimensional ground/foundation heat flow, and moisture transportation at different timescales. But, heuristically this is only really neccessary for single-family residential models, since the impact of the ground is fairly marginal for large, multi-storey commercial buildings. In fact, the UWG doesn’t even model ground heat transfer for its building energy models, although it does model the ground temperature to model the amount of energy needed for service hot water.