[EP]How to set up humidity gain from infiltration or seepage for underground buildings

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You can define infiltration in OpenStudio in the Space Types model data tab on the left edge. You can use either a design flow rate or effective leakage area approach, and I would argue that design flow rate may be more appropriate for underground buildings.

EnergyPlus will normally only account for sensible heat transfer through underground surfaces. To account for seepage, you will need to set the Algorithm input field of the HeatBalanceAlgorithm object to "CombinedHeatAndMoistureFiniteElement" instead of the default "ConductionTransferFunction". You will also need to include special objects like MaterialProperty:HeatAndMoistureTransfer:Settings within the model. These other special objects are not accessible from the OpenStudio application, but you can use OpenStudio SDK methods to write an OpenStudio Measure to add them.