[EnergyPlus_Support] U-factor external floor on ground

["FrancescoP" <direzionecontraria@xxxxxxxx>]

In the report EnvelopeSummary I can read also the U-factor for the floor which divides the internal environment from the ground (EXTFLOOR). 
I'm wondering how EnergyPlus calculate it. 
I think it is 
U-factor = 1/Reff
where Reff is defined in EnergyPlus Engineering Reference as the effective thermal resistance in m^2*K/W
If I know the dimensions of the building, the thermal conductivity of the soil, thickness and thermal conductivities  of the layers composing the slab-on-grade floor, how can I calculate the U-factor (or Reff)?
Best Regards
Francesco Passerini




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