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 or the F-factor) of the floor in contact with ground?

I would like to calculate the U-factor (or Reff or the
F-factor) before I create the dynamic model in order to compare that value with
the limits of ASHRAE 90.1 Table 5.5. I know the method of the European
technical standard EN ISO 13370, but I think it is not compatible with ASHRAE
90.1, which was written in the United States.

Best Regards

Francesco Passerini