F-Factor for slab with insulation on top? – GreenBuildingAdvisor

Does anybody know how to determine the F-factor of a slab-on-grade with insulation on TOP?

I’m trying to find/calculate the F-factor of the assembly shown in the attached image (with and without perimeter insulation). The sources for F-factors that I’m aware of don’t include values for this condition — they all assume that the insulation is below the slab, which it typically is. Perhaps there’s an equation that can calculate this? Similar to the ASHRAE formulas used to calculate the U-factors for walls and ceilings.

This is for a new construction project (not a retrofit) in Climate Zone 4C, zip code 98370 (near Seattle).

Thanks! -Don