ASHRAE 90.1 Table A6.3 has values for horizontal insulation and vertical insulation, but nothing for slabs that have both vertical and horizontal insulation. What f-factor should I use? I don't consider it to be a 'fully insulated slab' since the horizontal insulation only extends 24" inwards, and A6.1.3 specifices "continuous insulation...completely covers the entire area under the slab."