F-factor discribes the thermal short-cut of heat transfer from the zone through the slab and sideways through some ground to the outdoor air environment (mainly around the perimeter). The exposed perimeter is the line traced around the slab in the horizontal plane. Bear in mind F-factor applies to "slab-on-grade" which is usually not for basement slabs, but for slabs within about a foot within ground surface level.
Mit freundlichen Grü�en- Sent from my iPhone (excuse the brevity)
i. A.
Jean Marais
b.i.g. bechtold