Slab on Grade Frost Protection
Cold storage buildings require frost protection in most areas in Canada. Slab on grade foundations require the face of the slab and an extension of 2" thick SM rigid insulation minimum 2' beyond the slab footprint to eliminate frost from penetrating the ground surrounding the building and coming up under the foundation. Heated buildings are no different.
Heated slabs require 2" minimum SM under slab and the 2" SM around the exterior.
Bottom line, the SM pushes the line of frost penetration away from the building footprint, eliminating heaves.