Insulating a small existing slab – GreenBuildingAdvisor

I’m renovating what used to be a detached single garage (12x24ft). The car door has been removed and I’ll be sealing and insulating it so I can efficiently heat it with electric.

As part of that I’ve done some heat load calculations and the floor is the main conduit for heat loss. As it was originally an old detached garage the floor is just a 6″ monolithic slab (edges the same thickness as middle) sat on a bed of gravel, with no insulation. The heat load calcs indicate vertical insulation around the edge of the slab is far superior to insulating the top of the slab, however I’m not quite sure what those calcs use to define this (from what I gather it’s more substantial/deeper than I would be able to do, i.e vertical insulation to frost line?).

The Plan/Question

Climate zone 7A (design temp -25C/-13f)

I plan insulating the edge of the slab (exposed area above ground to ~1ft below ground) with between 1.5 and 3″ (R6-R12) of comfortboard. In addition, I’ll then cover the top of the slab with 1/2″ XPS and 5/8 Plywood (R3 ish). Unfortunately there is no space for additional insulating wings (patio, walls in the way) and I only have 1 1/2″ of space above the slab due to the door inswing.

Would it make sense to dig deeper (within reason, more than ~1ft is a pain!) and use thinner insulation on the edges, i.e. 18″ of 1.5″, or or shallower with thicker insulation – 12″ of 3″?

Is the insulation on the slab going to do much? Based on the load calc software it suggest no, but logically it should provide a thermal break that would have a significant impact on heat loss into the ground.

Thanks