deerefarm - 2/6/2011 12:33 Insulation around the outside is much more important than underneath, but both is probably best, heat lost out the side is lost but heat that goes down is not totally lost (heat rises) some of the heat that goes down acts as reserve in the soil below the slab. We used a tiling spade to dig in 2' down around our shop for insulation. 8' down around the perimeter would be best. Good luck on your new (big) shop.