Hi Andy, Floating, slab insulated well under and all around, and don't forget pex tubing tied to rebar grid.

Or rebel trench, drained well, with concrete beam on grade, Full timber foundation. I applied 3 inches of foam to mine, strapped with 1-1/2"x2" straps, installed pex and placed concrete into the bays. I will apply a narrow spruce floor over the top.