Good idea. Houses lose a shocking amount of heat thru floors, slabs, and foundations. We tend to focus on the attic and forget the bottom end.
Have you studied
frost-protected shallow foundations? Standard construction technique in northern Europe, and catching on in North America. The idea is to provide a foam 'tutu' around the whole house, keeping cold air and water away from the footings or basement walls. Much more effective than gluing foam flat to the wall, because it creates a conditioned space around the whole building. And you don't have to dig as deep. Helps with drainage issues at the same time.
If you can swing it, I'd advise digging about 30" deep all the way around. Glue blue foam to the vertical foundation, then a 4'x8' sheet of 2"t foam as your tutu. It should slope down an inch or so; placing perf pipe and gravel at the outer edge will help carry off any extra water, if you have someplace to carry it off to. Squirt foam the tutu-to-wall seam, and leave 1/4" between panels so you can squirt foam those seams too. If you really want waterproofing, you can apply 'Ice&Water Shield' membrane to the vertical foam and lap it over the wall/tutu joint.
One down side is the difficulty planting foundation shrubs; but if you go down 30", you should be fine. Both my house and shop are on SFPFs; the footers sit less than 2' deep. You still want a good surface grade to move water away from the house quickly. Won't kid you, tho -- it's a big job!
