I'm with Jay J on the NE and other places for underfloor insulation although I see the crawlspaces without vents cause the cold air can freeze plumbing very easy. Anymore around here, it is rare to build without a basement.
Without any floor insulation, would it be recommended to put some sheets of foam insulation under the plastic on the floor like what is done under basement floors and basement walls in some colder climates?
Going further would it make much of a difference to lay the foam on top of the plastic to keep it out of the mud?
Going further yet, would it hurt to have underfloor insulation with insulation on the ground (above or below the plastic) with further foam and plastic on the crawlspace walls?