Oldguy is right. With weather coming from down below, you can insulate til the cows come home and cold will radiate upward and you will be able to freeze butter by leaving it on the floor. I would guess it would help to have the insulation down there some, but no where near as much as if you build up a parameter, even if temporary thru the winter and take it down each spring, to STOP cold and WIND from entering under the cabin. Not only does this leave the sub floor accessible to you for wiring and plumbing, but it means you will prevent the cold from coming under and rising up.
It is hard to heat a box on the bottom. But remarkably easy to heat once the surround is blocked off and preventing the cold from accessing the underside of the cabin. And what about the water line and sewer lines exposed to the cold? Just hard to do this effectively and have it work the way you want it to.
Each year I use a log cabin all winter on odd weekends. Until there is enough snow to "bank" the walls, the underside is open to the cold and I can tell you walking in your bear feet (in the cabin) is an eye opener until you bank it, and then the whole cabin warms faster and will stay warm with minimal wood burning. I can only imagine a good tight sub wall going around would be even more effective in keep the wood use down and the floor warm.
Consider a plywood surround that you can put down in the late fall, and take off in the spring before snowmelt....no mold issues, no animals taking up residence, and blocks the cold from getting down there....the earth does a remarkable job of staying warm if you prevent the air from getting in from the outside.
Ground doesn't freeze much if snow gets down and blankets the ground before really hard cold sets in. Leave it exposed and it will freeze up and heave up an amazing amount.
