Well building directly on the ground without a major foundation can be a big heat sink.

I lived in a cabin similar to this for a few years up in the NW. It was also raised off the ground. During the time I lived there I put some lattice skirting around it to keep out the dogs and fiber glass insulation between the floor joists with a barrier.

It was very cozy - when it snowed the snow would pile up to my porch rails - no cold air infiltration problems there! ;)