This links provides analysis of winter hive temperatures. The study was conducted in a 5 year span in Madison, Wi. The data allowed the analysts to come to a number of conclusions which are at the end of the article. The ones I thought significant were:
1) The hive makes no attempt to regulate the temperature outside of the winter cluster.
2) Insulating does permit the cluster to be somewhat looser; meaning it is spending less energy to maintain cluster temperature.
3) In the Madison area both insulated and uninsulated hives should have no survival issues under normal winter conditions.