Wow! That's cold. We don't get it that low here, probably only down to +10F at worst. Had 2 TBH's in the back yard for the past 5 years and not lost a colony to winter yet. As Michael suggests, insulation on top of the roof is I think, the best place to put it. That's all I do, nothing else inside the hives. As is often quoted, its not the cold that kills bee, its the damp and the mould.