I attached a chart of winter so far in a few places in North America.  You will notice I added a line at which some insulation would be strongly recommended to ensure winter survival. Be it with or without a top entrance :). I would say a varying optimal R-value could be determined for hives based on this coldness factor for those below the line. I don't think any proper studies have ever been done to test different colour wrappings and measure the effect of solar radiation. 

I would say if people are losing hives with plenty of honey left more often than not they need to investigate better wintering strategies. I reviewed the Canadian overwintering survey results (http://www.capabees.com/capa-statement-on-honey-bees/) and Ontario's - http://www.omafra.gov.on.ca/english/food/inspection/bees/2018winterloss.htm.

Weather and Starvation make up a lot of perceived "reasons" for their winter losses. I would say the root causes would be the hives are not being prepared in a way to ensure they survive the winter for their climatic areas by the beekeepers (Poor Management). If you add in Varroa's impact on their fat bodies (vitellogenin) late winter broodrearing / effect on thermoregulation efficiency and possibly the beekeeper's quest to harvest a fall honey in many parts of North America (lack of winter prep time).

<The honey in the third box would then bee available for late winter usage without requiring cluster movement to one side or the other.>
The key to successful wintering is the ratio cluster size to volume of hive. My 5 frame nuc has survived 65 hours of <-40C and has been clustered since mid September. It still has a month to go to prove my concept. The reason a 5 frame nuc with ~30lbs of honey, a single with ~50lbs and double with ~120lbs can survive my winters with only consuming half to 2/3rds of their stores is due to insulation, no top vent, healthy bees (very low mites) and an August winter prep (1-2 months prior to steady <0C temperatures). My approach is to reduce the space, add one or two extra sugar syrup honey frames (I try to have a least one supers worth per 10 hives) doubles down to single, singles down to a 6 frame nuc. I usually get one warm enough day in late March/early April to attempt this. I can then be confident that it will make it to late April when I start feeding those low on stores. In warmer climates you may get away with adding a super but your spring start will be slowed. Bees still need to maintain a broodnest of 35C. In wood hives this requires a healthy cluster for larger spaces (volume).

The second image shows the degree hour difference between an insulated hive (orange line) and a non-insulated hive (blue line). The difference between the line is proportional to the energy requirements to maintain a brood nest temperature. As you can see the insulated hive does not require much "heat" to maintain nest as the loose cluster's "waste" heat is enough to keep inside temps relatively warm. I will be able to confirm this in a few weeks when I can re-install my hive scales and do my 1st inspection. My current consumption estimate for my double is ~65lbs of honey. One question I am asking myself is how much of the honey consumed is for heating  (maintain the nest at 35C) and what proportion is for brood rearing?

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