Unlike most insects that "hibernate" for winter through diapause, a dormant state that allows them to withstand cold temperatures. Honey bees are fully functioning within the beehive, even into the coldest of winter nights. In the upper Midwest, we just went through 2 weeks that never reached above 5ºF and fell into the -40ºF ranges over night for several days in the winter of 2020/2021.
The Cluster
The bees use the heat of the cluster to survive the winter and harsh cold temperatures. The bees cluster around the center of the brood boxes ( the lower section of the hives) to join into a ball like state, where bees vibrate to help generate heat. This heat is dissipated through the walls and ventilation of the hive. Not enough insulation or too much ventilation will result in more BTU's needed to survive, and therefore more food reserves are needed to produce the calories needed to make BTU's.
What's a BTU?
BTU is an acronym. It stands for British Thermal Unit. It’s a form of measurement that measures energy. One BTU refers to the amount of energy that’s required to increase the temperature of a pound of water by 1° F. It is most frequently used when talking about heating and air conditioning units.
BTU to Calorie
Calories is the BTU of life, and so many calories are needed to live and for bees, produce heat. 252 calorie is the same as 1 BTU and a BTU is the equivalent of 0.3 watts.
Considering all of these confusing conversions. Heat is generated as a result of life consuming calories, and therefore if a hive needs to heat more, the more calories they need, so they go through more food reserves. So, it's super safe to say, the less insulation and more ventilation a beehive has the more food reserves it will go through. If you want to further your odds of your hives surviving such cold weather, insulated hives with low to no ventilation may be key.
Other Considerations
A strong hive with large a large population going into winter has a huge advantage over a smaller hive population as the larger cluster can more easily produce heat due to population advantages. Consider each bee as a super small heater, the more heaters the better. The less, the more affected the bees are to heat loss, as they can only produce so much heat.
Wrap up
I have 5 hives in my yard, and am testing the results of insulated to un-insulated hives, and none of my hives with a small population survived the 2020/2021 winter, however in my 2 strong hives only the insulated hive survived. Granted none of the bees where proven cold hardy, it is potentially telling on the potential of insulated hives in helping your colonies winter survival rates.