I’m amazed at how hardy bees are in the cold. Usually bees die in the winter due to varroa, nosema, condensation, and other factors. Here’s an interesting explanation from HoneyBeeSuite about winter clusters and how the bees stay alive during the cold weather.
The cluster stays warm, the hive does not
Natural systems do not waste energy and honey bees are no exception. To survive the winter, a cluster of bees must keep itself warm. While it does this efficiently, it makes no attempt to heat the entire space within the hive.
The warmest place within a hive is in the center of the cluster. The temperature of the cluster decreases as you move toward the outside. The bees on the outside get so cold that they must rotate to the inside. If the inside of the hive were uniformly warm, this rotation would be unnecessary.
Of course, there is some heat lost from the cluster into the surrounding air, and because heat is lost, the bees must continually generate more. If you put your hand close to a heated iron, for example, you can feel the heat. The heat loss from the iron is similar to the heat loss from the cluster. You don’t have to move far away before you no longer feel it. The same is true inside the hive: the temperature drops rapidly as you move away from the cluster.
Nevertheless, the air inside the hive is slightly warmer than the ambient outside air. This is because the hive box itself provides a small amount of insulation. But the R-value of a pine board is not much, which means the difference in temperature between the inside air and the outside air is not great.
Heat rises from the cluster
There is one place in the hive that is warmer than the others, and that is the space immediately above the cluster. That is because warm air rises. One beekeeper in France measured the temperatures in his hive when the outside air temperature was 44°F. He measured 95° in the center of the cluster, 71° immediately above the cluster and 52° in other empty portions of the hive. Other beekeepers have found similar temperature gradients.
For this reason, an insulating layer placed above the bees reduces the rate of heat loss from the hive. Styrofoam, wood chips, or a layer of another material will slow the loss through the roof. But even this has its limits. For one thing, as the internal temperature gets warmer in comparison to the outside air, more heat is lost through the walls, so overhead insulation alone does not conserve as much heat as insulating the top and sides. It is a complex system with no easy answers.