The evolution of honeybees has taken place over many millions of years in tree hollows with a respectable thermal insulation of the surrounding wood. It is to be expected that they have adjusted perfectly to this climate inside the tree cavities, which is much influenced by the thermal insulation. Only in modern beekeeping did they have to forgo the advantages of such thermal insulation. With the lack of thermal insulation itself, the honeybee gets along surprisingly well, which is mainly because its honeycomb structure has excellent thermal insulation. This leads to the assumption that the lack of thermal insulation only means an increased energy expenditure for the bees. This is a mistake: The lack of external thermal insulation also leads to a change in moisture in the beehive. In a tree cavity with massive wooden walls with an average thickness of well over 30 cm, the dew point is deep in the wood of the side walls. With hives made of only a thin layer of wood and without more external insulation, the air humidity in honeycomb construction increases. This in turn leads to an increased risk of mould in the hive.
Using established calculation methods of building physics, areas of potential condensation can be determined in house construction. The same calculations can also be made for hives. Of course, these calculations cannot exactly predict the air humidity or the amount of condensation. There are simply too many variables for this. However, the influence of each factor can be seen very well from these calculations. Thus, the following calculation shows the influence of the external thermal insulation, or the thickness of the hive wall, on the moisture in honeycomb construction.
The same external conditions were used to determine the curves shown. Only the thickness of the wooden wall was set differently: Once according to a tree cavity with 40cm and once according to a thin-walled bee hive with 2.5cm. It was assumed that there are 2 unoccupied honeycombs between the bee cluster and the hive wall. At the edge of the bee cluster a temperature of 10°C and a relative humidity of 70% were assumed. The external climatic conditions were chosen in such a way that condensation water arises in the tree cavity.
The result is not surprising. It corresponds to what we know from house construction. An external thermal insulation shifts the dew point outwards. If there is poor or no external insulation, this dew point is already in the inner insulation. And this inner insulation is the honeycomb structure in bee hives. The air humidity between the unoccupied honeycombs is up to 100%, condensation already occurs during honeycomb construction.
Between the unoccupied honeycombs of the tree cavity, the calculated humidity is only slightly higher than at the edge of the bee cluster. The condensation water accumulates deep in the wood of the tree and is absorbed and distributed capillary there.
A similar picture results when looking along the honeycomb lanes, but here the condensation water in the beehive mainly accumulates on the inside of the hive wall, less on the honeycombs. But also here the humidity between the honeycombs is up to 100%.