Thermal insulation of bee hives & tree cavities

Adaptation of the honey bee to the tree cavity climate

The evolution of the honey bee has taken place over many millions of years in tree hollows with considerable thermal insulation of the surrounding wood. It can be assumed that they have perfectly adapted to this tree cavity climate, which is significantly influenced by the thermal insulation. Only in the modern beekeeping honey bees had to do without the advantages of such a thermal insulation. With the lacking thermal insulation itself the honey bee gets along surprisingly well, which is above all due to the fact that its honeycomb construction with the air layers has an excellent thermal insulation itself. This leads to the assumption that the missing thermal insulation only means an increased energy expenditure for the bees. This is a mistake: Due to the lack of external thermal insulation, there is also a change in moisture in the hive.

Thermal insulation of tree cavities

The wood surrounding the cavity has excellent thermal insulation. The tree cavity found by the honeybees in the course of evolution in the uncultivated forests were as well insulated as our houses. For example, a 250 year old beech tree at a height of 10m still has a trunk diameter of 80cm (see picture below). This height can be assumed as a usual height of woodpecker cavities, which is subsequently also used by honey bees. Such trees were surely no rarity, oaks and beeches can become many hundred years old. The side walls of the individual cavities are likely to have had average wall thicknesses of over 30cm and thus heat transfer resistance of at least 2.045m²K/W. This value even fulfills the heat insulation regulation of 1995 for new buildings of houses in Germany. The cavities are even better insulated at the top and bottom, these components even meet the requirements for for ultra low-energy buildings. This good insulation has many advantages:

  • Lower energy losses lead to lower honey consumption and thus also to less water accumulation in the tree hollow by the metabolism of the bees.
  • The dew point is deep in the wood of the tree. There possibly condensing water is taken up capillary and distributed in the wood. This free water is far away from the beehive and cannot lead to a load of the bees, particularly since the wood walls are coated with propolis and thus a penetration of free liquid water is almost impossible.
  • No mold danger between the unoccupied honeycombs, since the relative humidity is clearly lower here.
  • The honey remains drier, its water content stands in the balance with the relative humidity of the surrounding air.
  • In theory, the bees remain valuable time for important works, they are forced to take the winter formation at lower temperatures.
Sample cross section of a tree with a tree cavity, here with an average wall thickness of 40cm
250a beech
Approx. 250 years old beech in the Königsforst near Cologne, height 36m, BHD 135cm, in 10m height D=80cm, in front of it small human with 2m folding rule

Thermal insulation of bee hives

The thermal insulation of thin-walled bee hives is many times lower than that of tree cavities. The bee clusters mostly remain their own honeycomb structure in these hives as thermal insulation to the sides and downwards. Surprisingly, the bees are even able to cope with this in terms of heat insulation. A bad influence on the bee health might have however the resulting increased humidity. The advantages of the good thermal insulation listed above with the tree hollows logically result in disadvantages with the bee hives with a bad thermal insulation in the reverse conclusion.

Only the lid of the bee hives is reasonably well insulated. Conventional lids of bee hives usually consist of several thin layers. Usually a wood fibre insulation board of 2.0cm thickness is included. The usual construction shown below has a heat transmission resistance of 0.832m²K/W. This is of course not comparable to the top of a tree cavity, but it corresponds to a wall thickness of about 8cm beech wood.