Thermal insulation of honeycombs

When considering the thermal insulation of bee hives, we also have to consider the honeycomb construction with its air layers in between. These layers themselves act as thermal insulation. The more honeycombs between the bee cluster and the side wall, the higher the insulation. I.e. with regard to the side thermal insulation, the broad constructions of the modern hives are quite helpful.

Thermal insulation of honeycombs in hives

The heat-insulating effect in magazine hives is slowed down by the typical beespace. There is weak air circulation within the hive. These reduce the thermal transmission resistance of the honeycomb construction and the air layers between. The more honeycombs are free of bees, the higher the air movements and the lower the thermal insulation effect of the individual honeycombs and the air layers in between.

We can get a good idea of the thermal insulation within a hive by looking at the temperatures between the honeycombs.

Temperature between the honeycombs, 15.01.2014

The diagram shows the measured data of the HOBOS bee colony in Würzburg from January 15, 2014 at 0:30 am. The temperature difference on the side walls shows that these are not “strongly ventilated” honeycombs in the sense of building physics. If they were, the temperature difference on the side walls would be zero and the honeycombs and the layers of air between would not function as thermal insulation.

By comparing the gradient of the temperature curve in the wood of the side walls (λ=0,13W/mK) with that of the neighbouring honeycombs, we can draw a direct conclusion about the thermal conductivity of the honeycombs.

The bee colony should be in beeway 3, 4 and 5 at the time shown. In beeways 2 and 6 there are probably no or only a few bees. Beeways 1, 7, 8, 9, 10 and 11 are certainly free of bees. On the left side, the gradient of the temperature curve between the beeways 1 and 2 is the same as that between the inside and outside of the wooden side wall. Here the thermal resistance is about the same (2.5cm wood has the same thermal resistance as 3.5cm honeycomb including air layer). On the right side, the gradient of the temperature curve between the honeycomb alleys 6 and 11 is about half the gradient of the temperature curve through the side wall. In this slightly more ventilated part, the thermal conductivity of the honeycomb structure is thus calculated:

λ611=0,13WmK×2×3,52,5=0,36WmK

In the less ventilated part on the left, the thermal conductivity of the honeycomb construction is calculated:

λ12=0,13WmK×3,52,5=0,18WmK

Following still briefly the view of other periods and the measuring station in Schwartau:

  • The average measured values from January 2014 result in slightly higher thermal insulation of the honeycombs.
  • In winter 2014/2015 the colony is much stronger, they hibernate on 5 honeycombs. The average measured values from January 2015 result in somewhat higher heat transfer resistances, analogous to the above calculations.
  • The query of data from the apiary in Schwartau is not so informative, as no temperature sensors were placed in the beeways 2, 3, 9 and 10. However, temperature differences are also present on the side walls here, which proves the insulating properties of honeycomb construction and a “weak ventilation”.

Feature freestyle combs

There are less air movements in hives with freestyle combs. The absence of Beespace above and at the sides of the honeycombs virtually eliminates air circulation. We can apply at least the same thermal resistance to the honeycombs as to wood. This is a tentative estimation for the freestyle combs, I consider a twice as high thermal resistance to be more likely (a completely stationary air layer would have a thermal conductivity of 0.025W/mK, i.e. a thermal resistance about 5 times greater than softwood).

Thermal transport along the honeycomb channels

I can only make one assumption about the thermal insulation along the honeycombs and beeways (i.e. perpendicular to the previously considered direction), since no measurement data are available for this. I estimate the heat transport in this direction to be about the same. On the one hand, the air circulation in the beeways filled with bees is much lower and the layers of air are likely to be considered “stationary” in terms of building physics. This increases the insulating effect of the air layers compared to those in the unoccupied beeways. On the other hand, the transition resistances between the different media are eliminated in the beeway. Although the beeways make up only about 25% of the heat transmission surface under consideration, the transition resistances in a thermal transport perpendicular to the honeycomb account for up to 50% of the total resistance. In order to be able to make a more precise statement, temperature sensors would have to be placed along the entire length of the honeycomb.