Water vapor diffusion
Wood is a diffusible material That means it has low resistance to water vapor diffusion. If there is a different water vapor pressure on one component on different sides, the water vapor pressure tries to balance itself by diffusion through the component. The component itself opposes this diffusion with a resistance (water vapor diffusion resistance Z, depending on the material-specific water vapor diffusion resistance value µ and the component thickness). The largest possible water vapor pressure in air (with constant air composition and geographical location/height) is only variable by the temperature.
An example: At 15°C the water vapor saturation pressure is 1706 Pascal. If the relative humidity is 60%, the water vapor pressure is 60% of the water vapour saturation pressure (0,6×1706=1024 [Pa]). If a temperature of 4°C and a relative humidity of 100% is present on the other side of the component, the water vapor pressure is 813 Pa. The water vapor pressure is clearly higher on the warmer side despite lower relative humidity. Moisture is transported by diffusion from the warmer to the colder side.
This example illustrates that there is usually a moisture transport from the warmer to the colder side. In inhabited houses as well as in bee hives and tree cavities. However, the water vapour diffusion is strongly reduced by the propolis layer of the bees.
Water vapor diffusion plays a minor role in tree cavities. Although there is a clear difference in temperature to the outside air in the upper part of the cavity, the thickness of the outer walls allows only a small amount of water vapor transport. The propolis coating additionally slows down this water transport mechanism.
In the context of sorption and desorption processes of wood and honey, i.e. the interaction of the humidity of these substances with the relative air humidity, water vapour diffusion of course plays an important role.
Modern bee hive
With the modern bee hives it looks a bit different. Theoretically, water could escape the beehive through the thin walls by means of vapor diffusion. As it usually does not:
- There is no temperature difference at the side walls due to the poor thermal insulation. Indoor and outdoor temperatures are almost identical (see HOBOS temperature graph). With higher relative humidity on the outside (approx. 90% in winter), this even places an additional load on the interior. For this reason it makes sense to paint thin-walled magazine hives on the outside.
- There is a temperature difference at the lid, but here the diffusion is usually completely prevented by cover sheets and aluminium lids.
The graph shows the temperature curves of a beehive in Schwartau during the first week of January 2017. The diagram shows the temperature of the middle comb aisle (red), the temperature of the two outermost comb aisles and the temperature of the outside. Although the temperature inside the hive is about 20°C, the temperature in the vicinity of the side walls is at the same level as the outside air.
With the attachment of a D-lid, about 1-3l of water can be drained off in winter. This amount of water depends strongly on the thickness of the permeable layer and the inside climate.
The amount of water would correspond to a large part of the humidity occurring in the stock air in winter time.