The antiseptic function of propolis is generally known. This property slows down the development and spread of pathogenic germs in hives and tree hollows. However, propolis also fights condensation and mold in a different way, which is probably unknown to beekeepers. It is a simple physical effect: In the propolis layer there is no capillary condensation, propolis is not hygroscopic.
Mold can only grow in the presence of free water. For non-hygroscopic surfaces, condensation does not occur until there is a relative humidity of 100% near the surface. With hygroscopic surfaces (e. g. wood) capillary condensation already occurs when a relative humidity of 80% is reached in the areas near the surface. The condensation that accumulates in the wood is largely bound by the wood and is only available as free water in very small quantities. However, these small amounts are enough for mold to grow on the surface of the wood.
The coating of the Propolis changes the surface condition of the tree hollow or hive wall. The surface is no longer hygroscopic. Condensation on this surface only occurs at about 100% relative humidity. The critical level for the relative humidity, from which a mold danger exists, is thus shifted upwards by 20-30 percentage points.
The thin propolis layer (approx. 0.1-0.3mm) allows water vapour diffusion. Internal tests confirm the diffusion permeability of the tested specimens. The interaction of wood and air humidity is only slightly slowed down by the propolis layer. Under the propolis layer, capillary condensation continues to occur. It is part of the sorption process up to fibre saturation.
The layer of wood underneath the layer of propolis is downright mummified by the propolis, growth of mold is practically impossible.