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: Propolis prevents capillary condensation.
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 sufficient for mold to grow on the surface of the wood.
The application of a layer of propolis changes the surface of the tree hollow or hive wall. The surface is no longer hygroscopic. Condensation on this surface only occurs at about 100% relative humidity.
In an experiment to determine the resistance of propolis to water vapor diffusion, results indicate that it is “permeable” (µ=4-10). A 1mm layer of propolis thus has the same resistance as a layer of air with a maximum thickness of 10mm. This resistance is negligible.
As a result of this permeability, capillary condensation continues to occur in the wood. The interaction of moisture in the wood and humidity in the air remains nearly unaffected by the layer of propolis. It is only slightly decelerated.
The layer of wood underneath the layer of propolis is downright mummified by the propolis, growth of mold is practically impossible.