Lime has an open molecular structure.
A lime plaster becomes ‘wet’ once in contact with water. However, this doesn’t mean that there is a risk of damage. Water that is encrusted in the coating or joint (f.ex. when it rains), will dry easily.
This open structure will also mean good permeability and low capillary action. A wall always has a certain content of moisture, coming from the internal air, but also from the construction materials themselves. Additionally the moisture content will increase by upward water and all kind of infiltration through cracks. In order to keep the humidity level as low as possible, it is of major importance that the air circulation remains possible through the wall.
Permanent water in a wall not only facilitates the growth of algae and moss, but also decreases its insulating value.
The low capillary action (= water suction power) of lime will make that water won’t be retained, which on the contrary is the case with cement.
Open structure also leads to a good thermal insulation. As already explained, the water will not long remain locked, leaving room for stationary air.