Modelling climate related performances of building wall coatings and understanding the portability of the "Kunzel" rule in different climates

How may a coating affect the hygrothermal performance of the building envelope in different climates? Years ago, Helmut Kunzel, one of the fathers of Building Physics, proposed, a simple, well-known rule, relating two characteristics of a coating: its water absorption coefficient and its vapour diffusion. The "Kunzel rule" (and the associated diagram), based on a model confirmed by field tests in the German climate, set an upper limit to both parameters and their product, became a German standard and a practice among experts, practitioners and manufacturers, in many European countries. This paper proposes the results of an analysis aiming to verify its portability in other climates and is based on an extensive simulation of the hygrothermal performances of a reference wall in six different climatic conditions.