Decay of wooden beam ends in masonry walls is still a field of practice and scientific discussion. In literature many possible causes for decay are given. This paper delivers a comprehensive literature review about the hygrothermal performance of old masonry with and without interior insulation. In old walls without insulation the reasons for decay at wooden beam ends are often damaged downpipes, leaking roofs and lack of protection against wind driven rain. Furthermore the analyses of several interior insulation projects with wooden beam ends demonstrate that moisture load from outside by wind-driven rain (wdr) has an important influence on the behavior of moisture content of the beam end. In addition, hygrothermal simulations are carried out to detect and analyse the influence of the micro climate around the wooden beam end. A construction is simulated with and without convection from inside as well as with and without interior insulation. To interpret the results a simplified wood decay model based on the Viitanen model is used. The model considers the temperature dependency. This indicates that simple thresholds in national standards as 20 M-% moisture content is not suitable.