In the Viennese building stock, the timber beam ceiling represents the most widespread ceiling construction. Many of these buildings, constructed during the Wilhelminian period, are now in need of refurbishment. The beam head in the outer wall represents a thermal bridge, which must be given additional attention. Especially for buildings with a subsequently installed interior insulation, it should be investigated why some of these beam heads are still in perfect condition, while others are already mouldered. For this purpose, this thesis examined the effects of an air flowed beam head more precisely and investigated the effects of an airflow on the durability or mould susceptibility of such a construction. In the first step possible causes and paths of an airflow were investigated. The flow path with the highest probability arises between two stacked rooms but also a direct flow of air to or from the outside would be in the range of possibilities. Another point is the basic question, whether an airflow between different floors is even plausible. For this purpose, experiments were carried out in a house build in the Wilhelminian period. The air volume flow, in the area of beam bearing, from one room to the room above was measured. The investigations have shown that strong airflows can occur, but with thoroughly plastered ceilings only minimal airflows can be detected. For performing hygrothermal simulations of a beam head detail, the differences in pressure for the different flow causes were considered. As soon as an airflow to or from the outside of the building is possible, there is a significant increase in the pressure differences that occur. The largest pressure differences come about as a result of wind. However, their illustration is difficult because they occur as alternating pressure and suction stress and they are directly dependent on the orientation of the building. The simulations themselves were performed with the finite-element-program HAM4D. They determined the steady state vibration of moisture in the timber, with regard to diffusion and convec-tion. Different combinations of indoor climates and flow directions were varied. The simulations have shown that the historical structures are fully functional and that there is no critical humidity. Nevertheless, the occurring humidity is higher than in simulations without convection. Which can result in problematic humidity when driving rain arises or the component structures get changed. Wall constructions with a retrofitted internal insulation are problematic, since already causes the air flow mould growth. In such constructions boundary conditions such as dry indoor climate or a particularly air-tight executed beam head detail are of crucial importance, if it comes to mould growth. The simulations also showed that airflow around the beam head can produce positive effects. For that phenomenon the interior climate and the direction of flow are important.