This article examines whether transient two-dimensional simulations are sufficient or three-dimensional simulations are necessary for the hygrothermal analysis of the three-dimensional detail of a beam support. Detailed measurement series from a test house with various interior insulation systems available for the investigation allow the results of two- and three-dimensional simulations to be verified with the measured data. In both the cases, a very good agreement between measurement and simulations was found. With the thermal simulations, agreement with the measured data is similar in both two- and three-dimensional calculations. However, the hygric measurements agreed slightly better with the three-dimensional calculations. For planning real tasks, two-dimensional simulations should be sufficient, provided that the simulation settings are selected within the secure margins. The actual comparison is initiated by the validation of the software DELPHIN 6 by means of a three-dimensional, stationary test case from EN 10211.