ABSTRACT: The paper presents interim results of a research project carried out at ift Rosenheim in co-operation with Arbeitsgemeinschaft Mauerziegel and Adolf Wurth GmbH & Co. KG. The project is funded by the research initiative "Zukunft Bau" of the Federal Institute for Research on Building, Urban Affairs and Spatial Development (Reference No. SWD- The purpose of the research project is the development of appropriate design methods for the fastening of highly thermal insulating windows in modern thermal insulating single-leaf clay unit masonry. A methodology for the evaluation of fastening systems was devoloped, considering the load bearing capacatiy as well as the durability.Within first small scale tests load-bearing capacities of dowels installed in thermal insulating clay units were determined. The suitability and durability will be verified in large scale tests with windows in masonry walls. The investigations covered uniform wind load as well as local load concentrations.