From October 2018 to August 2021, the University of Bamberg, in collaboration with the Fraunhofer Institute for Building Physics (IBP) researched innovative solutions to improve the energy efficiency of historic glazing and glass windows. One reason for the project was the fact that in the preservation of secular monuments, historic glazing and windows from the era of industrialization are today still considered to be of little value, often being regarded as disposable assets. On the other hand, the production of glass requires considerable resources and energy. The aim of the project was to increase the value of industrially produced flat glass installed in secular buildings and to investigate the relationship between the resource-saving improvement of existing windows and their replacement. For this purpose, the life cycle of glazings was analysed. The key criteria of this evaluation were the consumption of material resources and primary energy, and the recycling rate of glazings. During the project, the improvement of existing windows was tested, climatically monitored and evaluated using examples of adaptive (retrofitted) window systems. The test site was the Alte Schäfflerei in Benediktbeuern. The advantage of retrofitting windows is the resource-saving preservation of the existing windows. The measure allows to minimize heat losses and thus lowers the demand for heating energy as well as the CO₂-footprint. The results were summarized in a practice-oriented guideline on the resource-friendly handling of historic glass and historic windows. The guideline presents a suitable procedure for the quality assurance in the handling of existing buildings and considers aspects of resource efficiency and environmental balance.