A window is an ‘opening especially in the wall of a building for admission of light and air that is usually closed by casements or sashes containing transparent material (such as glass) and capable of being opened and shut’ (Merriam Webster, 2021). Until medieval times windows were openings closed by leather, fabric or wooden shutters in most parts of Europe. Windows are essential functional elements of a building and, at the same time, they are comprehensive architectural and design elements of the massive building envelope. As a building component, the window has to fulfill a wide variety of tasks and requires the collaboration of different craftsmanship techniques in its manufacture. The diversity of the historical, national and regional types of windows is large and as diverse as architecture in general (Koolhaas, 2018). They represent different design characteristics depending on time and region and they include technical and structural developments that respond to different functional requirements, e.g. impermeability, ventilation possibilities, light permeability and operability. Despite or precisely because of this diverse functionality and significance, windows are subject to a particularly high pressure for change (VDL, 2017). Similar to other heritage authorities, Historic England has stated that unsympathetic replacement of windows and doors represents the number one threat of heritage buildings, and that this affects no less than 83% of defined conservation areas (Pickles et al., 2017, p.1). Traditional windows are threatened for many different reasons: they are often completely replaced to improve a building’s energy efficiency, even when simple technical and thermal upgrading options (e.g. draught-proofing, secondary glazing, shutters) would be also feasible at much less cost. Other reasons are safety and security requirements, material decay or the simple wish for change and modernization. […]