The Eyes of the City as a Threatened Asset of Cultural Heritage : Improvement of Historic Casement Windows with Vacuum Glazing

Abstract

Traditional casement windows are considered not only as aesthetically sophisticated window constructions, but also have been subjected to century-long optimization processes. Rough estimates state that there is a large number of these windows still to be found in the building stock of European cities, e.g., Vienna. However, the number is in constant decline, as casement windows often are considered expensive in maintenance and dissatisfactory in their thermal performance and fulfilment of current comfort desires of building occupants, and thus are often demolished and replaced by new windows. Moreover, many building retrofit subsidy programmes do not consider the restoration of windows as a feasible measure and only window replacement is subsidized. Needless to say, the replacement of casement windows with modern-day single layer multi-pane windows in historic facades is often a fundamental distortion in the corresponding façade appearance and thus a threat to the built cultural heritage of cities. One option to satisfy both the performance expectations of occupants and the appearance upkeep required for cultural heritage protection is employing vacuum glass as replacement for float glass. The implementation of vacuum glass products in existing window constructions – however – brings up a set of questions, such as the possible thermal performance improvement of the window, thermal bridge effects in the glass/frame/Wall joints, and constructive integration possibilities. A collaborative R&D project started in 2019 (VAMOS – Vakuumglas-Kastenfenster: Performance – Monitoring in Sanierungsprojekten) focussed onto the construction implementation of vacuum glass in demonstration windows, which was accompanied by comprehensive simulation and monitoring of the demo site windows.