Anne Maas, Branca Arthur Delmonte, Frank Scholzen, Stephan Leyer

• Renovation decisions are often based on Energy Performance Certificates (EPCs), which rely on standardised input parameters that may not fully represent real operating conditions of existing buildings. As a result, heat losses and energy-saving potentials can be misestimated, leading to renovation measures that are less effective than predicted. To meet climate targets, there is a need for rapidly deployable measures whose effectiveness is demonstrated under real operating conditions. This study demonstratesthat the effective U-values of older single-glazed windows with aluminium frames are typically lower than normative assumptions and investigates how closing external blinds overnight can further reduce heat losses as a simple operational measure. In-situ heat-flux measurements are combined with analytical calculations and dynamic TRNSYS simulations for an existing school building in Luxembourg. The results show a significant, costeffective and user-friendly measure for reducing heating demand.