A simplified approach to assess the effect of open and closed external blinds on the effective U-Value of single glazed windows

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.