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• Comparative in-situ monitoring of two architecturally identical Mediterranean schools. • ETICS retrofit moderates surface and indoor air temperature behavior under free-running conditions. • Short-term monitoring captures envelope thermal response under controlled conditions. • Qualitative thermography confirms improved envelope continuity after façade retrofitting. • Findings are transferable to widely replicated public school typologies in southern Spain. This study presents a comparative in-situ assessment of the thermal behavior associated with the application of an External Thermal Insulation Composite System (ETICS) in public primary school buildings located in a Mediterranean climate. Two architecturally identical schools in Seville (Spain) were monitored under free-running conditions, one in its original state and the other retrofitted with ETICS. The analysis relies on directly measured temperature-based indicators, including interior and exterior surface temperatures and indoor air temperatures, recorded during representative summer and winter school-week periods. The results show that the retrofitted building exhibited a moderated thermal response relative to the original case in both seasons. In summer, lower interior surface temperatures and reduced daily temperature amplitude were observed in the retrofitted façade, while in winter higher interior surface and indoor air temperatures were recorded, indicating enhanced buffering under heating-dominated conditions. Qualitative thermographic inspection supports these findings by revealing a more homogeneous surface temperature distribution in the retrofitted façade. The studied buildings belong to a standardized public school typology widely replicated across southern Spain, which enhances the potential transferability of the observed trends to similar educational buildings in Mediterranean climates. Overall, the study contributes empirical evidence on the envelope-driven thermal response of ETICS retrofits in existing schools under free-running conditions, supporting passive renovation strategies in warm Mediterranean contexts.