To improve the passive performance of the building envelope it is required to provide materials with high thermo-optical performance and flexible implementation capability in several contexts. This research study comprehensively evaluated the performance of a thermal insulating plaster smooth for building walls by combining in-laboratory and in-field experimental analysis. Laboratory tests were conducted to assess the real performance of this material in terms of thermal properties, infrared emittance, and solar reflectance – for application as the external finishing layer. Moreover, the in-field application on a real building was carried out to assess the thermal resistance of the material. This approach provides valuable insights into the complex interactions between building components and environmental conditions. The key findings from the laboratory analyses showed that the material has low thermal conductivity yet higher than expected (0.101 W/m K) and solar reflectance equal to 78.5%. However, the real in-field test demonstrated a small difference in thermal resistance for the same wall with a thin layer of and without the thermal insulating plaster (0.356 vs. 0.350 m2 K/W).