Validation of 3D thermal simulations of the Double C Block, a novel composite masonry unit, using in-situ U-value measurements

•3D thermal simulation applied to innovative prefabricated slender composite masonry units with embedded insulation.•Simulations enhanced by materials’ dry thermo-physical properties and thicknesses measured in laboratory conditions from DCB samples.•Site-specific boundary conditions using locally recorded weather files for Malta as well as indoor test cells’ DBT and RH recorded onsite.•Validation of simulations according to uncertainty indices such as of MAE, RMSE, MBE and cvRMSE.•Predicted U-value overlaps experimental values. Energy efficient building envelopes, particularly innovative prefabricated building components, can significantly contribute to lower the building industry energy-related emissions. In this research the composite masonry unit Double C Block (DCB) is quantitatively tested by means of three-dimensional thermal simulation validated by in-situ U-value measurements carried out in both summer and winter conditions. The simulations were enhanced by the experimental measurement of the dry thermo-physical material properties (specific heat capacity, thermal conductivity, and density) in laboratory conditions for both concrete and polyurethane. The samples were extracted from real-scale DCB prototypes. Furthermore, site-specific boundary conditions of Malta — representative a case study of a Mediterranean climate − were utilised in the simulations. Both laboratory and in-situ assessments of thermal transmittance, or U-value, had their uncertainty specified as part of the calculations. The validation exercise was performed by means of hourly recorded surface temperature and the selected indices were the root mean square error and mean absolute error. Results showed that these indices were within the recommended accuracy thresholds during both winter and summer assessments. For the hourly-based heat flux measurements, the mean bias error and coefficient of variation of the root mean square error were found lower than values reported in literature, thus supporting the robustness of the overall simulation outputs. Hence the 3D CVM simulation was experimentally validated by in-situ assessments. The convergence value of the experimental U-value provided respectively 1.51 ± 0.20 W/(m2K) (winter) and 1.44 ± 0.19 W/(m2K) (summer). Then, the validated 3D CVM model was adapted to calculate the theoretical U-value of DCB, using monthly values of recorded surface temperatures obtained by the test cells (in both seasons). The predicted thermal transmittan