Thermal Deformation of External Wall Insulation Systems Using EPS, XPS and PU Boards: A Combined Numerical and Experimental Study

Under extreme steady-state temperature gradients, external thermal insulation composite systems (ETICSs) are prone to thermal deformation, which can cause mortar cracking, hollowing, and even delamination and detachment of insulation boards, thus degrading building envelope performance and threatening structural and personal safety. In this study, a combined method of numerical simulation using ANSYS software and experimental testing was adopted to investigate the thermal deformation characteristics of three commonly used insulation materials: Expanded Polystyrene (EPS), Extruded Polystyrene (XPS), and Polyurethane (PU). The effects of temperature difference from 10 °C to 30 °C, insulation board thickness from 30 mm to 100 mm, and surface mortar thickness from 5 mm to 10 mm on strain distribution and deformation mechanism were systematically analyzed. Experimental validation showed good agreement with the simulation results, quantified by an estimated relative error of less than 15% across the investigated insulation thicknesses and steady-state temperature conditions. The results indicate that the strains of EPS, XPS, and PU boards all increase significantly as the temperature difference across the board rises. Under outdoor temperatures of 30 °C, 40 °C and 50 °C with a constant indoor temperature of 20 °C, the thickness-direction strain at the EPS–mortar interface increases by approximately 35% when the temperature difference increases from 10 °C to 30 °C. Increasing both insulation board thickness and mortar protective layer thickness effectively reduces thermal deformation. Specifically, when the EPS board thickness increases from 30 mm to 100 mm, the thickness-direction strain decreases by approximately 73%; and when the mortar thickness increases from 5 mm to 10 mm, the interfacial strain decreases by approximately 32%. Due to differences in linear expansion coefficients, the three insulation materials exhibit distinctly different thermal deformation behaviors, with the thickness-direction strain following the order EPS > XPS > PU. These findings provide a theoretical basis and data support for material selection, structural optimization, and safety design of external wall insulation systems.