Numerical Assessment of Thermal Performance in Hollow Clay Masonry: Continuous Insulation Layers versus Partial Cavity Fills

This study presents a comparative analysis of thermal retrofit strategies for hollow-clay-brick walls under high-intensity summer conditions. A numerical model coupling conduction, convection, and radiation simulates six configurations: an uninsulated wall, systems with 50 mm continuous extruded polystyrene (XPS) layers (external/internal), and full and partial cavity fills. The model is driven by a diurnal cycle with high ambient temperatures (25–45 °C) and significant solar irradiance. Results demonstrate a pronounced performance gradient. The continuous XPS layers are most effective, reducing thermal transmittance by 74% and providing superior dynamic performance. External XPS yields the largest thermal lag and smallest indoor swing while internal XPS offers comparable amplitude reduction. All cavity-fill strategies provide more modest U-value reductions (12–13%) and dynamic behavior similar to the reference wall. This work provides a critical dataset for selecting insulation strategies, concluding that continuous layers are optimal for peak performance, while cavity fills offer a viable supplementary or alternative solution where architectural constraints apply.