This study presents a novel approach to fabricating flat foam composites for use in thermal insulation systems and sandwich panels. The primary objective is to investigate the effect of partially replacing the polyurethane foam matrix with hollow glass microspheres on the apparent thermal conductivity and density of the resulting composite. In the experimental procedure, resin and isocyanate were poured into a free-flow casting mold containing pre-placed hollow glass microspheres at volumetric fractions ranging from 10% to 60%. The fabricated composites were characterized through multiple tests. The microstructure and phase distribution were analyzed using scanning electron microscopy (SEM). Apparent thermal conductivity measurements were conducted using a TH-FOAM device, with each sample tested in three 10-minute repetitions. The experimental results were compared with theoretical predictions from Maxwell’s model. The findings indicate that incorporating 60% hollow glass microspheres with an approximate diameter of 1000 µm reduces the apparent thermal conductivity by up to 28% compared to pure polyurethane foam. Furthermore, densitometry analysis reveals that increasing the hollow glass microspheres content from 10% to 60% results in approximately a 75% increase in composite density. These results highlight the potential of hollow glass microspheres-reinforced polyurethane foams as high-performance thermal insulators for industrial and construction applications