Rigid polyurethane foams are extensively employed as thermal insulation materials in construction, valued for their low thermal conductivity and favorable mechanical properties. Research has increasingly demonstrated that the integration of bio-based fillers derived from agricultural residues can augment both compressive strength and thermal insulation efficacy. This study examines the effects of integrating brewers’ spent grain, coffee grounds, and soybean husk—along with their respective combustion-derived ashes—on the structural and functional properties of rigid polyurethane foams. Additionally, it assesses the impact of coal-processing additives, specifically microspheres and fly ash, on the characteristics of the resulting composite matrix. Following foam synthesis via manual mixing and casting, morphological characterization using optical microscopy confirmed a predominantly regular, pentagonal cellular structure, while infrared spectroscopy revealed no evidence of new chemical bond formation between the polymer matrix and the fillers. Although mechanical evaluation exhibited significant variability, the introduction of fillers did not fundamentally alter the intrinsic cellular or chemical composition of the matrix. Furthermore, this research investigated how varying organic and inorganic additive compositions influenced the thermal properties of the composites. Ash particles acted as nucleating agents, decreasing foam density and enhancing polymer chain mobility, thereby delaying the onset of thermal decomposition. Ultimately, this work elucidates the correlation between specific biomass and ash compositions and resulting performance metrics, aiming to optimize sustainable insulation formulations and support waste valorization strategies.