The increasing demand for sustainable construction materials has driven interest in utilizing waste biomass within polymer composites. Rigid polyurethane foams, widely valued for thermal insulation, exhibit a significant flammability issue. This study investigates the impact of incorporating various waste biomass materials, including brewers’ spent grain, coffee grounds, and soybean husk and their combustion ashes on the selected properties of rigid polyurethane foams. The primary objective is to assess the potential of these eco-friendly additives as replacements for traditional raw materials, aiming to enhance fire resistance and thermal stability and thereby promoting circular economy principles in the construction sector. Composite foam samples were fabricated using a mixing and casting technique, incorporating 5% wt. of fillers into the polymer matrix. Thermal stability and flammability were evaluated using cone calorimetry and thermogravimetric analysis. The findings indicated that while biomass inclusion did not significantly improve char formation, the addition of ash substantially increased char yield, a critical factor in fire suppression. Although biomass and ash may influence flammability, they do not inherently bolster the intrinsic thermal stability of the polyurethane matrix itself.