This paper investigates the morphology, mechanical and thermal insulation properties of textile air conditioner dust-filled polyurethane (PU) rigid foams. Textile air conditioner dust occurs during the spinning process of staple fibres. It consists of cotton fibre impurities as boll, leaf, and short fibres that generate a vast environmental load. This dust is used as a filler and dispersed in polyurethane up to 40% wt. The cell -size of foams decreased and cell -density increased by the increase in filler ratios in which the dust acted as a heterogeneous nucleation site during cell formation. The dust agglomerated in polyurethane foam and induced irregular cell wall and hence non-uniform cell formation and many broken cells. The flexural strength and modulus decreased in higher dust content (40% wt.). The lower thermal insulation of cellulose-based dust and damaged structure/uniformity of foam cells resulted in an unstable/lower thermal conductivity of dust-filled polyurethane rigid foam composites.