In this study, rigid polyurethane foams that contain up to 5.0 wt % fly ash (FA) being a by-product of thermal power stations and being cheap source were successfully produced using a polyurethane injection machine. The effects of FA content on the thermal conductivity, compressive strength, and flammability were investigated. The morphology of the cell was observed under a special microscope. The incorporation of FA in rigid polyurethane foams may dramatically decrease production costs and reduce environmental pollution. In addition, the effects of intumescent flame retardant composed of ammonium polyphosphate and pentaerythritol were examined in pure rigid polyurethane foams and FA-rigid polyurethane foams. It was found that 5.0 and 7.5 wt % intumescent flame retardant loadings enhanced the thermal stability and improved the flammability resistance of the foams. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2095-2102, 2011