Rigid polyurethane foams (RPUFs) are one of the high-performance insulation materials with high mechanical strength, good chemical durability, easy processing and excellent thermal insulation properties. However, RPUFs generally possess low thermal stability, high flammability and produce toxic and asphyxiant gases during burning. Flame retardants help the formation of a char layer on the burning surface, can quench flammable radicals released from the material and may also act as heat sinks, depending on their chemical compositions. Flame retardant additives have to be added in high amounts to provide good retardance and they are usually not environmentally friendly and harmful to human health. These can be partially replaced by other additives to provide better flame retardance. Halloysite nanotube (HNT) is a low cost, naturally abundant clay mineral. With its unique structure, it stands as an environmentally friendly alternative to various synthetic tubular nanofillers. During burning, HNTs can reinforce the char layer formed on the composite surface and entrap flammable decomposition products in its lumen. Similar to other clays, homogeneous dispersion of HNTs in the polymeric matrix is a critical step in obtaining these ideal structures. This study presents the functionalization of the HNTs’ surface with 3-isocyanatopropyltriethoxysilane (IPTES) and their subsequent addition to rigid polyurethane foam formulations. Modified HNTs and their dispersions in polyol and isocyanate components were analyzed by FT-IR and NMR spectroscopies, thermo-gravimetric analysis (TGA), visible light microscopy, scanning electron microscopy (SEM) and viscometry. The effect of the HNT incorporation on the morphology, flame retardance, mechanical and thermal properties of produced nanocomposite foams was investigated by detailed characterizations using thermal conductivity, SEM, density measurements, mechanical compression and fire performance tests. This research is supported by Turkish Ministry of Science, Industry and Technology under grant number 115M033.