One of the biggest disadvantages of rigid polyurethane foams is its low thermal resistance, high flammability, and high smoke production when burning. Greatest advantage of this thermal insulation material is its low thermal conductivity, which at 20–25 mW/(m·K) is superior to other commercially available insulation materials. In recent years polyurethane materials from renewable resources have been widely studied. But their use on industrial scale was limited due to inconstant performance and relatively high price of raw materials. Different bio-based raw materials, such as rapeseed oil and tall oil, could provide abundant feedstock for PU foam production. Decrease of flammability of PU materials conventionally is achieved by addition of flame retardants, halogen-containing compounds, and phosphates. It can be considered that halogenated fire retardants could have several health hazards, such as volatile compound emission from materials and toxic gas release during burning process. Expandable graphite could be an answer to this flammability problem. This chapter describes development of bio-based rigid polyurethane foams and their flammability reduction using sustainable flame retardants. Different expandable graphite intumescent flame retardants provided significant flammability reduction while maintaining low thermal conductivity of insulation materials.