The limitation of fire class and halogen-containing flame retardants are two significant issues for polystyrene-based insulation materials. In this chapter, the necessity of external thermal insulation composite systems and related concepts are briefly introduced. Then, based on the approaches in the literature to halogen-free flame retardants for polystyrene, melamine-derived and expandable graphite-based substances on polystyrene production and properties are investigated. Flame retardants were added both during and after the polymerization reaction. Chemical and mechanical properties are investigated by means of elemental analysis, high-temperature gel permeation chromatography, thermal gravimetric analysis, tensile and flexural strength together with Young's and elastic moduli. Moreover, the horizontal fire class tests were also performed. According to results, except virgin PS all blended materials passed UL 94 HB test.