Thermal conductivity is the most important property for low-density polymer foams in thermal insulation applications. It is well known that thermal conductivity of polymer foams is attributed to different parts: conduction of solid polymer via cell walls and struts, conduction through kinetic energy of the entrapped cell gas, thermal radiation and convection (negligible if the cell size is less than 4mm). The drawback of low-density foams is the significantly increasing IR transparency and the thermal radiation part of the overall thermal conductivity. To overcome this problem, the addition of nanoparticles such as graphite and carbon black in little amount, helps to reduce thermal conductivity due to their infrared absorbing / reflecting effect. The particles can simultaneously act as nucleating agent leading to increased cell density and a reduction of the cell size. In order to achieve this, the foam extrusion parameters, blowing agent and nanoparticles employed must be carefully selected and optimized since they affect the cell morphology and the dispersion quality of the nanoparticles in the polymer matrix. In this study, Polystyrene foams modified with selected carbon-based nanoparticles i.e. graphene, carbon black, carbon nanotubes were produced by batch processing and subsequently via foam extrusion using a mixture of CO2 and Ethanol as physical blowing agents. By selecting constant foam processing parameters, the influence of different nanoparticles and the degree of dispersion in varying concentrations on the foam structure and nucleation density is investigated. The nanoparticles were found on the cell walls and struts of the foams. With graphene the nucleation density compared to neat Polystyrene was dramatically reduced by several decade at a density of 50 Kg/m3.