Steady state permeability coefficients P of carbon dioxide (CO2) in polystyrene (PS) and PS nanocomposites and their corresponding foams at 0.10 MPa (gauge pressure) and three different temperatures were measured. Permeability coefficients of foams are about 1 order of magnitude higher than those of corresponding plates. Three PS nanocomposites were used: PS + 5% 20A (treated nanoclay provided by Southern Clay), PS + 5% CNFs (carbon nanofibers), and PS + 5% MHABS (nanoclay treated with 2-methacryloyloxyethylhexade-cyldimethylammonium bromide). For both plates and foams, the permeability coefficient decreases considerably with addition of nanoparticles. Foam structure is shown to play a more important role than nanoparticles in determining the permeability coefficient values for nanocomposite foams.