Diffusion coefficients (D) of blowing agents and air in cellular plastic insulation are the key parameters in determining the thermal quality and dimensional stability of the foams. Measurement methods available for this purpose are briefly discussed. An apparatus based on the so called diffusion chamber method has been designed in this study. Strategies for preparing samples and for calculating the D-values are suggested in order to save measurement time. Diffusion of carbon-dioxide (CO2), oxygen (02) and nitrogen (N2) within five types of zero ozone depletion potential (ODP) foams will be shown. Results also show the dependence of D-values on temperature and foam densities. The D-temperature curve follows the Arrhenius type relationship. Different density foams at same temperature show different Ds, but no identical relationship is reached. Foams produced by different companies have dramatic differences in gaseous diffusion property.