This study elucidates the discrepancy in gas permeability between bulk films and asymmetric membranes of semi-crystalline cellulose acetates (CAs) from perspectives of thickness-confinement and crystallization suppression. CAs are the workhorse membrane materials for industrial CO2/CH4 separation. Bulk films of CAs often exhibit CO2 permeability values of 1.8-6.6 Barrers at 35 degrees C, which correspond to permeance values of 36-132 GPU for asymmetric membranes with assumed selective layers of 50 nm. However, commercial CA membranes can have CO2 permeance values as high as 200 GPU with a CO2/CH4 selectivity comparable to the bulk polymers. We hypothesize that as the CA films become thinner, the thickness confinement inhibits crystallization and thus increases gas permeability while retaining gas selectivity. To validate this hypothesis, freestanding cellulose diacetate (CDA) films with thicknesses ranging from 218 nm to 120 mu m were prepared, and their crystallinity was determined using Differential Scanning Calorimetry and Wide-angle X-ray Diffraction analysis. Gas solubility and permeability can be satisfactorily correlated with the crystallinity using the empirical equations available in the literature. We demonstrate that the micro-or nano-confinement and dynamics in thin-film polymers are instrumental in understanding gas transport properties of industrial asymmetric membranes.