A polymeric material is being considered as a coating film to protect an optical storage disk from water vapor damage. Suppose we need to protect a disk surface from an ambient medium that will be at 45% relative humidity at 25° C. How thick must the film be if the concentration at the disk/protective film interface is to be below 0.07 wt% water in the film for a period of 5 years? The film density is 1.224 g/cm3. At 100% relative humidity (25 C) the equilibrium uptake of water in the film is 6.2 wt%. The absorption isotherm for water in this polymer is linear. The diffusion coefficient of water in the film is 9.5 x 10 cm2/s.