Describes a procedure using a closed cell to evaluate rapidly, at any normal or elevated temperature, the water vapor transfer rate (WVTR) of packaging materials in sheet form, especially barrier films of coated paper. It involves clamping a specimen sheet between a high-humidity chamber (90% RH) and a dry chamber (5% or less RH) and determining the rate of change of humidity in the dry chamber. By means of a calibration curve these dynamic test results can be converted to grams of moisture per square meter-day.