A measure of paper or paperboard used for packaging that determines its efficacy as a moisture barrier. The purpose of many types of packaging materials is to prevent the penetration of moisture, ensuring that the material contained in the packaging is not damaged. The water vapor transmission rate is the weight of the water that permeates from one side of a one-square-meter area of paperboard to the other in a one-day period. Variations in temperature and relative humidity can provide an accurate moisture-transmisison profile for a particular paperboard. Various additional materials can be added to a packaging paper or paperboard—such as wax, aluminum foil, plastic, etc.—to increase its effectiveness as a barrier against moisture.