DIN EN ISO 15106-1: Plastics – Film and sheeting, determination the water vapour transmission rate,
Part 1: Humidity detection sensor method

Water Vapour Transmission Rate (WVTR), which is also known as moisture vapour transmission rate (MVTR), is the rate of a steady-state water vapour flow per unit time through a unit area of a body under specified temperature and partial pressure conditions.  It is a measure of the permeability of a material and is widely expressed in many types of units (see Versaperm Conversion Units).

ISO 15106 is a standard test method for measuring the vapour permeability of a film or plastic sheeting. The method uses a dry chamber which is divided from a wet chamber by a sheet of the film or sheet under test. One side is saturated with water vapour at a known temperature, the water vapour permeates through the vapour and the resultant humidity created on the other side is measured by a humidity sensor. The time it takes for the humidity to increase by a given amount determines the water vapour transmission rate (WVTR), which is the inverse of the vapour permeability. The system operates at a specified relative humidity.

The WVTR is critically dependent on temperature and relative humidity ( partial pressure) and these must be stated-be recorded.  Care must also be taken to ensure there are no wrinkles or slackness in the specimen/.