Water vapor permeability is a measure of the passage of water vapor through a material. It is also known as water vapor transmission rate or moisture vapor transmission rate, It is basically the mass of water vapor transmitted from an area measured in a certain unit of time under certain conditions of temperature and humidity. Water vapor permeability is a result of the vapor pressure difference between two surfaces at a given humidity and temperature.
Vapor permeability, often referred to as breathability, describes a material's ability to allow water vapor to pass through it. Unlike bulk water retention, which refers to water in liquid form, vapor permeability relates to water in gaseous form.
The measurement of the water vapor transmission rate is calculated according to the test protocol defined in the ASTM E96 standard developed by the American Testing and Materials Organization (ASTM). This method shows how much moisture can pass through a barrier in a 24-hour period (ASTM E96-00e1 Standard test methods for water vapor transmission of materials). The test methods described cover the determination of water vapor transmission in materials for which water vapor transmission is important, such as paper, plastic film, other board materials, fibreboard, gypsum and gypsum products, wood products, and plastics. These test methods are limited to samples not exceeding 32 mm in thickness.
This standard defines two basic methods for measuring permeability: the desiccant method and the water method. Both methods include wet service conditions on one side and low humidity service conditions on one side and high humidity service conditions on the other. However, harmony is not expected between the results obtained with different methods, the method that is closer to the conditions of use is chosen.
Since the measurements made are affected by the vapor pressure, it is necessary to adjust the vapor pressure across the sample to determine the water vapor permeability. The ASTM E96 standard is used to obtain a relative rating of materials that indicates how resistant each of them is to allowing water vapor to pass through.
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