Vapour Resistance

Vapour resistance testing measures how well a material resists the passage of water vapour. It’s commonly applied to building materials, membranes, textiles, films, and packaging to assess breathability and moisture control, crucial for performance and durability.

One of the most widely recognised standards for this testing is ASTM E96, developed by the American Society for Testing and Materials (ASTM). It outlines procedures for determining the water vapour transmission rate (WVTR) of materials, using various methods. A common method under this standard is the wet cup method, also referred to as the desiccant method.

In the wet cup test, a material sample is placed over the opening of a sealed cup containing a desiccant such as anhydrous calcium chloride. The desiccant draws moisture through the material, creating a humidity gradient that mimics real-world conditions. As water vapour moves from the more humid side to the drier side, the desiccant absorbs the moisture.

The test measures the weight gain of the cup over time, which reflects the amount of water vapour passing through the material. The WVTR is then calculated based on this weight gain and the surface area of the sample. Results are typically reported in g/m²/day, perm (US customary units), or ng/Pa·s·m² (SI units).

ASTM E96 specifies the procedures for consistent testing, including sample preparation, environmental conditions, and calculation methods. It also includes variations like the inverted cup and calcium chloride methods to accommodate different materials.

While ASTM E96 is a standard in many industries, other international standards such as ISO 15496 and JIS L 1099 also provide valid approaches to vapour resistance testing. For reliable and comparable results, it's essential to follow the test standard precisely and choose the method that best suits the material being evaluated.