> For the complete documentation index, see [llms.txt](https://docs.betterbuilding.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.betterbuilding.io/support-and-training/other-support/faqs/design-and-specification/materials/vapour-resistance.md). # 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. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.betterbuilding.io/support-and-training/other-support/faqs/design-and-specification/materials/vapour-resistance.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.