Moisture permeability testing is used to evaluate the ability of a fabric to transmit water vapor under certain conditions. A moisture permeability cup containing a desiccant or water and sealed with the fabric sample is placed in a sealed environment with specified temperature and humidity. The moisture permeability rate and moisture permeability are calculated based on the mass change of the moisture permeability cup (including the sample and the desiccant or water) over a specified time. Moisture permeability rate represents the mass of water vapor passing vertically through a unit area of the sample within a specified time under specified temperature and humidity conditions on both sides, expressed in grams per square meter per hour [g/(m²·h)] or grams per square meter per 24 hours [g/(m²·24h)]. Moisture permeability rate represents the mass of water vapor passing vertically through a unit area of the sample within a specified time under specified temperature and humidity conditions on both sides, with a unit water vapor pressure difference, expressed in grams per square meter per pascal per hour [g/(m²·pa·h)].
Higher values for both indicators indicate better moisture permeability of the fabric. The main difference between GB/T12704.1–2009 "Textiles - Test Methods for Moisture Permeability - Part 1: Hygroscopic Method" and GB/T12704.2–2009 "Textiles - Test Methods for Moisture Permeability - Part 2: Evaporation Method" lies in the following: in the hygroscopic method, a desiccant is placed in the permeation cup, while in the evaporation method, distilled water is placed in the permeation cup. The evaporation method is further divided into the upright cup method and the inverted cup method; the inverted cup method is only applicable to waterproof and breathable fabrics. In the standards mentioned above, there are multiple options for the temperature and humidity conditions of the sealed environment. Therefore, using the same test method on the same sample, different temperature and humidity conditions will yield different results.
Exercise generates a large amount of sweat, while outdoor activities inevitably involve wind and rain. This presents a contradiction: the body must be able to prevent rain and snow from soaking through while also being able to expel sweat effectively. Fortunately, the human body emits water vapor in a single-molecule state, while rain and snow are aggregated liquid droplets, and their sizes differ significantly.
