AATCC 204-2019 is similar in concept to other "upright cup" methods; however, it was developed specifically to evaluate comfort of vapor permeable textiles used in outdoor products. The temperature differential between the heated bath and the ambient air creates a driving force to promote water vapor transmission. This driving force is likely greater than in actual use, but it serves as an effective condition to differentiate materials by their water vapor transmission performance. Measuring water vapor transmission through test fabric as a percentage of transmission through control paper improves precision of the results, even with small fluctuations in test time or temperature. The control paper can also serve as a verification tool to ensure consistent results. Purpose and ScopeThis test method is used to determine the relative rate of water vapor transmission of textiles.