Moisture Vapor Transfer (MVT) in waterproof fabrics occurs by two processes: Driving Force is the difference in the level of heat and humidity on one side of the material compared to the other side.
The weight is then extrapolated to show the number of grams of sweat passing through a square meter fabric in 24 hours.
A desiccant, potassium acetate, is put into a cup and sealed with a piece of ePTFE (Teflon/Stedfast/Gore-Tex film).
The weight is then extrapolated to yield the number of grams of fluid passing through a square meter of fabric in 24 hours.
While B-2 is a good test, it eliminates the effect of fabric when in direct contact with water.
The lower the Ret value, the less resistance to moisture transfer and therefore higher breathability.
They had real people wear garments made with the fabrics of varying Ret values and work out on a treadmill.
They gathered the comments of the testers and correlated this to the Ret values of the fabrics and came up with a Comfort Rating System.
Typically hydrophobic coatings perform better than hydrophilic laminations on the Ret and A1 Test.