Breathability

Breathability is the ability of a fabric to allow moisture vapor to be transmitted through the material.

Air Permeability is the ability of a fabric to allow air to pass through it. While Air Permeable fabrics tend to have relatively high moisture vapor transmission, it is not necessary to be Air Permeable to be breathable.

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. Also known as the Differential Pressure. By 2nd law of thermodynamics moisture will move towards dry. Therefore, warm, moist air will flow towards cold, dry air until there is an equilibrium.

Due to body heat and moisture there is almost always higher heat and humidity inside a clothing system. This creates a differential pressure forcing the heat and humidity toward the outside. The greater the difference between the heat and humidity inside the clothing system and the outside, the greater the Differential Pressure to push that heat and humidity out.

Also known as JIS L 1099, JIS Z 0208, ISO 2528, Desiccant Method of ASTM E96, JIS K 6328 (JIS is short for Japanese Industry Standards). The A-1 method uses Calcium Chloride solution to simulate sweat while A-2 method uses just water. A desiccant, calcium chloride, is put into a cup. A piece of fabric is then secured over the cup and placed in a controlled environment. Then after period of time the cup is weighed to see how much water has been “pulled” into the cup through the fabric. The weight is then extrapolated to show the number of grams per 24hours per sq meter of fabric that will pass through the fabric.

Typical maximum with current technologies are in 15,000 range. High-performance fabrics may get A1 test scores in 10,000 to 15,000 gr/24hrs range and usually shows that a fabric has a fairly quick release of moisture, but may not be the best over longer periods of use.

Also known as JIS L 1099 is similar to the ASTM E96-BW test method. A desiccant, Potassium Acetate, is put into a cup and sealed with a piece of ePTFE (Teflon/Gore-Tex film). The fabric to be tested is then placed over the cup with the fabric side to the cup.

The cup is then inverted into a pan of water. Then after period of time the cup is weighed to see how much water has been “pulled” into the cup through the fabric. The weight is then extrapolated to show the number of grams per 24hours / per sq meter of fabric that will pass through the fabric.

The B-1 variant of test method puts membrane in direct contact with water while B-2 variant adds an ePTFE film between the water and the fabric. While B-2 is a good test, it eliminates the effect of fabric when in direct contact with water. When sweat condenses on the inside of a fabric with a hydrophilic laminate, the lamination will actively pull the water through the fabric reducing condensation. This can be a tremendous addition to the comfort of the user. The B-2 test is also best used for non-waterproof fabrics, so the water in the pan does not pass directly through the uncoated fabric.

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Wikipedia