A buyer compares two membrane laminates. Supplier A's data sheet says 9,000. Supplier B's says 20,000. Supplier B looks more than twice as breathable — until you read the small print and find that the two figures come from different methods, different conditions and, possibly, different units.
Water vapour transmission is one of the properties where the method is part of the number. Two methods come up often in textile and coating work and are frequently confused with each other and with ASTM E96: JIS L 1099 and ASTM D1653.
JIS L 1099: one standard, several different tests
JIS L 1099 is the Japanese standard for water vapour permeability of textiles; its current edition is JIS L 1099:2021. It contains two families of methods, and they work on different principles.
| Method | Principle | Commonly quoted conditions |
|---|---|---|
| A-1 | Calcium chloride desiccant in an upright cup, specimen over the top | 40 °C, 90 % RH |
| A-2 | Water in an upright cup | 40 °C, 50 % RH |
| B-1 | Saturated potassium acetate in an inverted cup; specimen in contact with water | Short test, typically around 15 minutes |
| Other B methods | Variants of the B approach for particular materials | See the standard |
The A methods are classic gravimetric cup methods: the cup gains (A-1) or loses (A-2) mass over a period of hours, and the transmission rate follows from the slope.
The B-1 method is different in kind. The potassium acetate solution is a very strong moisture absorber, and the specimen sits against liquid water rather than humid air. The driving force is different, and the test is short. A-1 and B-1 can therefore rank the same set of materials differently — patent literature on breathable fabrics goes as far as stating that A-1 and B-1 results do not correlate. There is no factor that converts one into the other.
Trap 1: the method letter is missing
"Moisture permeability 15,000 g/m²·24h (JIS L 1099)" tells you almost nothing. Is it A-1 or B-1? A data sheet that omits the method letter cannot be compared with anything, including a data sheet from the same supplier last year.
When you write a specification, name the method: "JIS L 1099:2021, method A-1". When you receive a figure, ask for the method before you compare.
Trap 2: per hour or per 24 hours
Gravimetric methods measure mass change per area per time, and both g/(m²·h) and g/(m²·24 h) are in use. Between them is a factor of 24 — large enough that nobody should miss it, small enough to be missed when a number is copied into a comparison table without its unit. Convert everything to one unit, and keep the unit next to every figure.
Trap 3: comparing JIS with ASTM E96
ASTM E96 also has desiccant and water methods, but at different conditions (the common procedures run at 23 °C and 50 % RH) and with different cup geometry. A JIS A-1 result at 40 °C and 90 % RH has a much larger humidity driving force. The methods measure the same phenomenon, but they are not interchangeable and there is no fixed conversion between them.
For clothing comfort research, a further step is the sweating guarded hotplate of ISO 11092, which reports water-vapour resistance (Ret) rather than a transmission rate. It answers a related but different question again.
ASTM D1653: written for coatings, not fabrics
ASTM D1653 is often listed next to textile methods, but its scope is the water vapour transmission of organic coating films — paint, varnish, lacquer and similar — either as free films or applied to porous substrates. It has two methods:
- Method A, dry cup — preferred where high relative humidity is not expected near the barrier.
- Method B, wet cup (the "Payne cup") — preferred where high humidity is expected.
The standard is explicit on three points that apply equally to coated textiles and leather finishes:
- Agreement should not be expected between results from different methods or test conditions; choose the method closest to the conditions of use.
- Results are only comparable when the coatings are tested under the same closely controlled temperature and humidity and at equal film thickness.
- Transmission is not a linear function of film thickness, temperature or relative humidity.
Point 2 is the one most often ignored. A coating supplier who quotes a transmission rate without the dry film thickness has given you half a result.
What a laboratory should do
- Ask which method the specification really means, including the letter (JIS) or method (ASTM).
- Run the specified method at the specified conditions rather than a nearby method you happen to have set up.
- Report units in full, with the time base.
- For coatings, measure and report film thickness alongside the transmission rate.
- Use a reference material that you test regularly, so drift in cup sealing, chamber humidity or air flow shows up in your own data. Some fabric methods, such as BS 7209, build a reference fabric into the procedure.
Equipment note
Our eight-cup water vapour permeability tester for clothing fabrics includes water containers, specimen holder rings and a reference fabric arrangement. Our gravimetric WVTR testers — single-station, six-position with a built-in balance, and eight-position with an external balance — run the cup methods, and all of them are produced in our own workshop in Istanbul.