MVTR Explained

MVTR — moisture vapour transmission rate, also written WVTR for water vapour transmission rate — is a mass flow. It states how many grams of water vapour crossed one square metre of fabric during the test period, conventionally normalised to 24 hours. A figure of 20,000 g/m²/24h means the sample passed the equivalent of 20 kilograms of water vapour per square metre per day under the conditions of that particular test.

That framing already contains the catch. The number is not a property of the fabric alone; it is the outcome of a fabric, a method and a set of temperature and humidity conditions. Change the conditions and the same fabric produces a different figure, legitimately.

The dominant family is the gravimetric cup test. ASTM E96/E96M covers determination of water vapour transmission rate by two basic methods: the desiccant method, where a dish of drying agent is sealed under the fabric and gains weight, and the water method, where a dish of water loses weight through it. Inverted variants place the water in contact with the fabric.

JIS L 1099 is the Japanese standard, and it is behind a large share of the figures quoted for outdoor fabrics. Its method A-1 uses calcium chloride desiccant at 40 °C and 90% relative humidity with air movement above the specimen kept below 0.8 m/s, and reports in grams per square metre per hour. Method B-1 uses a potassium acetate inverted cup and is applied specifically to waterproof textiles, which is why it tends to generate the largest headline numbers.

ISO 15496 is a third route, and unusually candid about its own scope: it describes a comparatively simple method intended to give manufacturers a recognised procedure for in-plant quality control.

This is not our inference. ASTM E96/E96M's significance clause states that a WVTR result obtained in one method under one set of conditions cannot be used to predict the result that would be obtained using the same method under different conditions, or using the other method.

ISO 15496 goes further and states that its results cannot be compared with other dry desiccant methods because they will not correlate, referring the reader to an annex for the reasons. It also states that it is not applicable for classifying water vapour resistance against values relating to physiological effects specified in product standards, and that it has limitations relative to ISO 11092.

The practical size of the effect is visible in a document that does disclose its methods. Gore's technical sheet for its PYRAD protective fabrics states requirements of not less than 600 g/m²/24 hours under ASTM E96-13 procedure B, and not less than 3,600 g/m²/24 hours under procedure BW. One standard, two procedures, a sixfold difference in the number.

In our verified database, Rab publishes 20,000 g/m²/24hrs for the men's Downpour Mountain and names no test standard, no procedure and no test conditions. That is not a criticism of the fabric; it is a statement about what can be concluded from the figure, which is: less than it appears.

Patagonia names MVTR as one of the measures its H2No programme uses, but publishes neither a value nor a method on its own pages. The North Face's technology page explains the FUTURELIGHT membrane without an MVTR figure. Helly Hansen publishes none for Helly Tech. Arc'teryx and the GORE-TEX platform generally publish no breathability figure at all.

Pertex is worth quoting here because it argues against its own metric: its Shield Air page states that standard MVTR tests do not capture that fabric's full in-use comfort benefit. A fabric maker saying the industry number understates its product is a useful corrective to reading MVTR as a ranking.

You will find tables across the outdoor web presenting 5,000 g/m²/24h as poor, 10,000 as good and 20,000 as excellent. We searched for a standard, testing institute or published methodology behind those thresholds and did not find one. Without a named method they are not even internally consistent: a 10,000 figure from the potassium acetate inverted cup and a 10,000 figure from an ASTM desiccant cup do not describe the same fabric behaviour.

What is true is that the bands describe roughly where marketed fabrics sit, because brands select methods that flatter their products and because the market has converged on round numbers. That makes them a description of labelling conventions, not of performance.

Our position is that the honest way to read an MVTR figure is as follows: with a named method and conditions, it is one useful data point about the fabric. Without them, it tells you the brand believes the fabric is competitive. Neither reading supports ranking two jackets from different brands.

Treat a published MVTR as a rough indication of intent. A brand quoting a high figure is signalling that the fabric is aimed at active use; a brand quoting nothing is not signalling that the fabric is poor, since some of the most respected laminates publish nothing at all.

Compare figures only within one brand's own range, where the method is likely to be consistent even when unstated. That gives you a defensible reading of which of that brand's fabrics is more permeable, which is what most buyers actually want to know.

Then weight the things that are verifiable: layer construction, venting, hood and cuff design, and how maintainable the outer fabric is. On a wet day at effort, those decide comfort more reliably than any figure on a hangtag.