Reading Waterproof-Breathable Spec Sheets: HH, MVTR, and RET

Every waterproof-breathable shell ships with at least one lab number. Often three. Those numbers come from narrow, specific test conditions. They do not convert into each other. They do not transfer cleanly across brands. Treat them as starting points, not verdicts.

What follows is an account of what each test actually measures, where the numbers break down, and how to use them when matching a shell to the way you move.

Hydrostatic Head: The Water Column

Hydrostatic head (HH) expresses waterproofing as the height of a water column—in millimetres—that a fabric can support before liquid passes through. A sample is placed over a tube; water pressure rises steadily until three drops appear on the outer face. That height is the rating. A fabric rated 20,000 mm held the pressure equivalent of a 20-metre column before leaking (ChiuVention, c. 2025).

The dominant standards are ISO 811 and AATCC 127. Both use rising pressure and a three-drop failure threshold, but differ in procedural detail. GB/T 4744 and JIS L 1092 also appear on spec sheets; JIS L 1092 contains two distinct test methods within the single designation (Testex, c. 2023). The conversion: 1 mmH₂O = 9.80665 Pa (ChiuVention, c. 2025).

For orientation: tent canopies commonly rate at 3,000 mm or above, tent floors above 5,000 mm, and military-performance fabrics above 10,000 mm—vendor guidance thresholds, not standard requirements (ChiuVention, c. 2025). Shell jackets for sustained precipitation typically land between 10,000 mm and 28,000 mm depending on membrane type and construction.

HH measures membrane and fabric in isolation, under static pressure. Sustained mechanical abrasion—a pack hipbelt pressing a wet shoulder seam—transfers pressure in ways the lab test does not model. Seam tape quality and DWR condition matter alongside the membrane rating.

MVTR: The Cup Tests

Moisture Vapour Transmission Rate (MVTR) is expressed in grams of water vapour per square metre per 24 hours (g/m²/24h). Higher means more vapour passes through. The problem: MVTR is not a single test. It is a family of tests whose members produce results that do not correlate.

The Test Landscape

The main methods in use (SGS / Textile World, 2013):

ASTM E96 runs at 23 °C and 50% relative humidity. Procedure A is an upright cup with solid desiccant, suited to water-permeable fabrics. Procedure BW is an inverted cup with water in contact with the membrane, better suited to impermeable membranes (SGS SafeGuards 162/13, 2013).

JIS L 1099 offers upright-cup methods (A-1 and A-2) at 40 °C, and an inverted-cup method (B-1) at 30 °C with a potassium acetate solution in direct contact with the sample (ScienceDirect Topics). The high temperature and direct-contact conditions of B-1 produce significantly higher numbers than ASTM E96.

ISO 15496 is an inverted-cup, double-membrane method.

BS 7209 uses an upright cup on a turntable.

None is more correct than the others. They model different conditions. The consequence is stark: independent testing reported by a tester on Backpacking Light found results substantially lower than JIS L 1099 B-1 figures and substantially higher than ASTM E96 figures for the same fabrics (Backpacking Light forum, c. 2022). The same thread noted two fabrics commonly sold as equivalent posting JIS L 1099 B-1 values of 19,344 and 78,264—a four-fold difference attributed to variations in face fabric, backer, and bonding rather than the membrane alone.

The divergence is formally documented: two fabrics that both score 10,000 g/m²/24h on JIS B-1 can score 10,000 and 4,000 respectively on JIS A-1 (Wikipedia: Breathability). RET, A-1, B-1, and B-2 results do not correlate with each other.

The Practical Read

When a spec sheet lists an MVTR, the first question is: which method? A figure without a stated method is not useful for cross-brand comparison. PT Outwear advises specifying method alongside the target value—for example, “MVTR ≥ 15,000 g/m²/24h per JIS L 1099 B-1” or “RET ≤ 6 per ISO 11092” (PT Outwear, c. 2026).

A second variable: face fabric weight, weave tightness, and laminate bonding all influence the result. The membrane is not the only factor.

RET: The Sweating Hotplate

Resistance Evaporative Transfer (RET) comes from a different category of test entirely. ISO 11092 (also published as EN ISO 11092:2014) defines a sweating guarded hotplate held at a constant surface temperature while moisture evaporates through the specimen. The instrument measures the power required to maintain that temperature. RET is the water-vapour pressure difference across the specimen divided by the resulting evaporative heat flux per unit area: units are m²·Pa/W (SATRA Bulletin). Lower RET means less resistance and more breathability.

The companion metric from the same standard is Rct—thermal resistance in m²·K/W, measuring insulation rather than breathability. A dry hotplate run produces Rct; a sweating run produces RET (Centexbel).

The Comfort Bands

RET was developed by correlating instrument measurements against human testers’ comfort reports. The resulting bands are approximate, but they map to activity intensity in a way raw MVTR numbers do not (Wikipedia: Breathability):

Decathlon, which publishes ISO 11092 RET values on its jacket spec sheets, uses a similar scale with the first-to-second band boundary at 12 rather than 13 (Decathlon, c. 2021). Treat the boundaries as approximate.

One finding from the comfort study is worth holding: testers could not distinguish between fabrics within the same band. A RET 40 and a RET 55 felt identical in use. The bands are a meaningful unit of resolution. Chasing differences within a band is not productive.

RET and MVTR do not convert into each other. Different test principles, different temperatures, different driving forces (PT Outwear, c. 2026). A high MVTR does not guarantee a low RET.

Air-Permeable Membranes: A Separate Category

Standard MVTR and RET tests assume a membrane that blocks bulk airflow. Air-permeable membranes—pore structures large enough to allow some convective air exchange—move vapour by a different mechanism. Cup tests and hotplate tests do not fully capture this.

Air permeability is measured separately, typically by ASTM D737 or ISO 9237, which report airflow in cubic feet per minute (CFM) or litres per square metre per second at a fixed pressure differential. The practical effect: air-permeable membranes can flush heat during high-output movement in ways that MVTR or RET figures alone do not predict.

Confirmed airflow specifications from manufacturer technical data sheets were not available at time of writing. Any figures in marketing copy should be checked against the stated test standard before comparison. Treat air-permeable membranes as a distinct category—not simply a higher tier of standard waterproof-breathable construction.

Matching Ratings to Activity

The numbers become useful when mapped to actual output level.

Low-output activities—trail hiking at moderate pace, belaying, on-piste skiing—generate modest heat. A RET in the 13–20 range is unlikely to cause distress. An MVTR in the 10,000–15,000 g/m²/24h range (method-dependent) is adequate. HH above 10,000 mm covers sustained rain without seam failure.

High-output aerobic activities—fastpacking, ski touring, technical scrambling—generate heat rapidly and continuously. The RET band matters most here. The 6–13 range is the functional target. An air-permeable membrane may outperform a conventional membrane of nominally similar RET because convective flushing supplements diffusive vapour transfer. HH can sometimes be traded downward in exchange for lower RET or less weight, depending on precipitation intensity and duration.

Stop-start activities with precipitation exposure—alpine climbing, mountaineering—demand both high HH and low RET, which tend to pull against each other. Decathlon states the trade-off plainly: the more waterproof a product is, the less breathable it tends to be (Decathlon, c. 2021). No spec sheet resolves that tension. It is a design compromise the activity and the individual must weigh.

Four Questions for Any Spec Sheet

1. Which HH standard? ISO 811, AATCC 127, JIS L 1092, or another. The number is not portable across standards without this.

2. Which MVTR method? ASTM E96, JIS L 1099 A-1, JIS L 1099 B-1, ISO 15496, or something else. A JIS B-1 figure and an ASTM E96 figure for nominally similar fabrics can differ by a factor of four or more. Without the method, the number is not comparable.

3. Is RET reported? Place it in the comfort-band table and map it to expected activity intensity. This is the most direct route from lab data to real-world use.

4. Is the membrane air-permeable? If so, treat MVTR and RET as partial information. Airflow data under ASTM D737 or ISO 9237, if published, adds to the picture.

Spec sheets are lab snapshots. Face fabric construction, seam tape quality, DWR condition, and fit all influence performance in ways no single number captures (T3, c. 2023) (Fiona Outdoors, 2026). The numbers narrow the field. They do not close it. When a brand omits the test method from a rating, ask for the technical data sheet—or treat the figure as unverifiable.