RET Explained

RET — resistance to evaporative heat transfer, written Ret in the standard — describes how much the fabric obstructs evaporative heat leaving the skin. Because it is a resistance, the scale runs the opposite way from most jacket numbers: a lower RET means less obstruction and therefore better breathability.

The unit, m²·Pa/W, encodes what is happening. It relates the area of fabric and the vapour-pressure difference across it to the evaporative heat flow that results. This is a physiological framing: the question is not how many grams got through, but how much the fabric interferes with the body's own cooling mechanism.

ISO 11092:2014 is the standard behind it. Its scope covers the measurement of thermal resistance and water-vapour resistance under steady-state conditions for fabrics, films, coatings, foams, leather and multilayer assemblies used in clothing and similar products. Thermal resistance Rct and water-vapour resistance Ret are two sibling outputs of the same procedure.

The apparatus is a heated, porous plate that simulates sweating skin. It is held at a controlled temperature in a controlled airflow, water is supplied so that the surface evaporates, and the fabric sample is laid over it. The electrical power needed to maintain the plate against the evaporative heat loss, with and without the sample, gives the resistance the fabric adds.

The standard notes that measurement is bounded by the equipment as well as the sample: it names apparatus-dependent ceilings, for example 2 m²·K/W for thermal resistance and 700 m²·Pa/W for water-vapour resistance.

We deliberately do not quote the exact plate temperature, air temperature, humidity and air speed the standard specifies. Those numbers are widely repeated across the web, but they sit in clauses of ISO 11092 that are behind the standard's paywall, and we could not read them at the primary source. Rather than restate figures we have not verified, we say so.

EN ISO 24232:2025, the European standard for protective clothing against rain, classifies water vapour resistance in four classes. Class 1 covers Ret above 40, class 2 above 25 up to 40, class 3 above 15 up to 25, and class 4 covers 15 or below. Class 4 is the most breathable.

This matters for two reasons. It is a published, normative classification rather than a marketing convention, and it shows how coarse a genuinely standardised grading is: everything from a high-performance mountain laminate to a merely decent one lands in class 4 together.

The same standard also classifies resistance to water penetration in four classes, and the two sets of classes run in opposite directions — higher is better for waterproofness, lower is better for Ret. That reversal is a common source of confusion when reading a garment's pictogram.

Almost every outdoor retailer publishes a version of this table: RET under 6 extremely breathable, 6 to 13 very breathable, 13 to 20 breathable, above 20 poor or unsuitable for sustained activity. We set out to find its origin.

It is not in ISO 11092, EN ISO 24232, ASTM or JIS. The most authoritative trail we found runs through Gore's own technical sheet for its PYRAD protective fabrics, which footnotes its breathability rating as a Hohenstein rating system using Ret results and human subject feedback on comfort, citing a specific page of the Elsevier textbook Waterproof and Water Repellent Textiles and Clothing (Williams, 2018).

So the bands are traceable, but not to a standards body: they originate with a private German testing institute's comfort scheme, and by that footnote's own description they blend measured Ret values with subjective wearer feedback. We could not read the cited book page, and found no Hohenstein publication stating the bands directly.

Our conclusion, stated plainly: the bands are a reasonable industry convention with a genuine research lineage, and they are not a scientific classification. We will not reproduce them as though ISO or CEN published them, and neither should anyone else. This is exactly the kind of unsourced outdoor-industry folklore that gets repeated until it looks official.

Published RET figures are rare on consumer product pages. In our verified database, Montane publishes a RET of under 3.5 for the women's Phase Nano. Montane does not name the test standard, so while RET as a quantity is defined by ISO 11092, we cannot confirm from the product page that this figure was produced to it.

Patagonia names RET, alongside MVTR, as a measure used in its H2No programme, and publishes no value for either. Most GORE-TEX garment pages publish no breathability figure at all. Professional and workwear garments certified to the European rain standard are where a reader is most likely to encounter an explicit Ret class, because the standard requires it. Note that this standard changed identity recently: EN ISO 24232:2025 supersedes EN 343:2019, which the CEN register records as withdrawn with effect from 22 October 2025. The four Ret classes are unchanged, and we have read them in table 3 of ISO 24232:2024 itself.

Where a manufacturer publishes a figure without a method, our records say so, and we do not rank that figure against another brand's number. Where no figure exists, our records read "Not publicly specified by the manufacturer".

A low RET is genuinely informative about a fabric's potential during hard, sustained effort, which is where evaporative cooling matters most. It is the figure we would rather see than an MVTR number, because it has one dominant standard behind it and a physiological rather than a gravimetric framing.

But it remains a steady-state laboratory measurement of a flat fabric sample. A real jacket adds a lining, seams, a pack pressed against it, a face fabric that can wet out, and a wearer whose output swings. None of that is in the number.

So our reading order stays the same: what the jacket is for, then construction and venting, then maintainability of the outer fabric, then whatever figures the brand chose to publish and the methods it chose to name.