RET at a glance
- Test
- ISO 11092
- Extremely breathable
- Under 6
- Barely breathable
- Over 20
What RET measures
RET stands for resistance to evaporative heat transfer. A heated plate simulating skin is covered with the fabric, and the test measures how much the fabric impedes evaporation from it. Because it measures resistance, a low number is the good one, which catches people out constantly.
Why it is more trustworthy than MVTR
Its advantage over MVTR is that there's one method, ISO 11092, usually run by the Hohenstein Institute. Two RET figures from two brands are genuinely comparable, which is not something you can say about MVTR.
The bands worth knowing
The bands are widely quoted and worth knowing. Under 6 is extremely breathable, the territory of high-end mountain shells. Between 6 and 13 is very breathable and covers most good hiking shells. Between 13 and 20 is breathable enough for moderate effort. Above 20 the fabric is effectively not breathing, which is where cheap coated rainwear sits.
What this means when you're choosing
- An RET figure at all, which relatively few brands publish
- Under 6 for genuinely breathable, under 13 for usable in effort
- That you are reading RET and not MVTR, since lower is better here
The common mistake
Reading it like a score
Higher is worse. A jacket advertising RET 4 is more breathable than one advertising RET 12, which is the reverse of how MVTR, hydrostatic head and almost every other spec on the hangtag work.
Common questions
Is a lower or higher RET better?
Lower. RET measures resistance to moisture escaping, so less resistance means a more breathable fabric.
Why don't more brands publish RET?
The test costs money and the number is unflattering for cheaper fabrics. MVTR is easier to produce and easier to make look impressive, which is why it dominates hangtags.
Where this is defined
- ISO 11092, thermal and water-vapour resistance of textiles, International Organization for Standardization