MVTR (Moisture Vapour Transmission Rate)
MVTR is the mass of water vapour that passes through a square metre of fabric in 24 hours, written as g/m2/24h. Higher is more breathable, but only within a single test method.
The unit looks standard and is not. At least four common methods produce substantially different MVTR figures from the same piece of fabric, and most brands do not say which one they used.
The methods, and what they do to the number
| Method | How | Effect |
|---|---|---|
| JIS L 1099 A-1 | Upright cup, calcium chloride desiccant | Conservative, lowest figures |
| JIS L 1099 B-1 | Inverted cup, potassium acetate, fabric in contact with liquid | Aggressive, often several times an A-1 result |
| ASTM E96 procedure B | Inverted cup, water | Mid range |
| ASTM E96 procedure BW | Inverted, fabric touching water | Higher than procedure B |
A brand can raise its published breathability substantially without changing the fabric at all, simply by testing to a more generous method. That is a choice, not a fraud, and it is rarely disclosed.
What a good number looks like
Because the method drives the figure, bands are only meaningful within a method. As a rough guide under the aggressive inverted-cup methods, 10,000 g/m2/24h is a reasonable floor and 20,000 is good. Under upright-cup methods the same fabrics would report far lower and still perform identically.
The practical limit
None of it matters much when the outer face wets out, because a saturated fabric stops passing vapour whatever its rating. Nor does it matter in hot humid air, where there is very little vapour pressure gradient to drive the transfer. Mechanical ventilation beats material breathability under real exertion.
Related terms
- Waterproof and breathability ratings explained: the full comparison problem with sources
- RET: the resistance-based alternative
- Wet-out: why the rating stops mattering in rain
- Glossary: all terms