To find out whether a fabric is breathable, we test its evaporate resistance or RET (ISO 11092 standard). The lower the fabric's resistance, the more moisture is evaporated, and therefore the higher the fabric's breathability.
We consider that if:
RET < 9 = very breathable fabric
9 < RET < 12 = breathable fabric.
The values for the Trek 500 jacket’s breathability are as follows:
> More resistant fabric: RET < 12 (or 3621 g/m²/24 h, JIS1099A standard)
> Lightest fabric: RET < 8 (or 2972 g/m² /2