The breathability of a component is measured by its "Thermal Evaporative Resistance" (RET).
This refers to the fabric's capacity to allow the water vapour generated by the body during exercise to escape, so that water doesn't accumulate on the skin. The lower the RET value, the more breathable the fabric. RET ≤ 6: very breathable fabric
RET ≤ 12: breathable fabric
RET ≤ 20: fabric with low breathability
This product has an RET rating = 7.4