To determine whether a fabric is breathable, we measure its evaporative resistance (test based on standard ISO 11092).
The lower its resistance, the more the fabric will let water vapour produced by the body escape, and therefore the more breathable it is.
For example: RET < 9 = extremely breathable fabric.
Machine wash at 30°C. Synthetic programme. Turn the garment inside out. Use minimal laundry detergent. Do not add softener. Line dry in a warm, ventilated area. Do not tumble dry. Do not dry clean. Do not bleach. Do not place on a heater to dry.