Made from fiber that doesn't absorb sweat but conducts it to the outer layers, where it evaporates. The direction is set by the moisture gradient and fiber geometry: liquid pulls from wet toward dry, toward where the capillary channels are narrowest and the surface area greatest.

Moisture travels along the surface of the fiber without soaking in. In practice, a property of hydrophobic fibers only.

Water vapor moves from higher vapor pressure toward lower, and vapor pressure rises with temperature. Next to the skin it is warm and humid; further out, cooler and drier — so vapor pushes outward. The body works like a heat pump. And because evaporation binds heat, it cools the skin at the same time.

Not ventilation, but the rate of water vapor transmission, measured on the MVTR scale (g/m² per 24 h). Under 5,000 is a modest figure, around 10,000 is considered good, over 20,000 is enough for running and the mountains, and the best membranes reach about 40,000.

Synthetic fiber — polypropylene, polyester, polyamide (e.g. ECONYL®).

Natural fibers are mostly hydrophilic: they absorb moisture into themselves. Cotton binds the most; linen dries faster and feels cooler. Bamboo/viscose is regenerated cellulose that binds as much moisture as cotton, although it is often marketed as moisture-wicking.

When cotton binds moisture, air gets pushed out — and air is the insulating layer that keeps a human warm. Soaked fabric moves heat off the skin roughly 25 times faster than air: water conducts heat at about 0.6 W/(m·K), air at about 0.026. In mountain conditions, the heat loss that follows can be fatal.

Of the natural fibers, merino wool manages moisture best — by a different mechanism than synthetics. It binds moisture as vapor and stays warm even when wet.

No. In quality technical underwear the pattern and the knit matter as much as the fiber: the best hydrophobic fiber fails if the knit or the cut is done poorly