Pertex

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Pertex is a versatile, wind resistant, durable wicking fabric with more breathing ability than waterproof membranes. It is typically found in winter clothing and sleeping bags.

The breathability of Pertex comes from the fabrics use of capillary action using denier gradients. Pertex combines two yarns with different properties. The inner yarn has larger filaments and outer yarn has smaller filaments. Capillary action moves moisture from larger filaments to smaller filaments without passing through the air.

Imagine a bundle of very fine fibres next to an equal weight bundle of thicker fibres. The same weight of material can make more thin fibres than an equal amount of thick fibres, thus there are more thin fibres on the outer side (and a tighter weave) than there are thick fibres on the inside. The bundle of finer fibres has greater surface over which moisture can spread - so they will evaporate moisture more quickly. The moisture from the inside is directed to the thinner fibres from thicker fibres by, again, capillary action. The driving force behind all this is the temperature difference between your own body heat on the inside and the air temperature on the outside.

Pertex is windproof because of the tight weave of the outer surface. The breathing ability of the fabric is not based on air passing through the weave of the fabric, but by moisture absorbed in the inner yarn passed to the outer yarn (which passes from the outer yarn to the outside air). Adding a waterproof coating would trap the moisture in the yarn, instead of letting it pass through the yarn and evaporate into the air.