Pertex is a durable, versatile and wind resistant wicking material that offers high breathability and is used in a wide range of products which typically include bivvy bags and sleeping bags, as well as down jackets and other water resistant clothing.
It was originally created and manufactured by Perseverance Mills Ltd., based in Manchester, England. However, when the company went into liquidation back in 2005, the rights to Pertex were purchased by Japanese company Mitsui & Co.
The breathability of Pertex comes from it’s use of a capillary action, an action which works to move moisture from larger filaments to smaller ones without passing through the air, which uses denier gradients, combining two yarns of distinctly different properties. The inner yarn uses larger filaments, whilst the outer yarn uses smaller ones.
The construction of Pertex is based on the combination of very fine fibres used on the outer layers and thicker fibres on the inner layers. The thinner, finer fibres have a greater surface area than thicker fibres, allowing moisture to spread, causing them to evaporate far more quickly. Moisture is then directed from the inside to the thinner fibres from thicker fibres, using a capillary action. What drives this action is the temperature difference between your body heat, which increases the head inside the bag, to the air temperature outside.
Pertex is windproof thanks to the tight weave used on its outer surface. You may think that this would impede the breathability of Pertex, however the tight weave plays no part in this as the breathing ability of this material is not based on air passing through this weave, instead, it works by moisture being absorbed by the inner yarn and then passed to the outer yarn, passing to the outside air.
Pertex is typically not treated with any additional waterproof coating as this tends to trap moisture in the yarn instead of letting it pass, impeding the materials ability to work as it is designed to. It’s important to add that Pertex is considered water resistant, not waterproof, which is why it’s most commonly used to as an outer shell for bivvy bags, helping to prevent water build up which would saturate and soak through other waterproof materials, preventing them from doing their job.