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When we talk about jackets being 'breathable' we are referring to the extent to which the garment allows moisture vapour to escape.
In a ‘non-breathable’ garment even light activity will result in you quickly overheating. Your body’s response to exertion is to sweat, but because there is no way for that vapour to escape, the interior of the jacket becomes very wet. This overheating is soon followed by a dramatic reduction in body temperature as the trapped moisture acts a thermal conductor transferring your body heat nearly 25x faster than that of dry clothing. The result would, at best, leave you feeling very uncomfortable and in extreme cases lead to dehydration or even the onset of hypothermia.
When considering the problem of breathability our designers will typically use either Microporous or Hydrophilic technology.
Tiny, microporous holes exist in the membrane which are small enough to allow steam to escape, but too small to result in liquid water permeating from the outside.
Hydrophilic membranes work by conveying moisture at a molecular level. The process relies on the pressure inside the garment exceeding that outside, so wearers have to be active for the membrane to become breathable. As a result jackets that use hydrophilic membranes adapt to your activity level and retain heat well.
It’s worth mentioning that all breathable systems rely on a difference in humidity and pressure between the inside and the outside of the garment in order to work. This means that even the most breathable system will be less efficient when the outside atmosphere is full of water (i.e when it’s grim out).
There are many ways to measure breathability. Our standard method is to record the amount of evaporation through a square metre of fabric over a 24 hour period eg 20,000mm/sq m/24hrs. Even the most breathable systems will be less efficient when the outside atmosphere is full of water though, so aside from lab-based measurements, thorough field testing plays a central role in the design and development of our breathable garments.