Over the last 40 years, the makers of cold-weather garments have relied on so-called "breathable" waterproof fabrics. Yet, even today, people are straining to blow air through such fabric to find out if it really is breathable. The trouble is, air does not pass through a breathable membrane (or coating)
Rather, the principle is based on permeability to water vapour , what makes up sweat, rendered possible by micropores or the polymer’s highly hydrophilic nature. The membrane's ability to release water vapor, driven by a vapor pressure differential, favors comfort by reducing sweat condensation inside the garment while preventing the wind from passing through it. Being drier, the body is less likely to cool down during periods of rest or low activity. In other words, it's when you catch your breath that your breathable fabric plays its role best.
A technology that is certainly relevant, but which, like all the others, has its limits. You can see this with any "breathable" piece of clothing as soon as you exercise moderately. This is the reason why some garment makers chose to introduce vents in their design. In the end, when you get too warm, nothing beats unzipping your coat.
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