eVent® is a proprietary waterproof/breathable membrane fabric. Like all waterproof/breathable membranes, its micro-fine pores allow the passage of water vapour, but not liquid. There are a number of waterproof breathable fabrics available, all with different characteristics. Traditionally, waterproof/breathable PTFE (polytetrafluoroethylene) membranes got clogged easily by body oils, which rendered them useless after a little use. The solution was to add a PU backing to protect the PTFE membrane. When thin enough, PU still allows vapour transmission, but at a much lower rate. Some manufacturers avoided the membrane and just used a PU coating. The PU has to be relatively thick to ensure sufficient waterproofing, resulting in even lower breathability.
The genious of eVent was to make the membrane oleophobic (oil repellent), meaning it wouldn’t clog with oils, therefore didn’t need the inhibiting PU layer. At this juncture, eVent® is the most breathable waterproof (20,000mm) fabric we know of. It is sufficiently breathable that a pressure gradient can force air through it.
We use eVent in this way as a purge valve in the bottom of our eVac Dry Sacks and eVent Compression Sacks.
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