You pull on your rain jacket, step out into the storm, and within half an hour, your undershirt is soaked. The jacket you purchased as “waterproof” seems to have stopped working, and all the marketing claims feel a bit suspect.
In reality, the jacket probably hasn’t failed overnight: a mix of how it’s built, the exact level of water protection it offers, and years of sweat, skin oil, and dirt have all played a part.
But there are a few simple ways you can care for your rain jacket to ensure you stay dry, even when it’s pouring.
The science behind rain jackets
Most proper rain jackets are built around a waterproof “membrane” sandwiched inside the fabric. Gore-Tex is the most popular technology used, which includes a very thin layer of chemicals known as PTFE (polytetrafluoroethylene) or expanded PTFE (ePTFE), which are full of microscopic pores.
Those pores are much smaller than liquid water droplets. But they’re big enough for individual water vapor molecules, so rain on the outside can’t push through, but sweat vapor from your body can escape outwards.
Other fabrics use solid, non-porous membranes made from polyurethane or polyester that move water vapor by absorbing it and passing it through the material molecule by molecule rather than via tiny holes. This can make them a bit more tolerant of dirt.
The outer fabric is sometimes treated with a very thin chemical finish that makes water roll off the surface instead of soaking into the fibers — a bit like wax on a car. This finish is known as “Durable Water Repellent” and helps to reduce the saturation of water in the exterior of the jacket.
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