Hiking rain jackets don’t keep you dry in the rain; they keep you warm. If you don’t believe me, go hiking in the rain for 4+ hours and see if the clothing inside your jacket stays dry. Unless you’re hiking someplace with extremely low humidity, I bet the clothing under your jacket will get wet, if not soaked-through by sweat and condensation. Like so-called waterproof hiking boots, hiking rain jackets are another item of hiking gear where the expectations don’t match the results.
Which is why, you should think of a hiking rain jacket as a wet suit that will keep you warm when you’re wet, instead of preventing you from getting wet.
How Wet Suits Work
Wet suits are not meant to keep you entirely dry. Instead, they trap a thin layer of water between your body and the suit. This layer of water is warmed by your body, to prevent you from loosing too much heat while in the water.
Wet suits are made of neoprene, which is made of small closed cells that are filled with air and provide insulation against cold water by trapping heat in.
How to Make Your Rain Jacket Work Like a Wet Suit
The key to making your rain jacket work like a wet suit depends on what mid-layer insulation you wear. Since rain jackets are only like an outer skin, you’re going to want to wear a mid-layer that can trap air that you can warm with your body heat, but that won’t be compromised by the condensation that occurs on the inside of your jacket when it begins to rain.
The best mid-layers for this purpose are fleece or wool pullovers, worn over a synthetic or wool baselayer. Fleece is a hydrophobic material, holding less than 1% of its weight in water and retains much of its insulating powers even when wet. It also dries much faster than wool. While Wool also retains warmth when wet, it absorbs up to 30% of its weight in water and takes longer to dry.
Whatever you do. Don’t wear cotton, hemp, bamboo, or other garments made with plant-based fibers as mid-layer and base-layer garments under a rain jacket. These materials soak up so much water that they can’t retain warm air. The water fills up all of the air spaces in the fabric and your body can’t generate enough warmth to warm it up.
What Good is a Rain Jacket Then?
Why bother with a rain jacket if it just makes your mid-layer wet in rain from condensation or by trapping your sweat? Good question.
Rain shells, like house wrap, trap heat when worn so it doesn’t escape. They also help prevent something called evaporative cooling, which occurs when wind or air comes in contact with your torso and legs and causes evaporation to occur. Evaporation sucks heat out of your body – which is why the process of sweating cools you.
You can break the “evaporative chain” by wearing an insulating mid-layer like a fleece pullover, which preserves the air space around your body, and keeps the thin-film of wetness (caused by condensation) between its outer surface and inside of your rain jacket.
While the combination of an insulating mid-layer and rain jacket doesn’t work exactly the same way as a wet suit, the underlying principles are quite similar. Your body heats the air trapped by your mid-layer, while your rain jacket keeps the resulting warmth from escaping.