Why do I get wet inside my rain jacket?

Why do I get wet inside my rain jacket?

The leading causes of wetness inside your rain jacket are condensation and perspiration. Condensation occurs water vapor encounters a colder surface and changes from a gaseous form into a liquid one. When the air is humid it has a lot of water vapor in it. If the external surface of your rain jacket is cooler than its interior, the water vapor will condense on the inside your jacket and make your clothes wet.

Perspiration also produces water vapor and condenses by the same process. You can reduce the amount you perspire when wearing a rain jacket by taking off clothing layers worn underneath it or by generating less body heat, by being less active. For example, mountain climbers deliberately slow down their pace to reduce the amount they sweat so they don’t soak their insulation and reduce its effectiveness.

Pit Zips

A rain jacket acts as a thermal envelope and makes you warmer by trapping body heat. If your rain jacket has pit zips, you can open them to shed some of the warmth so you perspire less. You can do this without getting wet from the rain because your arms cover the pit zips and prevent rainwater from dripping into them. It’s not perfect, but it can reduce the amount of water vapor that’s produced when you sweat, thereby reducing the amount of condensation that makes your clothes wet.

Wearing a rain hat instead of using a jacket’s hood is another way to vent excess body heat since your blood flows close to the skin in your neck where it can be cooled by the surrounding air. If you don’t mind getting your hands and wrists wet, you can also push up your arm sleeves, because there’s also a lot of blood flowing near the surface of your wrists.

If you have a rain jacket that doesn’t have pit zips, there’s really no way to reduce the amount of heat trapped inside your jacket without stopping and shedding clothing layers. Unfortunately, that’s often not practical if its pouring rain.

Waterproof/Breathable Rain Jackets

Waterproof/breathable jackets are designed to release the water vapor that accumulates inside your jacket when you zip it closed. These jackets have a breathable layer with microscopic pores that vent water vapor while preventing rain drops from entering. They only release water vapor in its gaseous form, however, so you’re stuck with any liquid condensation that’s already formed inside your jacket.

A waterproof/breathable jacket is made with several different fabrics and materials that are sandwiched together. A  waterproof/breathable layer is often sandwiched between other fabrics or materials that protect it since its very thin and delicate. It’s important that these remain clean and don’t obstruct the passage of water vapor through the waterproof/breathable layer. This requires frequent washing and treating the exterior surface of the jacket with a chemical coating called DWR, which stands for durable water repellent.

The DWR coating makes rain bead up and run off the exterior of the jacket. If the DWR coating wears off, then a jacket’s outer fabric can get saturated, blocking the waterproof/breathable layer’s microscopic pores. The waterproof/breathable layer will still prevent water from penetrating deeper into the jacket to make you wet, but the soaked outer fabric will prevent water vapor transmission. With nowhere to go, the water vapor will condense and makes your clothing wet.

Unfortunately, the DWR coating deteriorates a little every time you use the jacket, fold it up, or stuff it into your backpack. While you can reproof it occasionally with a product like NikWax TX Direct, the DWR will never be as effective as the day you bought it.

If you own a waterproof/breathable jacket and your DWR coating fails, having a rain jacket with pit zips will at least let you dump excess heat, resulting in less perspiration, and less condensation.

Summary

Condensation and perspiration are the primary reasons that you can get wet inside a raincoat. Condensation occurs when water vapor touches a colder surface, like the inside of your rain jacket which acts as a thermal envelope keeping colder external air from chilling you. You can reduce the amount of water vapor inside your jacket and the amount you perspire by opening pit zips which are designed to cool you off. While waterproof/breathable jackets can vent water vapor, they can only vent it in its gaseous form and not after it’s condensed into a liquid. The DWR coating on most waterproof/breathable jackets must be carefully maintained and reproofed to maintain a jacket’s breathability performance.

If you don’t have a rain jacket with pit zips, you should consider getting one. Some waterproof/breathable jacket manufacturers also include pit zips in their jackets because there’s no other way to release the body heat (when it’s raining) that leads to increased perspiration.

Here are a few popular examples:

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15 comments

  1. Great write. It helps to have an experienced and well thought explanation. It is hard to figure out whats wrong while reading the standard publications of the gear companies and unfortunately individuals repeat those uncritically.
    According to this article isn’t it best to have a non breathable waterproof jacket rather than the breathable ones?

    • I think that the decision to make is somewhat different – should you buy a jacket with pit zips or not?

      All rain jackets, breathable (and there are many different levels of breathability) or not suffer from condensation. That’s a law of nature like gravity. You can reduce the level of condensation by cooling yourself off with pit zips.

      In my experience, breathable jackets are higher quality than non-breathable ones, with better zippers, adjustable hoods, and taped seams. They’re also far more abundant in terms of choice. But they require considerable extra DWR and washing to maintain the breathability. Most people aren’t willing to take the time to do this. If you’re not willing to do it, buying an expensive breathable jacket is pointless.

      Even if you do maintain the DWR, breathable jackets are still not immune to condensation because they can’t keep up with the amount of water vapor in the air or that you perspire. Pit zips are the best way to dump extra heat, but they won’t let you dump all your extra body heat and you’re still likely to get damp inside your jacket, especially if you’re a hiker wearing a backpack. Skiers riding up chair lifts don’t sweat as much….

      There’s also the issue of fabric thickness which isn’t often talked about. Heavier jackets with thicker fabric insulate more so you sweat more.

      Ultimately, this all comes down to cost. if you buy a non-breathable rain jacket, you want one with pit zips. If you buy a breathable rain jacket you’re going to want one with pitzips. You simply cannot rely on breathability as the only means of venting all the water vapor in the air or that you create (through perspiration) inside a rain jacket.

      Is that clearer?

      The harsh reality is that you’re probably going to get damp inside with any rain jacket. The bigger issue is maintaining a comfortable level of warmth and pitzips provide a better option for self-regulating that than none at all.

      • Thanks for the elaborated and helpful. I’m definitely lazy enough to buy the non breathable ones.

      • I would love to know exactly what percentage of my perspiration actually exits through the “breathable” membrane. I, and most all of my outdoor partners, find that any jacket, mo matter how expensive or well-constructed, is mostly a perspiration barrier during even moderate exercise.

        Now a days we just wear unshelled fleece as long as the wind and precipitation allows.

        And i won’t buy any shell without pit zips or thigh vents.

  2. Thoughts on the Packa, the hybrid cagoule/poncho/packcover, with the huge pit zips, and the long and open ventable bottom?
    Works well for me in my trials.

    • I’ve never tried one, but I imagine it would be annoying in the wind (think mountains) and cold (including winter), since it probably doesn’t hold your heat very well. Personally, I like a rain jacket with pockets, that’s also light enough to use as a wind shirt. I could see using a Packa in summer on a heavily forested trail maybe.

  3. I’ve had to move to using a poncho and, only when necessary, chaps as I’ve never been able to stay cool enough in any jacket, even with pit zips (I sweat A LOT). The problem is most ponchos don’t offer a zip up or button up front to allow excess heat and vapor to escape. I wish more manufacturers would incorporate this small change. Plus, the poncho works quadruples as a pack cover, ground sheet, and cooking/sleeping tarp.

    Years ago when they were pushing their cagoules, ponchos, and chaps, I thought I remembered Sierra Designs saying that mechanical venting (including pit zips) was 40% more effective than even the best waterproof breathable fabric. Do you remember anything along those lines?

  4. Do you know anything about the environmental impact of aftermarket DWR? That gives me pause to re-treating my old marmot precip

  5. I know that many people rate them but I have never found pit zips that useful.
    My arms normally force the opening closed when I walk.
    In the past i have found tjat if I periodically bellow the front of the jacket manually and have a loose cut jacket it expels hot air from the body much more effectively.
    I now have a columbia jacket with forward facing vents and external membrane (can’t wet out) that is a better solution IMHO.

  6. This is a good write up and clear explanation. I’ve tried all of the expensive jackets, and for a long time I was a die hard Arcteryx user. Over the last few years though, I’ve gone back to a basic silnylon jacket, for many of the reasons you mention.

    I hike hard, and build up heat in ALL of my rain jackets. The best way for me to regulate heat is via the front zipper and pit zips. I currently use a Light Heart jacket in warmer seasons, and a Marmot Precip in colder temperatures. I like the straighter style of the man’s Marmot Precip because I can layer it better over other clothes.

    I don’t need to pay the price of the “breathable” jackets when I get almost the same results from less expensive jackets like Light Heart or Frog Toggs.

  7. Philip, what is your current personal pick for 3-season rain jacket? If I remember correctly, you had been using the Lightheart Gear, non-breathable one listed above. Still your favorite?

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