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.
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.
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:
- Waterproof Breathable Rain Jackets with Pit Zips
- Non-Breathable Rain Jackets with Pit Zips