Jackets with Pit Zips: When Are They Effective?

Jacket Pit Zips - When are they Effective?

Jacket pit zips are zippered openings located under the armpits on many rain jackets and winter shells. They’re designed to help you vent excess body heat, so you perspire less when wearing waterproof/breathable or non-breathable jackets. Pit zips are positioned in your armpits because your arms and shoulders shield them from rain. Major blood vessels also pass close to your armpits, providing an efficient way to shed excess body heat when exposed to colder temperatures.

Perspiration and Condensation

When you wear a rain jacket or winter shell and zip it up, it’s going to insulate you by trapping your body heat. When body heat has no place to go and the temperature inside your jacket exceeds the temperature outside, you’ll start to perspire, which is the body’s way of venting excess heat. You’re also likely to experience condensation when water vapor inside your jacket comes in contact with the inside surface of the external fabric, which is cooled by the outside air. Perspiration and condensation are the two ways that you can feel wet inside a rain jacket or winter shell (See: Why Do I Get Wet Inside My Rain Jacket).

The Enlightened Equipment Visp Rain Jacket comes with 11” pit zips
The Enlightened Equipment Visp Rain Jacket comes with 11” pit zips

But if you open your pit zips, you can shed some of that excess body heat and avoid or reduce the degree of perspiration you experience. The trick is to do it when you begin to feel warm, rather than waiting until you feel wet with sweat. Some people just keep their pit zips open all the time, even if they feel a little cool when they start hiking and before their body heats up.

Pit zips are most effective when there is a large temperature differential between the inside of your jacket and the outside world. Depending on where you hike, this will occur when temperatures are cooler in spring, winter, and autumn. But pit zips are unlikely to provide much relief in summer when the temperature inside your jacket is close to the temperature outside.

Comparison Table

Jackets without Pit Zips

Some rain jacket and winter shell manufacturers don’t put pit zips into their jackets at all, instead relying on waterproof/breathable membranes to vent water vapor before it condenses into liquid form. It’s important to understand that these membranes can only vent water vapor, not perspiration or condensate. When your skin feels wet, it’s too late for your waterproof/breathable jacket to provide much relief…unless it can boil off the liquid and turn in into vapor again which would require a lot of energy.

The Outdoor Research Foray Jacket has torso zips that run from the armpits to the waist for maximum temperature regulation.
The Outdoor Research Foray Jacket has torso zips that run from the armpits to the waist for maximum temperature regulation.

Why don’t manufacturers always put pit zips in rain jackets? It’s hard to give a definitive answer, so here are a few possible reasons.

  • Adding pit zips increases the cost of manufacture and results in reduced profitability.
  • Adding pit zips to a waterproof breathable/jacket diminishes the value of the breathable membrane.
  • Adding pit zips adds extra weight to a jacket.
  • Gore-tex prohibits us from putting pit zips in rain jackets that use their breathable membrane.
  • People will still buy a jacket that doesn’t have pit zips if we tell them it defies the laws of physics.

Limitations of Pit Zips

Despite their benefits, pit zips have their limitations, especially in hot and humid weather when it’s impossible to shed body heat if you have to wear a rain jacket. If there isn’t the potential for hypothermia, which can occur even in summer, you’re probably better off using an umbrella, which is like an UBER pit zip for shedding body heat, while keeping your torso dry.

But in conditions where there is a decent temperature differential between the warmth inside your jacket and exterior coolness, the bigger your pit zips, the better. For example, both the Montbell Versalite Jacket and the new Lightheart Gear Silpoly Rain Jacket have 18″ long pits zips, which are definitely on the long side. Some manufacturers like Outdoor Research even put torso zips on their jackets which run from the biceps down to the waist. The ones on the OR Foray Jacket, for example, are particularly effective. Even if pit zips don’t completely eliminate heat build-up inside a rain jacket or shell, they’re better than not having them at all.

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