Pit Zips and Ventilation
Pit zips are zipped openings under the arms that let you dump heat and moisture directly, bypassing the fabric entirely.
Under real exertion, mechanical ventilation moves far more moisture than any membrane does. If you are choosing between a higher breathability rating and better venting, take the venting.
Why they beat breathability ratings
A membrane passes vapour slowly and stops almost entirely once the face fabric wets out. An open zip moves air. On a climb, on a bike, or walking fast with a bag, opening pit zips changes how you feel within a minute in a way that a better rated fabric does not.
The forms of venting
- Pit zips: the most effective, and add weight and cost.
- Two-way front zip: open from the hem to vent the torso without exposing your chest to rain.
- Open cuffs and a loose hem: free, and surprisingly effective as air moves through.
- Mesh-backed pockets: modest venting, and a route for water on an unflapped zip.
When they matter most
Hot and humid conditions, where there is little vapour pressure gradient for a membrane to work with, and high-output activity in any temperature. They matter least on a slow walk in cool dry air, which is exactly the condition where membranes perform best.
Related terms
- Waterproof and breathability ratings explained: why ratings underdeliver in the heat
- Wet-out: what stops a membrane working
- What to pack for Singapore: where venting beats any rating
- Glossary: all terms