How Important Are Pit Zips?

Breathability figures — MVTR, RET — measure how fast water vapour passes through a fabric under laboratory conditions. Whatever the number, an open zip moves air, and moving air carries moisture out orders of magnitude faster than any membrane. This is not a controversial claim; it is why manufacturers that publish no breathability figures still fit pit zips to their mountain shells.

The practical consequence: venting strategy matters more than small differences in published breathability. Open the vents before you overheat, not after — once you are sweating into your base layer, the moisture has to be moved twice.

The clearest case is stop-start mountain use: a hard climb followed by a windy summit, in rain. You cannot remove the shell (it is raining), you cannot slow the weather, and your output swings wildly. Pit zips let you tune the jacket like a thermostat. Pack wearers get a second benefit: the underarm is one of the few vent locations a hip belt and shoulder straps do not block.

The opposite case is just as clear. A jacket for commuting, dog walks and spectating never sees the output levels that overwhelm a membrane, and its pit zips will never be opened. On that jacket they are weight, cost, stiffness and one more water-resistant zip to maintain — pure downside.

A two-way main zip vents from the hem upwards and works with any jacket that has one — effective, though it exposes the front to rain. Mesh-lined chest pockets double as vents when open, at the cost of pocket security. Full-length side zips — Outdoor Research's TorsoFlo, running from hem to bicep, is the best-known example — offer the largest venting area of any design and let the jacket open like a poncho.

None of this is either-or. Mountain shells increasingly combine a two-way front zip with pit zips; the Rab Downpour Mountain in this database pairs underarm vents with its front zip. The point is to match the venting to your actual output pattern, not to collect zips.