Gear
Rain shells
Most shell failures are not leaks. They are the membrane doing exactly what it was designed to do, badly.
A waterproof-breathable membrane moves water vapour outwards because the inside of the jacket is warmer and wetter than the outside. That gradient is the engine. Remove it and the jacket stops breathing, whatever the number on the label says.
Wetting out
The outer fabric carries a durable water repellent finish. When that finish wears off, the face fabric absorbs water and stays saturated. A saturated outer layer is cold and wet, which collapses the temperature and humidity difference across the membrane. Vapour stops leaving, condenses on the inside, and you conclude that the jacket leaks. It has not. It has wetted out, and the fix is washing it properly and reproofing it, not replacing it.
The two numbers
- Hydrostatic head, in millimetres, is how much water pressure the fabric resists before passing water. Rucksack straps and sitting down both apply far more pressure than rain does, which is why the number matters most at the shoulders and the seat.
- Breathability, usually grams per square metre per 24 hours, is measured under lab conditions that do not resemble a wet hillside. Useful for ranking fabrics against each other, close to useless as an absolute.
Two, two and a half, three layers
Two layer hangs a separate mesh liner inside the membrane: comfortable, heavier, and the liner holds moisture. Two and a half prints a protective pattern directly onto the membrane instead: lightest and cheapest, least pleasant against skin. Three layer bonds a knit backer to the membrane permanently: most durable, most expensive, and the only construction that really tolerates a decade of use.
Practical consequence
In persistently wet terrain, venting matters more than the membrane. Pit zips and a hood that turns with your head do more for staying dry than upgrading from one fabric to another. And a shell only works as part of a system that manages sweat, because the water you generate is usually more of a problem than the water falling on you.