Waterproof Ratings Explained: Hydrostatic Head and Breathability
By Trailbound Editors · Published · Photo: Wikimedia Commons
Table of Contents
Quick Verdict
A hydrostatic head figure describes how much water pressure a fabric resists, and almost every jacket sold for outdoor use already exceeds what rain produces. Breathability figures are laboratory numbers that rarely predict how a jacket feels on a hill. Fit, hood, and vents decide comfort; the surface treatment decides whether the fabric beads or wets out.
Waterproof clothing is sold on two numbers, and both of them describe less than the buyer assumes.
Hydrostatic head
This measures the water pressure a fabric resists before water passes through, expressed in millimetres. Rain delivers relatively little pressure; sitting on wet ground, leaning against a wet pack strap, or kneeling in a puddle delivers much more.
In practice, almost every shell made for outdoor use clears the figure rain requires. Beyond that, a higher number usually reflects a heavier and stiffer fabric rather than a drier walk.
Breathability
Vapour transfer figures are measured under laboratory conditions with a temperature and humidity gradient that your body under a pack does not reproduce. They are useful for comparing two fabrics and poor for predicting how either will feel on a climb.
What actually determines comfort is ventilation: pit zips, a two-way front zip, and a fit loose enough for air to move.
The surface treatment, and why jackets look like they fail
When water stops beading and the fabric darkens, that is the durable water repellent finish, not the membrane. The jacket is still waterproof; it just feels cold and clammy because the outer fabric is soaked.
Washing the jacket and applying heat as the care label allows restores the finish. Most jackets declared dead are simply dirty.
What to check instead of the numbers
- Hood that moves with your head, with one-handed adjustment.
- Vents, ideally pit zips.
- Cuffs that close over a glove.
- Seams taped and intact, especially across the shoulders where a pack rubs.
- Fit loose enough for a midlayer and air movement.
A mid-range jacket with a good hood and vents beats a high-rated jacket without them in every real condition you will meet.
Frequently Asked Questions
What hydrostatic head do I need?
For walking and general outdoor use, a figure in the low thousands of millimetres is enough for sustained rain. Higher numbers exist for sitting in a storm, for pack abrasion, and for marketing, and they usually come with a heavier, stiffer fabric.
Do breathability numbers mean anything?
They are measured under standard conditions that your body does not reproduce. A jacket with vents and a looser fit will manage moisture better than a higher-rated jacket with no way to dump heat.
Why does my jacket wet out?
The durable water repellent surface has worn or been contaminated by oils and dirt. Washing the jacket and applying heat as the care label allows restores it. The membrane itself rarely fails.
