Waterproof-breathable membrane at a glance
- Blocks
- Liquid water
- Passes
- Water vapour
- Breathes best
- When it is cold and dry outside
How a membrane blocks water but passes vapour
The trick rests on a size difference. A droplet of liquid water is enormous compared to a single water molecule in vapour form, so a film with pores in between the two blocks the first and passes the second. That's the mechanism behind ePTFE membranes like Gore-Tex, and it's why the technology exists at all.
Porous and non-porous membranes
There's a second family that works differently. Polyurethane membranes are solid rather than porous, and they move moisture by absorbing it on the inner face and releasing it on the outer, driven by the humidity difference across the film. They're generally cheaper, and their breathability depends more heavily on that humidity gradient being favourable.
Why breathability collapses in the wet
Both are limited by the same physics. Vapour only moves from wet to dry, so a membrane breathes well on a cold dry day and barely at all when the outside air is warm and saturated. That's why nobody stays dry inside a shell in humid rain no matter what they paid, and why ventilation features usually matter more day to day than which membrane the label names.
What this means when you're choosing
- Pit zips or other ventilation, which outperform any membrane when it's warm
- Whether the membrane is named and rated, or just described as waterproof-breathable
- How the laminate is built, since 2.5L and 3L feel different against skin
The common mistake
Blaming the membrane when the jacket wets out
A saturated face fabric stops the membrane working long before the membrane itself fails. The clammy jacket you are wearing is almost always a dead DWR finish, which is an hour of maintenance rather than a new shell.
Common questions
Why do I still get wet inside a waterproof-breathable jacket?
Because breathability depends on a humidity difference across the membrane, and in warm rain there isn't one. Your sweat has nowhere to go, so it condenses inside. Opening the jacket is the only thing that reliably helps.
Is Gore-Tex better than a brand's own membrane?
Not automatically. Gore's quality control is strict and its performance is well documented, but several in-house membranes now test comparably. Construction, fit and ventilation vary more between jackets than membranes do.