“Waterproof and breathable” is the most repeated phrase in outdoor retail and the least examined. Both properties are real. The problem is that they pull in opposite directions, they are measured in ways that do not match how you use a jacket, and the part of the specification most people compare is the part that matters least.
What the membrane actually is
A waterproof breathable membrane is a thin film bonded to the inside of a face fabric. Liquid water droplets are too large to pass through its structure, while water vapour molecules - the moisture evaporating off your skin - are small enough to diffuse through it. That is the whole trick, and every membrane does it in one of three ways:
- Microporous films, with pores small enough to block droplets. Expanded PTFE and some polyurethane films work this way.
- Hydrophilic, non-porous films, which move vapour through the polymer itself by absorbing it on one side and releasing it on the other. Solid polyurethane and polyester films behave like this.
- Electrospun or hybrid films, which combine a very fine fibrous structure with a hydrophilic coating.
Commercially, jackets are described by how the membrane is attached: two-layer (membrane protected by a separate loose lining), three-layer (face fabric, membrane and a thin protective scrim laminated together) and the various “two and a half layer” constructions where a printed or sprayed pattern replaces the scrim.
Three-layer laminates are usually more durable in the hand and don’t cling, which matters in cold, wet conditions. Two-layer jackets feel softer and are typically cheaper, but the lining adds weight and holds moisture near the membrane. Two-and-a-half-layer jackets are light and pack small, and are usually the least durable of the three over years of use.
The two numbers on the hang tag
Almost every technical shell quotes two figures: a hydrostatic head in millimetres, and a breathability number - either a vapour transmission rate or a resistance figure such as RET, where lower numbers mean more vapour passes through.
Both are laboratory measurements, and both are meaningful only inside the same test method. Different methods expose samples to different temperatures, humidity gradients and pressures, so a jacket measured under one protocol is not directly comparable with a jacket measured under another. Manufacturers also quote the best figure from a range of samples and tests.
This is why we do not publish those numbers as a ranking, and why we would not choose between two jackets on them. Use them within one maker’s own range to see which jacket is meant to be more protective, and treat cross-brand comparisons with suspicion. What the ratings do not tell you is how a jacket feels during a long climb in wind-driven rain, which is what you are actually buying.
Why “breathable” disappoints
Vapour only moves through a membrane if there is a gradient to push it: more humidity inside, less outside. Three ordinary conditions break that gradient.
The outside of the jacket is already wet. When the face fabric is saturated, water sits against the membrane. Vapour transport slows sharply, sometimes to the point where condensation forms on the inside instead. This is the single largest cause of a jacket that “used to breathe and now doesn’t”.
You are working hard and the rain is warm. In mild, humid rain your sweat output can exceed anything the membrane can pass. At that point the jacket is a moisture trap no matter what the specification says.
You are wearing too much underneath. A shell is not insulation. If you are wearing a heavy fleece under a shell while climbing, you will be wet from the inside before the membrane has anything to do with it.
The practical conclusion is that venting matters more than membrane branding. Pit zips, chest pockets that can be opened as vents, adjustable cuffs, a two-way front zip and a hem that can be loosened all move large volumes of air and vapour in seconds. A well-vented jacket with a modest membrane will keep you drier than a sealed jacket with an excellent one.
DWR is the part that fails first
The durable water repellent finish on the face fabric is what makes rain bead up and roll off. It is not waterproofing: it keeps the outer fabric from wetting out, which preserves the vapour gradient and stops the jacket feeling cold and clammy.
DWR is a wear layer. Abrasion, body oils, sunscreen and dirt all degrade it, and so does the detergent residue left in a washing machine. When it goes, the face fabric wets out, the jacket feels damp inside and the membrane gets blamed for a problem that a wash and a re-proof will solve. We cover that process in detail in our guide to keeping a rain shell working.
Construction is where jackets actually differ
Given that membranes from the same generation perform within a similar band, the money in a good shell is spent on:
- Taped seams. Any jacket that claims to be waterproof must have them. Seam tape quality and coverage vary, and untaped seams are the most common leak point.
- Zips. Water-resistant coated zips are adequate for most uses; proper waterproof zips are stiffer, heavier and more expensive. Flaps over zips add protection at the cost of some ventilation.
- The hood. This is the hardest part to get right and the part you notice most. It should adjust in at least two directions, turn with your head, keep the peak out of your eyes and still let you breathe.
- Wrist and hem closures. Elastic alone leaks; velcro or a drawcord holds an edge closed against the wind.
- Length. A shell that finishes at the hip exposes your mid layer when you reach up and lets water run down onto your legs. If you walk in real rain, look for a longer cut or a hem you can cinch.
Choosing for the weather you get
Sustained rain on an exposed hill is a different problem from a cold, windy morning with a passing shower. In the first case, durability, hood design, length and ventilation are the deciding features. In the second, almost any shell with a reasonable DWR is enough, and a light, packable jacket is the better choice.
There is no jacket that is both the lightest option and the most durable one, and there is no jacket that keeps you dry while you work hard in warm rain. Deciding which of those two failure modes you can accept is the real decision. After that, the specification on the tag is mostly decoration.
