Breathability Explained

The confusion starts with vocabulary. Four distinct properties are routinely collapsed into one word, and they are measured by different standards, in different units, on different equipment.

Water-vapour transmission is the movement of evaporated moisture through the fabric, driven by a difference in vapour pressure between the humid air inside your jacket and the drier air outside. Evaporative resistance is the inverse view of the same phenomenon: how much the fabric obstructs evaporative heat flow away from the skin. Air permeability is bulk airflow through the fabric, a genuinely different property with its own standard, ISO 9237. Ventilation is not a fabric property at all — it is holes you can open: pit zips, a two-way main zip, mesh pockets, cuffs, hem.

Air permeability deserves a note, because ASTM's own standard for it says the measurement can be used to indicate the breathability of weather-resistant and rainproof fabrics. That is technically true and practically misleading for shells: nearly all waterproof laminates are effectively air-impermeable, and a windproof claim is often expressed as an air-permeability figure. The deliberate exception is a small group of air-permeable membranes; Polartec publishes an air-permeability figure of about 2 l/m²/s for NeoShell and names ISO 9237 as the method, describing the airflow as imperceptible to the wearer.

A waterproof membrane blocks liquid water while allowing individual water-vapour molecules through, either through pores far too small for a droplet or by absorbing and desorbing moisture through a solid film. Either way, the driving force is a vapour-pressure gradient: warm, humid air inside, cooler and drier air outside.

That gradient is the part most explanations skip, and it is the reason breathability is conditional rather than constant. On a cold, dry, breezy day the gradient is steep and a modest fabric performs well. In warm, saturated air — heavy summer rain, or high humidity at low wind — the gradient collapses. There is nowhere for the vapour to go, and no membrane can create a gradient that the weather has removed.

This is also why a fabric maker can openly say its own metric falls short. Pertex states on its Shield Air technology page that standard MVTR tests do not capture that fabric's full in-use comfort benefit — a manufacturer confirming that a laboratory throughput number is not the same thing as how a garment feels.

In our reading of the technical documentation, most reports of a jacket "leaking" during exertion describe one of three mechanisms, none of which is a barrier failure.

The first is simple output. Working hard produces sweat faster than any membrane can pass it as vapour, and the excess condenses on the cool inner surface of the shell. It then feels exactly like water coming in, because it is water — from the wrong side. The fix is mechanical: open the pit zips, drop the hood, unzip the front, or slow down.

The second is wet-out of the outer face fabric. When the surface repellency is worn or dirty, the face fabric saturates and holds a film of water. The membrane still blocks liquid, but the wet layer chills the fabric and suppresses vapour transfer, so the inside becomes clammy. Washing the jacket properly and renewing the repellent finish restores this, and a great deal of perceived breathability loss is actually this and nothing else.

The third is the gradient problem above: warm, humid weather removes the driving force. A jacket that performs beautifully in cold rain can feel like a bag in a warm downpour, with no defect involved.

Moisture has to reach the membrane before the membrane can move it. That makes what you wear underneath, and how the shell sits, more consequential than most fabric comparisons admit.

A next-to-skin layer that spreads and moves moisture keeps liquid sweat off your skin and presents vapour to the shell. A heavy cotton layer holds it against you and no shell can rescue that. Very close-fitting shells give vapour less still air to cross but also less room to pool; very loose shells behave more like a tent, in both good and bad ways.

Mechanical ventilation is the largest single lever available to the wearer. Opening a pair of pit zips moves vastly more humid air than any difference between two good membranes. That is our practical position, not a measured claim: no standard we could find quantifies venting against fabric transmission.

The maintenance point is equally practical. A worn repellent finish costs more real-world breathability than choosing the second-best laminate, and it is the one variable fully in your control.

Across the jackets in our verified database, published breathability figures are the exception. Rab publishes 20,000 g/m²/24hrs for the men's Downpour Mountain, and names no test standard, procedure or conditions. Montane publishes a RET of under 3.5 for the women's Phase Nano, again without naming a standard.

Patagonia's H2No page names both MVTR and RET as measures the programme uses, but publishes no value for either and no test standard. The North Face explains the FUTURELIGHT membrane mechanism without publishing a figure or method. Helly Hansen publishes no breathability figure for Helly Tech. Arc'teryx publishes none for the Beta.

We record those absences as absences. Where a manufacturer has not published a figure, our jacket records read "Not publicly specified by the manufacturer" rather than borrowing a number from a retailer. And because the two published figures above come from unnamed methods, we do not place them in a ranking against each other or against anybody else's.

Start from your effort level and your climate rather than from a number. Hard, sustained aerobic work in cool conditions is where breathability genuinely decides comfort, and where a lighter, more permeable shell with real venting earns its place. Low-output use in heavy rain is where durability, seam quality and hood design matter far more.

If a figure is published with its method, treat it as one data point about the fabric. If it is published without a method — the normal case — treat it as an indication that the brand considers the fabric competitive, and nothing more precise than that.

Then judge the features that are not in dispute: how large the pit zips are, whether the main zip is two-way, whether the hood can be adjusted without help, and how easy the fabric is to maintain. Those are visible, verifiable and, in our view, the better predictor of whether you will be comfortable.