Rain jackets: what breathability numbers do and do not tell you

Two numbers on a rain jacket mean something and one does not. Hydrostatic head, measured by ISO 811, tells you how much water pressure a fabric resists. RET, measured by the sweating guarded hotplate of ISO 11092, tells you how strongly it resists water vapour leaving. MVTR, the figure most brands print, comes from several incompatible test methods and cannot be compared between manufacturers at all.

What the numbers on the hangtag actually measure#

Hydrostatic head (ISO 811). A fabric sample is clamped under a rising column of water until three droplets appear on the far side. The height of that column in millimetres is the rating. It is a laboratory pressure, not a weather forecast: static rain applies only a few hundred millimetres of pressure, and even kneeling on wet ground under a loaded pack is usually modelled in the low thousands. The same test underlies tent and groundsheet figures, worked through in waterproof ratings explained.

Evaporative resistance, RET (ISO 11092). A porous plate is heated to skin temperature and fed water so that it sweats. The fabric goes on top, and the machine measures how much extra power is needed to keep evaporation going. The result is a resistance in square metres pascal per watt: low numbers pass vapour easily, high numbers do not.

Because RET comes from one defined method, two RET figures are genuinely comparable. EN 343 uses the same logic, classing rainwear 1 to 4 on both water penetration and water vapour resistance, so a garment certified to that standard tells you both halves of the story.

Why MVTR numbers cannot be compared between brands#

MVTR, the moisture vapour transmission rate in grams per square metre per 24 hours, is the number most likely to appear on a swing tag and the least useful one there. It can be produced by an upright cup method, an inverted cup method, a desiccant method or a sweating plate variant, under different temperatures and humidities. ASTM E96 and the several methods within JIS L 1099 are not interchangeable, and the results from them are not on the same scale.

The practical consequence: a fabric quoted at 25,000 g per square metre per 24 hours is not necessarily more breathable than one quoted at 10,000. It may simply have been measured by a method that returns bigger numbers. Unless a brand states the test method and conditions, an MVTR figure is a marketing number. Where a brand publishes RET, use that instead. Where neither is published, assume nothing and judge the garment on venting and fit.

Breathability is a gradient, not a property#

A membrane does not pump moisture. Water vapour moves through it by diffusion, driven by the difference in vapour pressure between the inside and the outside. Warm humid air inside your jacket has a high vapour pressure; cold dry air outside has a low one; the difference does the work.

Now consider a warm wet day at 18 degrees C with the air near saturation. The gradient across the membrane is small, so the transfer rate collapses, and the jacket that felt superb in cold winter rain now feels like a bin liner. Nothing has failed. The physics simply stopped favouring you. This is the single most common source of the belief that a jacket has "stopped breathing", and no amount of reproofing or washing restores a gradient that the weather has removed.

Two conclusions follow. First, the warmer and wetter it is, the more you should rely on mechanical venting rather than the membrane. Second, jacket choice should follow your climate: in cool dry cold, membranes work well; in warm humid rain, they are always going to disappoint, and the honest response is to accept being wet with warm rain rather than wet with your own sweat.

Wetting out is not leaking, and you can tell the difference at home#

When the durable water repellent finish on the face fabric fails, the outer fabric absorbs water instead of beading it. The membrane behind is still intact, so nothing is getting in, but three things happen: the wet face fabric conducts heat away and feels cold and clammy, the vapour gradient collapses because the outer surface is now saturated, and condensation forms on the inside. People describe this as the jacket leaking. It is not.

Run this test with a tap and a kitchen towel:

1. Hold the jacket under a running tap and watch the outside. If water beads and rolls off, the DWR is alive. If it darkens and soaks in, the fabric is wetting out.
2. Turn the jacket inside out and stretch a section over a bowl. Pour water onto the membrane side and leave it for five minutes. If any water appears on the outer face, that is a real leak.
3. Check the seams first when you find water inside. Lift the taping and look for lifting edges, especially at the shoulders where a pack strap works the seam.
4. Look at the cuffs, hem and hood opening. Water tracking down your neck and up your sleeves when you raise an arm accounts for a large share of "leaks".

A jacket that fails step one and passes step two needs reproofing rather than replacing, which is a wash and heat treatment costing a fraction of a new shell.

The honest hierarchy of staying dry#

Ranked by how much difference each makes to whether you arrive dry, which is close to the reverse of the order they are advertised in.

1. Fit and venting. Pit zips, a two way front zip and a jacket cut with room over your layers move air by convection. Convection carries far more moisture than diffusion through a membrane ever will, at no cost in fabric technology.
2. Hood and collar. A hood with a wired peak, a rear volume adjuster and a collar that covers the chin keeps rain off your face and neck, which is where discomfort actually starts. A hood that does not turn with your head means you walk with rain on your face and the jacket half open.
3. Cuffs and hem. Adjustable cuffs stop water running up your forearms when you use poles, and a hem drawcord stops the jacket ballooning and pumping cold air.
4. DWR condition. A working repellent finish preserves both comfort and the vapour gradient. It is maintenance, not a purchase, and it needs renewing every few washes.
5. Seam taping. Fully taped beats critically taped in sustained rain. Critical taping covers shoulders and hood, which is enough for a shower and not enough for six hours of Atlantic weather.
6. The membrane itself. Real differences exist, and they are smaller than the four items above. A RET 5 fabric in a badly cut jacket with no vents loses to a RET 12 fabric with pit zips.

That ordering is the argument of this page. Most buyers invert it, spending on the sixth item and then wondering why they are wet.

Choosing by climate and activity#

The awkward row is the fourth. In warm humid rain no membrane performs well, so the honest options are heavy venting or a non waterproof wind layer plus dry clothes at camp. Pairing either with the insulation choices in layering matters more than the shell itself. For a fixed camp, see also camping in the rain.

Common mistakes#

Buying on hydrostatic head. Above roughly 10,000 mm the number describes durability of the waterproofing over time more than resistance to rain. A 30,000 mm rating does not make you drier this afternoon.

Comparing MVTR figures. Different methods, different scales. Treat any MVTR number without a stated method as decoration.

Washing the jacket rarely to protect it. Body oils, sunscreen and dirt on the face fabric are a leading cause of wetting out. Shells need washing more often, not less, with the right product.

Wearing a heavy insulated layer under a shell while moving. The shell traps the vapour that layer produces. The insulated jackets guide covers why an active insulation piece under a shell behaves differently.

Ignoring the trousers and boots. Water running off the jacket hem lands on your thighs and then in your footwear. See footwear and socks for what happens after that.

Frequently asked questions#

What is a good hydrostatic head for a rain jacket?#

For general hiking, 10,000 mm under ISO 811 is enough to keep rain out. Ratings of 20,000 to 30,000 mm mainly buy resistance to pressure points, such as pack straps and kneeling, and give more margin as the fabric ages. Beyond that, higher numbers rarely change how dry you are on a given day.

What does RET mean on a rain jacket?#

RET is evaporative resistance measured by the sweating guarded hotplate method of ISO 11092. It counts how strongly a fabric resists water vapour passing through, so lower is better. Under 6 is very breathable, 6 to 13 is breathable, and over 20 is uncomfortable during exercise. Unlike MVTR, two RET figures come from the same method and can be compared directly.

Why does my expensive jacket feel wet inside?#

Almost always condensation rather than leakage. Either the face fabric has wetted out because the water repellent finish has worn off, or the outside air is warm and humid so there is no vapour pressure gradient for the membrane to work with. Check the beading with a tap: if water soaks in rather than beads, the finish needs renewing.

Is a more breathable membrane worth paying for?#

Only after fit, venting, hood and seam taping are right. Moving from RET 13 to RET 6 is a real improvement at high output in cold conditions. It is a smaller improvement than adding pit zips, and it does nothing at all when the air outside is already saturated.

Fully taped or critically taped seams?#

Fully taped for sustained rain and multi day trips. Critically taped garments seal only the shoulders and hood, which is enough for short showers but not for hours of rain with a pack pressing on the seams. The weight difference is typically tens of grams, so full taping is cheap insurance for anything but a summer emergency layer.

How often should a rain jacket be reproofed?#

When water stops beading on the face fabric, which for regular use often means after every five to ten washes, sooner if the jacket carries a pack. Reproofing is a wash with a technical cleaner, a spray or wash-in treatment, and heat from a tumble dryer or iron on a low setting to activate the finish.

Do pit zips actually work?#

Yes, and by a different mechanism from the membrane. Zips move bulk air, which carries far more moisture per minute than diffusion through fabric. On a climb in cool rain, opening pit zips and the front zip to the sternum removes more vapour than any realistic upgrade in membrane specification.

Can I use a rain jacket as a wind layer?#

You can, but it is usually the wrong tool. A waterproof membrane has much higher evaporative resistance than a light windproof, so in dry cold you will sweat into your layers to gain wind protection you could have had for 90 to 150 grams and a much lower RET.