Waterproof Jackets Explained: Breathability Ratings and What They Mean for UK Weather

Pick up a waterproof jacket in any outdoor shop and you will find a swing tag covered in numbers: 20,000mm, 15,000g, RET 4. They look like a scoreboard, and it is tempting to buy the biggest number in the shop. That approach usually costs more and does not always keep you drier.

British rain is awkward. Most of the time it is not a tropical downpour but a slow, wind-driven drizzle at eight degrees, arriving sideways while you are halfway up a hill and working hard enough to sweat. That combination — light rain, near-saturated air, real physical effort — is exactly when breathability matters more than the headline waterproof figure.

Hydrostatic head: what the number actually measures

Hydrostatic head (HH) tests how much water pressure a fabric can hold back. A column of water is placed above a sample, and the height at which droplets start appearing on the underside is recorded in millimetres. The higher the figure, the more pressure the fabric resists before water is forced through.

The catch is that the test is done on a flat piece of fabric in a lab, not on a finished jacket. Seams, zips, worn patches and the pressure of rucksack straps all change the picture. A well-made 10,000mm jacket with fully taped seams and a decent hood will keep you drier on a real walk than a 20,000mm shell with leaky stitching. As rough guidance:

• Under 1,500mm — shower resistant at best. Fine for a city commute in light drizzle, not for a day on the hills.
• 1,500–5,000mm — handles short outings and light rain. Will start to fail in steady rain or under a pack.
• 5,000–10,000mm — the practical floor for general walking in British rain.
• 10,000–20,000mm — comfortable for full days out, wet grass, sitting down and carrying weight.
• 20,000mm and above — for sustained heavy rain and long days where the jacket rarely comes off.

MVTR: what breathability figures tell you

Breathability is usually quoted as MVTR — moisture vapour transmission rate — in grams of water vapour per square metre over 24 hours. A figure of around 5,000g is modest, 10,000–15,000g is good, and anything above 20,000g sits at the top of the market.

Two warnings. First, manufacturers do not all test the same way. Upright cup, inverted cup, desiccant and dynamic methods give different results, so a 15,000g jacket from one brand is not automatically comparable to a 15,000g jacket from another. Second, some brands use RET instead, where a lower number is better — roughly under 6 is very breathable, 12 or above feels clammy. If you are comparing two jackets, compare the same scale.

More usefully: breathability only works when there is a gradient to work with. Your body is warmer and more humid than the air outside, so vapour moves outwards through the fabric. Stop for lunch and that gradient fades, which is why you feel damp the moment you sit down. It is not a fault, it is physics.

Why good numbers can still feel clammy

The most common reason a breathable jacket stops breathing is the durable water repellent (DWR) finish on the outer fabric wearing out. When the face fabric wets out, it sits there as a cold, sodden layer with a film of water on top of the membrane. Vapour has to get through that film, and it mostly does not. The jacket has not suddenly become a bin bag, but it feels like one.

British conditions do not help. In drizzle at 95 per cent humidity there is barely anywhere for your sweat to go, however good the membrane. Add a thick fleece underneath and the mid-layer soaks up moisture before it ever reaches the shell. This is why ventilation often beats specification: pit zips, mesh-lined pockets, adjustable cuffs and a two-way main zip let you dump heat and moisture directly rather than relying on diffusion. A mid-range membrane with good vents will out-perform a top-end membrane with no way to let steam out.

Layering: build a system, not a jacket

A waterproof jacket is the outer layer of a system, and it can only manage the moisture that reaches it. Get the layers underneath right and the whole thing works better.

• Base layer: thin synthetic or merino. Cotton holds water against your skin and chills you when you stop.
• Mid layer: a light fleece or grid fleece for moving, with an insulated jacket in the pack for stops. One lighter mid beats one heavy one you have to take off every twenty minutes.
• Shell: waterproof and windproof, sized to fit over the mid layer with a bit of room for air to move.

Start slightly cool. If you are warm and comfortable in the car park, you will be overheating within ten minutes of walking. Insulated waterproof jackets — the three-in-one type — are convenient for low-level use but limit your options on a hill day, because you cannot shed the insulation without shedding the waterproofing.

Matching ratings to the rain you actually get

Ignore the worst weather imaginable and think about your realistic worst day. A low-level woodland walk in steady drizzle needs waterproofing around 5,000–10,000mm and as much breathability as you can afford. An exposed ridge in driving rain at four degrees needs a higher HH, a properly adjustable hood and a storm flap over the zip. A commute or a flat cycle ride is usually better served by something light and packable, since you are never far from shelter.

For multi-day backpacking, look at 20,000mm and up with fully taped seams and a tougher face fabric that will cope with pack straps rubbing all day. For summer showers, a 2.5-layer jacket packs down small and is comfortable enough, even if it is not the most breathable option available.

Fit, hoods and the details ratings ignore

Two jackets with identical numbers can behave completely differently on a wet hillside, and it comes down to construction. A hood should adjust in at least two directions, have a stiffened peak, and fit over a hat without pulling the collar up. Pockets need to sit above a hip belt or harness. Cuffs should cinch down, the hem should have a drawcord, and the zip needs a storm flap or a water-resistant finish behind it. A slightly longer back keeps rain off your seat when you sit down, and reach-through pockets are a genuine comfort on a cold day with wet hands.

Look after it and the numbers stay honest. Wash the jacket with a technical cleaner rather than detergent, skip fabric softener entirely, and follow the care label — a low tumble dry often reactivates the DWR. When water stops beading and starts soaking in, reproof it with a wash-in or spray-on treatment. A reproofed mid-range jacket will usually outperform a neglected expensive one.

A quick way to choose

1. Write down the wettest day you will realistically be out in, and for how long.
2. Set a floor of fully taped seams and an adjustable hood. Nothing else matters if water gets in at the shoulders.
3. Choose your hydrostatic head for the pressure it will face — pack straps and sitting down count.
4. Compare breathability figures only between jackets using the same test scale, and treat them as a tie-breaker rather than gospel.
5. Prioritise ventilation and fit over the last few thousand millimetres of waterproofing.
6. Try it on over the layers you actually wear, with a loaded rucksack if that is how you will use it. At home, stand in the shower for a couple of minutes and check the chest and shoulders.

Buy for the weather you walk in, not the weather on a marketing poster, and you will end up with a jacket that earns its place by the door for years.

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