Waterproof Jackets Explained: Ratings, Membranes and Breathability

A jacket's spec sheet can read like a spreadsheet: 20,000mm, 20,000g/m², 2.5-layer, PFC-free DWR. None of it tells you whether you'll be comfortable on a wet ridge in November. But the numbers do mean something — they simply need translating. Here's what each figure measures, where the marketing stretches things, and which specs matter for the walking, running or climbing you actually do.

What hydrostatic head actually measures

A square of fabric is clamped under a column of water, pressure is increased steadily, and the test records the height at which water starts to push through. The result is given in millimetres of water column. A 10,000mm rating means the fabric held back a theoretical ten-metre column before leaking.

As a rough guide to what you're buying:

• Up to about 1,500mm: weather-resistant rather than waterproof. Fine for drizzle, wind and a brisk walk to the shops.
• 5,000–10,000mm: copes with steady rain on a lowland walk or a commute.
• 10,000–20,000mm: the useful band for British hill days, where rain arrives sideways and settles in.
• 20,000mm and above: winter mountaineering, long days out, and conditions where a loaded pack presses damp fabric against your back.

Two caveats matter more than the headline figure. First, the test uses a flat, clean, unstitched piece of fabric — as soon as a jacket has seams, the seams are usually the weakest point. An untaped seam will leak whatever the fabric claims. Second, the fabric is tested new. Dirt, abrasion and a worn-out finish all reduce real-world performance. Beyond roughly 20,000mm, you're mostly paying for durability and features rather than extra rain protection.

Membranes, coatings and laminates

Waterproof-breathable fabrics keep rain out and let vapour through in one of two broad ways. Microporous membranes rely on holes far too small for liquid droplets to pass, but large enough for water vapour. Hydrophilic or monolithic materials have no holes at all; vapour dissolves into the polymer and diffuses through. Plenty of fabrics combine both approaches.

How the jacket is put together

• 2-layer: membrane bonded to the outer fabric, with a separate mesh lining. Warm, a bit bulky, robust and usually the cheapest option.
• 2.5-layer: a printed or sprayed protective layer instead of a lining. Light and packable, but can feel clammy against bare skin.
• 3-layer: face fabric, membrane and a thin inner scrim bonded as one. The most durable and the most expensive, and the sensible choice if you're out most weekends.

Don't get too hung up on brand names. In-house membranes from the big outdoor brands perform well. What separates a keeper from a regret is construction: fully taped seams, a storm flap over the main zip, a hood with proper adjustment, and a cut that leaves room for a mid-layer without flapping in the wind.

Breathability: RET, MVTR and the limits of the lab

Breathability describes how quickly sweat vapour moves through the fabric. You'll see two scales. MVTR is measured in grams per square metre per 24 hours, and higher is better — but different labs use different methods, so figures from different brands aren't directly comparable. RET, from the Hohenstein system, works the other way round: lower is better. Under about 6 is very breathable, over 20 is poor.

Here's the honest part. At high effort in cold, damp air, no membrane can shift vapour as fast as you produce it. Breathability helps, but ventilation helps more. Pit zips, a two-way main zip, cuffs and hem you can open, and a wicking base layer rather than a cotton T-shirt will do more for your comfort than another few thousand grams on the spec sheet.

DWR: the finish that keeps rain beading

DWR stands for durable water repellent — the treatment on the outer face fabric that makes water bead and roll off. It doesn't make a jacket waterproof; the membrane does that. Its job is to stop the face fabric wetting out. Once that happens, the jacket feels cold and clammy, and the membrane underneath struggles to pass vapour because the surface is already soaked.

If your jacket has started soaking up water instead of shedding it, the fix is usually maintenance rather than replacement:

1. Wash it with a technical cleaner — no fabric conditioner and no biological detergent, both of which leave residues that kill the finish.
2. Tumble dry on low, or iron through a cloth, if the care label allows. Gentle heat reactivates many DWR treatments.
3. If it still wets out, re-proof with a spray-on or wash-in product. Spray-on generally gives a more even result on the face fabric.

Fluorine-free, PFC-free finishes are now standard on most kit, and they're a better environmental choice, but they tend to need re-proofing more often. Keep a bottle at home and top up once or twice a season.

Matching the numbers to the walking you do

Commuting and dog walks: 5,000–10,000mm, taped seams, a decent hood, packable enough to live in a bag. Modest breathability is fine.

Three-season hillwalking: 10,000–20,000mm, 2.5 or 3-layer, pit zips, an adjustable hood you can cinch down one-handed in wind.

Winter and mountain use: 20,000mm and up, 3-layer, a hood that fits over a helmet, and chest pockets placed high enough to clear a rucksack hip belt.

Running and fast packing: a lower hydrostatic head is fine — 10,000mm handles most rain — but prioritise breathability and minimal weight over features.

Trying a jacket on properly before you pay

Shop fittings decide more than spec sheets do. Put the jacket on over the layers you'd wear on a cold day, then reach forward, raise both arms and swing them. If the hem rides up or the cuffs pull back past your wrists, you'll feel it every time you use a walking pole.

Check the hood with a hat on, and with a helmet if you'll be wearing one. It should turn with your head rather than leaving you staring at the inside of the fabric. Make sure the pockets are reachable with a pack on, and that you can work the cuff and hem adjusters wearing gloves. Run your fingers along the inside of the seams — you should feel tape, not stitching. Give the fabric a splash of water if the shop will let you, and watch whether it beads or soaks in straight away.

Finally, read the care label. A jacket is a system: fabric, membrane, finish, seams and fit, all kept working by how you look after it. Buy for your wettest realistic day, then wash and re-proof it when it needs it. That routine will keep a mid-range jacket performing better than a top-tier one that never sees a washing machine.

Photo: ChristopherPluta / Pixabay