Waterproof Fabrics: Coatings, Membranes and What the Ratings Mean

"Waterproof", "water-resistant" and "water-repellent" are printed on swing tags as if they were grades of the same thing. They are not. They describe different properties, achieved in different ways, and tested by different methods. This guide explains the three ways fabrics keep water out (dense weaves, coatings and membranes), what a hydrostatic head rating of 5,000 mm or 20,000 mm actually measures, how breathability is rated, and how to care for waterproof fabrics so they keep working. Where the numbers are textile arithmetic, it uses the site's Fabric Weight Converter and Thread Count & Cover Factor Calculator.

Three different words

• Water-repellent: the surface makes water bead up and roll off, usually because of a durable water repellent (DWR) finish. It says nothing about what happens under pressure or after long exposure; water can still get through.
• Water-resistant: the fabric slows water down. Tightly woven fabrics and light coatings are water-resistant. They cope with showers, not sustained rain or kneeling on wet ground.
• Waterproof: the fabric resists water under a specified pressure. For a garment to be waterproof in use, its seams, zips and closures must be too.

Way 1: a dense weave

A very tightly woven fabric, especially one made from fibres that swell when wet such as fine long-staple cotton, can resist rain without any coating. When the fabric gets wet, the yarns swell and close the gaps between them. Historically this approach was developed for demanding uses, and some tightly woven cottons are still made for outdoor clothing today. These fabrics are breathable, quiet and repairable, but they are heavier than modern laminates and their protection is lower and slower to start.

The cover factor shows why only dense constructions can do this. In the thread count calculator, a cloth of 150 ends and 150 picks per inch in Ne 40 cotton has a cloth cover of 27.34, close to the full-cover value of 28. An open cloth of 80 by 60 in Ne 20 yarn has a cover of 22.73, with far larger gaps between yarns. Only the first is in the territory where swelling yarns can close the cloth. Dense-weave rainwear fabrics use their own specialised constructions; the numbers here only show the principle.

Way 2: a coating

A coating is a polymer layer spread directly onto the fabric, usually on the inside face. Polyurethane (PU) coatings are common on budget rainwear, tent flies, bags and tent floors; PVC on heavy rainwear and tarpaulins; silicone on lightweight tents and tarps. A solid coating is very waterproof but not breathable. Microporous and hydrophilic PU coatings let some water vapour out, though generally less than a good membrane.

Coatings add weight. The guide to coated fabrics works through how much of a heavy fabric's weight can be coating.

Way 3: a membrane

A membrane is a thin film, made separately and laminated to the face fabric. Membranes are usually one of two kinds:

• Microporous, such as expanded PTFE or some polyurethanes: the film is full of pores far too small for liquid water droplets but large enough for water vapour molecules.
• Hydrophilic (solid), such as some polyester and polyurethane films: there are no pores; water vapour moves through the film by diffusion.

Membranes are laminated in different ways:

• 2-layer: membrane bonded to the face fabric, with a separate hanging lining to protect it.
• 2.5-layer: membrane bonded to the face fabric, with a thin printed or sprayed protective layer instead of a lining. Light and packable.
• 3-layer: face fabric, membrane and a thin backer fabric bonded into one. The most durable construction, common in mountain jackets.

Laminate weights are often quoted in GSM or ounces. A light 2.5-layer jacket fabric might be in the region of 100 GSM, which the converter shows as 2.95 oz/yd²; a robust 3-layer fabric might be around 180 GSM, or 5.31 oz/yd². These are illustrative figures; check the product's own specification.

Reading a hydrostatic head rating

The usual waterproofness figure is the hydrostatic head, quoted in millimetres. A sample is clamped under a column of water, or under equivalent rising water pressure, and the pressure at which water appears through the fabric is recorded as the height of water column it equals. A rating of 10,000 mm means the fabric held back a 10 m column of water in the test before leaking.

Some things to know about these numbers:

• Methods differ. Standards such as ISO 811 and its equivalents describe how to test, and the rate of pressure increase and the definition of failure affect the result. A number without a method is hard to compare.
• Real-world pressure is higher than you think. Kneeling, sitting on wet ground, or the pressure of pack straps on the shoulders pushes water through fabric harder than falling rain. That is one reason tent floors and trouser seats need higher ratings than a tent fly.
• Ratings drop with use. Abrasion, flexing and contamination with dirt and body oils reduce the waterproofness of coatings and membranes over time.
• Seams matter more than the fabric. A 20,000 mm fabric with untaped seams is not a waterproof garment.

For protective clothing for work in rain, standards such as EN 343 in Europe set classes for both water penetration and breathability. A consumer jacket marketed as "waterproof" may or may not be tested to such a standard; the label or the maker's specification should say.

Breathability ratings

Breathability is the rate at which water vapour, mostly your sweat, gets out. Two families of numbers are common:

• MVTR (moisture vapour transmission rate), in grams per square metre per 24 hours. Higher is more breathable. There are several test methods and they give very different numbers for the same fabric, so only compare figures from the same method.
• RET (resistance to evaporative heat transfer), tested to ISO 11092. Lower is more breathable. RET values under about 6 are commonly described as extremely breathable and values above about 20 as poorly breathable, though the bands vary between sources.

No waterproof fabric breathes as well as an open, non-waterproof one. In cold, wet, windy conditions, venting, layering and pace matter as much as the rating on the label.

Keeping waterproof fabrics working

• Keep the DWR working. When the face fabric wets out, it stays waterproof underneath, but breathability falls and the jacket feels clammy. Washing with a cleaner suitable for technical outerwear, and reactivating or renewing the DWR as the maker recommends, restores beading.
• Avoid fabric softeners and ordinary detergents with additives, which can leave residues that attract water and clog membranes.
• Follow the heat instructions. Some DWR finishes are reactivated by gentle heat; some coatings and seam tapes are damaged by high heat.
• Store dry. Coatings, especially some PU coatings, degrade faster when stored damp and warm.

Some older DWR chemistries based on long-chain fluorochemicals are now restricted in several countries because of their persistence in the environment, and many makers have moved to alternatives. Newer DWRs can need renewing more often.

Waterproofs as safety equipment

In the hills, on the water or at work outdoors, waterproof and windproof layers are part of what keeps you warm, and getting cold and wet can become dangerous quickly. Do not rely on a garment's rating alone: check its condition before you go, carry spare insulation, and follow the advice of the relevant outdoor, maritime or workplace safety bodies for the activity.

What the tools cover

The site's tools can convert laminate weights between GSM and ounces and show how densely a woven fabric is covered. They cannot predict a hydrostatic head, a breathability figure, or how long a coating or DWR will last. Those depend on the materials and the test method, so rely on the maker's tested figures and look for the method named alongside them.