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How does a waterproof jacket keep you dry?
Learn the ins and outs of waterproofing and breathability, and the science behind them.


How does a waterproof jacket keep you dry?
Learn the ins and outs of waterproofing and breathability, and the science behind them.

Waterproofs: From then, to now

In the past, fabrics were coated with oils and waxes in order to repel water. But these were often heavy on the fabric and needed to be constantly reapplied. 

Modern waterproof fabrics use a thin waterproof membrane. These can be engineered to allow some moisture, like sweat, to pass through them, resulting in a more comfortable garment. 

A durable water repellent treatment (DWR) is often applied to the outer face fabric to help maintain the waterproofness and breathability and to ensure the membrane works as efficiently as possible. But we’ll get to those later.

How to create a waterproof barrier

There are two ways we can turn a regular fabric into a waterproof one.

Waterproof Coatings

The first ever breathable, waterproof barrier was coated onto a piece of fabric and, in general, it’s the simplest way to create a waterproof fabric. 

A liquid coating is applied in a similar way that you would apply paint with a roller to form a flexible, durable coating. 

Waterproof Laminates

Laminates are fine membranes that are made separately as a sheet of material which is then glued or bonded (laminated) to the face fabric.

It’s important to select and pair the right membrane with the correct fabric and bonding process for the fabric’s ultimate intended use.

These waterproof barriers are incredibly thin. At Rab we use membranes that range between 7 and 30 microns in thickness – for reference, a strand of human hair is around 100 microns.

Finer membranes will allow for greater breathability for fast and light pursuits. Thicker membranes provide a greater level of waterproof durability.

How to build a waterproof

Waterproof fabrics, whether coated or laminated, are usually constructed in one of three ways. Each method results in different characteristics and possible uses.


What is it?
This is the simplest construction with a laminated or coated membrane being applied to a single layer of fabric.

Why is it useful?
2-layer waterproof fabrics are generally the lightest in weight, ideal for situations where low weight and small pack size are crucial.

Where we use it?
We use this method on our shelters and tents, where there is no direct abrasion on the membrane.


What is it?
A 2.5-layer construction is similar to a 2-layer construction with the addition of a light coating or print, which is applied to the surface of the membrane.

Why is it useful?
This additional coating adds durability and protection without affecting the breathability. It also adds softness and is a great value for money waterproof fabric.

Where we use it?
We use this method in some of our clothing that is used in situations where weight, packability and breathability have the priority over durability.


What is it?
In 3-layer construction the membrane is sandwiched between an outer fabric and an extremely light woven or knitted inner fabric.

Why is it useful?
The third layer offers more comfort, durability, and protection, but they are the most complex waterproof fabric to manufacture.

Where we use it?
3-layer construction is commonly found in Rab shell garments. This fabric allows us to create hardwearing and durable gear, that is still lightweight and relatively packable.

What is DWR?

Find out what the main function of a durable water repellent (DWR) treatment is – HINT: It’s not there to keep you dry!

How durable is my waterproof?

There are many factors that can impact the durability of a particular waterproof fabric:

The type of material and fabrics used (e.g. nylon or polyester)

The type of membranes used (e.g. PU, PE or ePTFE)

The denier and weight of the face fabric used

The method of the face fabric construction (e.g. woven or knitted)

In addition to the exhaustive durability tests carried out by our fabric partners, we test all our designs ourselves. We get all the toys out in our Rab Lab and we send them off to specialist testers and athletes to be put through their paces out in the field, from lofty mountain tops to freezing polar regions.

Testing for Durability in the Lab

The Martindale machine offers an industry standard test. It’s beauty is in its simplicity, subjecting a sample of fabric to a repeated rubbing motion that simulates long term wear.

The machine can simulate the pressure an item will have on the fabric, like backpack webbing, or the constant abrasion of wearing a 6kg backpack.

Testing for Durability in the Field

Our kit isn’t fully approved for use until it’s seen action in the places it’s designed for. From winter in the Scottish mountains to Himalayan expeditions, our test team provide us with real world evaluation to supplement our lab tests.

Feeding back to our designers, garment technologists and specialist fabric team, the testers put our gear through rigorous mountain use, ensuring that every aspect, from construction to fit, is working correctly.

How waterproof is waterproof?

You might have seen waterproofs classified as 10K, 20K or even 30K – but what does this actually mean? Find out about hydrostatic head, water pressure, and how we measure the waterproofness of our fabrics.

Changing water pressures that your kit deals with on every adventure

What is breathability?

When we talk about gear being ‘breathable’ we are referring to the extent to which it allows moisture to escape.

Even during light activity, the body responds by sweating. In a ‘non-breathable’ garment there is no way for this moisture to escape. It collects on your skin and on the fabric as condensation. 

Trapped inside your jacket, this moisture acts as a thermal conductor. It transfers your body heat nearly 25 times faster than dry clothing. This feels very uncomfortable, chilling and in extreme environments it can lead to hypothermia. 

A breathable garment will allow moisture to escape through the membrane. It reduces peaks and troughs in body temperature and keeps you comfortable for longer.

How can a membrane be waterproof and breathable?

Microporous Membranes

A microporous membrane has millions of tiny holes that are large enough to allow water vapour to pass through, but too small for water droplets to get in.

What is it made from?
In waterproof garments, these membranes are generally made from PTFE or PU, in a network of tiny interconnected pores.

How does it work?
Individual water molecules (around 700 times smaller than the pores) in sweat vapour can pass through the membrane.

Groups of water molecules (around 20,000 times larger than the pores) in rain droplets are too big to pass through the membrane.

Hydrophilic Membranes

A hydrophilic (water loving) membrane attracts moisture to its inner surface which it then moves to its outer surface.

What is it made from?
Often made from PU, this is a solid-state membrane without any microporous holes.

How does it work?
You might have heard of osmosis from your days in biology class. That’s exactly how a hydrophilic membrane works. Moisture from your sweat is absorbed into the membrane and then diffuse in to the atmosphere on the other side.

As you get warmer and produce more sweat, the diffusion gradient increases. The membrane responds by increasing the rate of moisture transmission, working to equal the balance of water vapour on the inside and outside of your gear.

How breathable is my waterproof?

Here’s where things get technical… You can measure the breathability of fabrics in a variety of ways in a lab.

You might have seen the MVTR rating quoted before? This refers to the Moisture Vapour Transfer Rate and is an industry standard test. We also use the Sweating Guarded Hot Plate – no that’s not a kind of French table grill! It's another industry standard test that simulates sweat transfer through fabrics.

Does breathability mean comfort?

Using the same test method, we may be able to say one product is more breathable than another in the laboratory, however this will not take into consideration its other properties, that will impact overall comfort.

Comfort can come from many elements, including the breathability, weight and stretch of the fabric. It may have specific features designed to improve ventilation, like pit zips to dump heat or a soft, fast-wicking lining.

We use these laboratory tests as a guide for understanding how breathable a fabric is. However, we know that comfort is personal and depends on the design of the garment as a whole.

Measuring breathability with MVTR

MVTR records the amount of evaporation through a square metre of fabric over a 24-hour period.

There are two kinds of MTVR test, inverted cup and upright cup, but the premise is the same. They calculate the change in weight created by the movement of moisture from one side of the fabric to the other.

This weight difference is calculated after the test to give a moisture vapour transfer reading of grams per square metre of fabric over 24 hours. This describes how much sweat will pass through the fabric.

What should I look for?
Figures can range from less than10,000 MVTR, to up to 40,000 MVTR.

High MVTR = High breathability

Low MVTR = Low breathability

Measuring breathability with RET

RET shows the fabric’s level of resistance to evaporation.

In this test, fabric is placed above a porous metal plate. The plate is heated to a constant temperature between 33°C and 36°C and water is channelled into the metal plate to simulate sweat.

As water vapour passes through the fabric it causes evaporative heat loss. The amount of energy that is needed to keep the plate at a constant temperature is used to extrapolate an RET figure.

What should I look for?
The better the breathability, the lower the RET value.

High RET = Low breathability

Low RET = High breathability

So, there you have it. A full journey through the world of waterproof gear. We’ve covered waterproofness, durability, and breathability. Why they are important, and how we test for them.

Next time you purchase some Rab waterproof gear, have a look for the HH, MVTR and RET values. Now you know what they mean, you can decide which gear is appropriate for your next adventure.