Rab Lab – Waterproofing
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WATERPROOFING
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.
Waterproofing: Then to Now.
In the past, fabrics were coated with oils and waxes to repel water. But these were often heavy on the fabric and needed constant reapplication.
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 (DWR) treatment is often applied to the outer face fabric to help maintain waterproofing and breathability and to ensure the membrane performs as efficiently as possible. But we’ll come to that later.
How to create a waterproof barrier
There are two ways to turn a regular fabric into a waterproof one.
Waterproof Coatings
The first waterproof and breathable barrier was coated onto a piece of fabric, and generally, it is the most straightforward way to create a waterproof fabric.
A liquid layer is applied similarly to how you would apply paint with a roller to form a flexible, durable layer.
Waterproof Laminates
Laminates are thin membranes that are manufactured separately as a sheet of material that is then glued or bonded (laminated) to the face fabric.
It’s important to select and combine the correct membrane with the right fabric and bonding process for the intended end-use of the fabric.
These waterproof barriers are incredibly thin. At Rab we use membranes varying between 7-30 microns thick; for reference, a human hair strand is around 100 microns.
Thinner membranes will allow for greater breathability for fast and light activities. Thicker membranes provide a higher level of waterproof durability.
How to build a waterproof
Waterproof fabrics, whether coated or laminated, are generally constructed in one of three ways. Each method results in different characteristics and possible uses.
2-Layer
What is it?
This is the simplest construction with a laminated or coated membrane applied to a single layer of fabric.
Why is it useful?
2-layer waterproof fabrics are typically the lightest, ideal for situations where low weight and small pack size are crucial.
Where do we use it?
We use this method on our shelters and tents, where there is no direct abrasion on the membrane.
2.5-Layer
What is it?
A 2.5-layer construction is similar to a 2-layer construction with the addition of a lightweight layer or print, which is applied to the surface of the membrane.
Why is it useful?
This additional coating adds durability and protection without affecting breathability. It also adds softness and is an excellent value waterproof fabric.
Where do we use it?
We use this method in some of our garments that are used in situations where weight, packability, and breathability take precedence over durability.
3-Layer
What is it?
In 3-layer construction, the membrane is sandwiched between an outer fabric and an extremely lightweight inner fabric.
Why is it useful?
The third layer offers more comfort, durability, and protection, but they are the most complex waterproof fabrics to manufacture.
Where do we use it?
3-layer construction is commonly found in Rab Shell garments. This fabric allows us to create strong, durable garments that are still lightweight and relatively compact.
What is DWR?
Discover the main function of a durable water repellent (DWR) treatment – HINT: It's not there to keep you dry!
How durable is my waterproof jacket?
There are many factors that can affect the durability of a particular waterproof fabric:
The type of material and fabrics used (e.g., nylon or polyester)
The denier and weight of the face fabric used
The denier and weight of the face fabric used
The construction method of the face fabric (e.g., woven or knitted)
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 construction method of the face fabric (e.g., woven or knitted)
Laboratory Durability Testing
The Martindale machine provides an industry-standard test. Its beauty is in its simplicity, subjecting a fabric sample to repeated rubbing motion that simulates extended wear.
The machine can simulate the pressure an item will have on fabric, such as rucksack straps, or the constant abrasion of wearing a 6kg pack.
Field Durability Testing
Our kit is not fully signed off for use until it is seen in action in the very places it is designed for. From winter in the Scottish mountains to Himalayan expeditions, our testing team provide us with a real-world appraisal to supplement our lab testing.
Responding to our designers, garment technologists, and fabric specialist team, testers put our gear through rigorous mountain use, ensuring every aspect, from construction to fit, functions correctly.
How waterproof is my waterproof?
You might have seen waterproofs rated as 10K, 20K, or even 30K – but what does this really mean? Find out about hydrostatic head, water pressure, and how we measure the waterproofness of our fabrics.
Water pressure changes your gear faces on every adventure
What is breathability?
When we talk about kit being ‘breathable’ we mean 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 the skin and on the fabric as condensation.
Trapped inside the jacket, this moisture acts as a thermal conductor. It transfers heat away from your body almost 25 times faster than dry clothing. This feels acutely uncomfortable, chilling and in extreme environments can lead to hypothermia.
A breathable garment will allow moisture to escape through the membrane. It reduces the peaks and troughs of 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 enter.
What is it made of?
In waterproof garments, these membranes are generally made of PTFE or PU, in a network of small interconnected pores.
How does it work?
Individual water molecules (about 700 times smaller than the pores) in sweat vapor can pass through the membrane.
Clusters of water molecules (about 20,000 times larger than the pores) in raindrops are too large to pass through the membrane.
Hydrophilic Membranes
A hydrophilic (water-loving) membrane attracts moisture to its inner surface and then moves it to its outer surface.
What is it made of?
Often made of PU, this is a solid-state membrane with no microporous holes.
How does it work?
You may have heard of osmosis from your days in biology class. This is exactly how a hydrophilic membrane works. Moisture from sweat is absorbed into the membrane and then diffuses into the atmosphere on the other side.
As you warm up and produce more sweat, the diffusion gradient increases. The membrane responds by increasing the rate of moisture transmission, working to equalize the balance of water vapor on the inside and outside of your gear.
How breathable is my waterproof jacket?
This is where things get technical… You can measure the breathability of fabrics in a variety of ways in a lab.
Have you seen the MVTR rating quoted before? This refers to the Moisture Vapor Transmission Rate and is an industry standard test. We also use the sweating guarded hot plate – no, it's not a type of French hot plate! It's another industry standard test that simulates sweat transfer through fabrics.
Does breathability mean comfort?
Using the same test method, we can say that one product is more breathable than another in the lab, however, this will not take into account its other properties, which will affect overall comfort.
Comfort can come from many elements, including breathability, weight, and fabric stretch. It can have specific features designed to improve ventilation, such as armpit zippers to release heat or a soft lining that quickly wicks away moisture.
We use these laboratory tests as a guide to understand 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 one square meter of fabric over a 24-hour period.
There are two types of MTVR tests, inverted cup and upright cup, but the premise is the same. They calculate the weight change 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 vapor transfer reading of grams per square meter of fabric over 24 hours. This describes how much sweat will pass through the fabric.
What should you look for?
Figures can range from less than 10,000 MVTR to 40,000 MVTR.
High MVTR = High breathability
Low MVTR = Low breathability
Measuring Breathability with RET
RET shows the level of evaporative resistance of the fabric.
In this test, the fabric is placed on a porous metal plate. The plate is heated to a constant temperature between 33°C and 36°C, and water is channeled onto the metal plate to simulate sweat.
As water vapor passes through the fabric, it causes evaporative heat loss. The amount of energy required to keep the plate at a constant temperature is used to extrapolate an RET figure.
What should you look for?
The better the breathability, the lower the RET value.
High RET = Low breathability
Low RET = High breathability
There you have it. A complete journey through the world of waterproof gear. We've covered waterproofing, durability, and breathability. Why they are important and how we test them.
The next time you buy Rab waterproof gear, look for the HH, MVTR, and RET values. Now that you know what they mean, you can decide which gear is appropriate for your next adventure.
