Waterproofs 101

To welcome the rainy season in the northern hemisphere, here is an article I wrote on my personal blog (Outdoors Father) almost 2.5 years ago but since proved to be an excellent corner stone article about waterproof fabrics and clothing. So here is a long, sometimes technical look into waterproofs, enjoy: 

Waterproof clothing has many many myths, claims and strong opinions around it; I have heard so many of them over the years that it still amazes me what some companies will sell as “waterproof.”

The properties of waterproof clothing

Lets start with the first truth that no one likes to admit: making something waterproof is extremely easy and cheap – just pick up any plastic bag and you have a waterproof item. Or better, take a big bin liner, make holes for your head and arms and you have a perfectly waterproof garment. It might not be pretty or fashionable, but it will keep you dry.

The only problems with a bin liner (besides style) is that it is very flimsy and offers no breathability at all. When we are being active, we sweat a lot in order to cool ourselves. This sweat, when wearing a bin liner, will stay inside and keep us wet, rendering the waterproof part redundant. So the greatest challenge with waterproof garments is making sure it is breathable. The other thing that a bin liner has none of is durability – walk next to a branch with a bin liner and most of the bag will stay on the branch. Any chafing from backpacks, skis or ice axes will completely ruin your magnificent bin liner attire.

The two things we can give a bin liner all the credit for is being utterly waterproof, really, bombproof even; and it is also very light. You can get slightly thicker bin liners, but they will still remain amazingly light compared to most jackets out there.

So the 4 key characteristics we want from waterproof clothing are: waterproof-ness (I know this is not a word, but we are going with it), breathability, durability and lightness. The last factor will be price. As you can imagine, having the combination of the four properties at the highest quality will cost you dearly, very dearly. Those high end super expensive waterproofs? They have all those great characteristics, this is why they are expensive…..

Waterproof construction – the “only 2” theory

Again here, the universe of construction method claims is endless: bonded, laminated, printed, sprayed, waxed, weaved and more- all have been used to make a waterproof garment, but are they really all unique methods construction? Well, not really.

To make a waterproof and durable item, we need a method that will allow the movement of heat and perspiration out while not allowing water to get in – pretty simple. We can see how the bin liner won’t work here, so what will? We need a fabric that has holes to allow the heat and sweat out, but won’t allow water in. The trick is achieved by using the effects of the surface tension of the fabric (making a fabric hydrophobic, or water repellant) while combining it with a porous membrane to make it breathable.

You might have read/heard about the big drop with tiny holes idea (the idea that a drop of water is too big to fit into the pores of a membrane from the outside in, but that the water vapour from your body heat and sweat is made up of smaller molecules and so will be able to escape), but that is sadly not true. If the fabric on your waterproof clothing is losing that surface tension, the fabric will not be waterproof anymore, and may lose the breathability all together.

Back to the construction claims – as I mentioned, there are essentially only two methods of making breathable and waterproof clothing: laminating and coating. Both methods can be very simple but the varieties within each method makes them complicated.

Lamination

Lamination is the expensive option in the world of waterproof clothing. Laminated clothes tend to be durable and very breathable, which leads to a higher price point. Laminated fabrics are essentially a “sandwich” of three layers:

1. External treated durable material – this is usually a hydrophobic (non absorbent) material that is very durable such as Polyamide, Polyester or Nylon. The external fabric is treated with a DWR finish (explained further below).
2. Membrane – the “heart” of the fabric. Made from a very thin synthetic material, the membrane can feature different configurations (depending on the fabric) of microscopic pores that allow vapour to pass though and blocks water drops from coming in.
3. Internal lining – can be a loose fabric or laminated to the membrane on the inside. The lining’s purpose is to keep the membrane from getting contaminated from body oils, salt and grime.

The big names in the world of membranes are Gore-Tex and eVent. Most of the big companies will also have their own membrane fabric that has been developed in house such as HyVent (The North Face), MemBrain (Marmot) and many more.

Coating

Coating has a couple of variations: a standard PU (Polyurethane) coating that has very limited diffusion ability, and a printed porous membrane that has more breathability to it (also known as 2.5 layer fabrics). Coating is the cheaper option and is offered by pretty much any company that claims to make outdoor gear.

PU coating uses the same Diffusion concepts as an ePTFE (Elastic Polytetrafluoroethylene) membrane, which is explained in more detail in the Diffusion Membranes section below, but because PU needs to be thicker to bond with other fabrics (like Nylon, Polyamide etc), the PU coating ends up being much thicker than the laminated membranes so diffusion is slower (aka less breathability).

Printed PU membranes are the new age of waterproof clothing. PU’s have much greater diffusion abilities compared with ePTFE membranes and can be printed into thinner, more complex patterns that are robust yet breathable. Printed PU membranes have great breathability while being combined with a variety of face fabrics to allow super-light running clothing or robust heavier ski or mountaineering garments.

Measuring waterproof-ness

With the constant promise for garments that are more and more waterproof, it seems that all we get is a numbers game, aiming to show who “has the biggest”. The most common method of measuring waterproof-ness is called Hydrostatic Head (HH). In this test, a tube with both ends open is placed on the waterproof fabric (on the exterior) and filled with water. The tube usually measures 1″ x 1″ and the measurement taken is how much water can be filled (in millimeters) until water starts leaking through the fabric. Most HH tests are now standardized but variations between labs can exist.

Hydrostatic Head measurement brackets:

• Up to 1,500mm – fabric is not waterproof
• 1,500mm – 10,000mm – waterproof fabric as defined by European ISO standards for waterproof fabrics
• 10,000mm – 20,000mm – outdoors industry standard for waterproof fabrics
• 20,000mm+ – completely waterproof fabrics as made by the highest end manufacturers

Hydrostatic Head might seem like a good way to measure waterproofing, but there some issues here:

• HH is a laboratory test, so real life conditions do not apply
• Not all labs comply with the same standard of testing
• The test does not add the internal pressure made by using the garment (such as joints, or when sitting etc)
• The test does not add external pressures such as packs, wind factors etc
• Usually tests are on brand new fabrics and do not calculate the reduction of performance over time
• Final garments have stitches and tapes that have different waterproof ratings and might damage the performance

Using PSI to measure waterproof-ness:

PSI (Pounds per Square Inch) is a different method of looking into waterproofing. PSI measures how much pressure can be applied on the fabric before it fails (water seeps through). As a rule of thumb HH and PSI can be converted at 704mm=1 PSI, yet this is not a satisfying conversion. Compared to HH, PSI testing is based on active movement, so it can include more factors than just the presence of water, such as wind, movement compared with drop impact (your “collision” with the rain), internal pressure from pressed fabrics etc.

Industry standard considers waterproof fabric as 3 PSI and up, yet this will amount to a light rain with no wind while standing around. To get something that will fight a Scottish mist (what we call a torrential storm), at least 30 PSI should be sought after to stay dry.

I prefer using PSI measurements when it is offered as I think it tends to represent reality a little better, even if tested in lab conditions. There are some things that are important to remember when looking at PSI: when we bend our knee and stretch the fabric of our trousers, 2-4 PSIs are being placed on the fabric from the inside. If we place that knee on the wet ground, we need to add the external pressure that is placed on the fabric from our weight being pressed to the ground, that can be quite a lot too. Between those two factors, you can easily see how 10 PSI on waterproof trousers won’t protect your knees (or bum) when in contact with the wet ground.

DWR – the must-know acronym for waterproof-ness

All the waterproofing technology amounts to nothing without the most important part of the equation: Durable Water Repellent (DWR). DWR is a chemical coating that is treated to the face fabric (the exterior layer) in order to make it completely hydrophobic. Without DWR, the exterior fabric can get “wetted out” and suffer from reduced breathability or even cause a reverse effect in that moisture is “sucked in” to the fabric, causing the garment to get completely soaked inside and out.

DWR is a chemical treatment (usually silicon based) that is applied on the fabric in the factory, but it wears off over time. The main reason for DWR wear is abrasion, so areas that suffer in particular are where straps, harnesses, gaiters and other bits of gear create friction on the fabric. In order to keep the fabric from wetting out, it is important to re-apply the DWR on a regular basis, along with giving the garment a good clean.

DWR restoring

I recommend washing and treating your waterproofs regularly. I personally wash my waterproofs (if used) after 10 days of total use (can be a 3 day trip, 5 days out and then 2 day hikes) and treat them with an external spray. I use a cotton cloth to spread the spray evenly and remove the excess. After the DWR has dried, a short round in the drier on very low heat helps “cure” the DWR and restore its repellency.

Limitations of Waterproof-ness

When we are talking about waterproof clothing, it is not just the fabric and its ability to repel water that matter; there are other factors that influence waterproof-ness:

• All waterproof garments have big holes that are weak points: collars, hoods, sleeves, waist bands or anywhere you actually fit into the clothes. Those weak points are inevitable and we just have to accept that with enough rain over enough time, water will seep through those weak spots.
• Zippers are very rarely waterproof, but can be water tight. Again, these are weak spots the can be partially solved with storm flaps (fabric that covers the zip), but those will fail too in some point.
• If you don’t keep the DWR in good condition, water might be sucked into the garment or just saturate the fabric, making it not waterproof anymore. Keep an eye on that DWR.

Breathability

Breathability is both a key feature that must be present in waterproof clothing, and really tricky to measure. First, we need to distinguish between what many companies will call breathable and what it actually is – for a garment to be breathable, the fabric itself need to allow the movement of heat and moisture away from the body. The important thing is to understand that having vents or mesh or zips won’t make a garment breathable and waterproof, but will just make it open (for heat to get out and rain to come in….).

For a fabric to be breathable it needs to have some form of a pore to allow that heat and moisture we generate to escape the garment. The two main technologies out there use the porous structure but with a slight variation that creates two distinct forms of breathability: Ventilation (air-permeable membranes) or diffusion (PU based membranes).

When talking about breathability, the correct term is actually Moisture Vapor Transmission (MVT) and it is the way that a lot of companies will present the information. I will continue using the term breathability for the sake of convenience but remember that we are actually looking at MVT.

Ventilation membranes

Ventilation has always been the “original” idea around keeping a fabric more breathable, starting from treated cotton such as Ventile all the way to high-tech fabrics such as Polartec Neoshell. The mechanism behind this technology is based on having a good balance between “straight” pores that allow air to flow into the garment and angled pores that block the wind. The more straight pores there are, the more air can flow into the waterproof clothing – but then there is also a higher chance of water “slipping” in. For instance, Polartec claims that the Neoshell is 97% windproof, but that comes with a waterproof level of 10,000mm hydrostatic head (explained above) compared to 20,000+mm by other companies.

Ventilated fabrics are really useful when less rain is expected and you are going to be doing a higher impact activity. Many times waterproof clothing using this technology will be considered softshells rather than the traditional hardshells and will be sold as a more active-use garment.

Diffusion Membranes

Diffusion is the movement of a substance from a high-density state to a low-density state. When it comes to waterproof clothing, that means that hot and humid sweat vapor is diffused through the fabric and expelled on the other side (from the inside to outside). Diffusion works on a basic chemical mechanism that occurs inside ePTFE (Elastic Polytetrafluoroethylene) membrane fabrics. These fabrics usually use a hydrophilic internal layer (absorbing water vapor) that in turn moves through the ePTFE membrane and is then “pushed” by the diffusion to a lower concentration area on the outside layer.

To simplify it: hot sweat vapour on the inside of the garment moves through a 3 layer fabric that contains a membrane on the outside of the fabric that is not as hot and humid as the inside layers (so the vapour naturally moves away from the body and out to the external shell of the garment).

The biggest problem with diffusion is that it is very slow in moving moisture out and the diffusion only occurs when moving from dense and warm to dry and cold. When using diffusion-based fabrics in an environment that have the same humidity and heat as the inside of the garment, diffusion won’t work and the fabric won’t “breathe.”

On the other hand, fabrics that use diffusion have a greater waterproofing ability as the pores are not straight and tend to be smaller in size compared to fabrics that use ventilation.

Measuring breathability

When measuring breathability, there are more tests in the outdoor industry than there are for waterproofing. As a result, the number of different data options is mind boggling, making it almost impossible to try and compare various fabrics and their performance. To name some:

• ASTM E 96 BW: Inverted Cup Method (g/24hrs/m²)
  
• ASTM F 2298 DMPC Diffusion Test Method (g/24hrs/m²)
  
• ASTM F 1868: Sweating Hot Plate Method (m² · Pa/W)
• JIS L 1099: Desiccant Inverted Cup (g/24hrs/m²)
  

There are many other methods, but these are what the big companies are using. In the brackets are how the results are presented: you can see that 3 out of the 4 methods present results in g/24hrs/m², and one present in m²*Pa/W.

g/24hrs/m²

Results presented in this manner are showing the maximum grams of moisture that passed through a square meter of the fabric using that specific method. The test is conducted over a 24 hour period. The biggest problem with this way of measuring is that it is spread over a period of 24 hours, when in reality we sweat in rounds of high activity that can be very, very hot and then have a cooling down time when the fabric will barely function.

Despite the fact that the results are presented in the same way, the tests do not correlate, so there is not really a way to compare them. The most commonly used test in this category is the inverted cup test. Expected ratings for that test are:

• Under 10,000mm – low breathability
• 10,000-20,000mm – good breathability
• 20,000mm+ – excellent breathability

m²*Pa/W = RET

The Sweating Hot Plate Test (SHP) has become known as the best simulation of real life as it depends on the heat and moisture transfer in every single second rather than over a long period of time. The results are presented as the Resistance To Evaporation (RET) and they are very simple to understand: the lower the number, the less resistance there is and the more breathable the fabric is. Expected ratings:

•  0-60 – extremely breathable
• 60-130 – very breathable
• 130-200 – acceptable breathability
• 200-300 – breathable
• 300+ – not breathable

Limitations of breathability

• Most membranes work on reaching an equilibrium between the hot and humid conditions inside the garment, and a colder and drier environment outside. If the difference between the two environments is very small, breathability is dramatically reduced, and might even reverse in extreme conditions.
• If DWR is not well maintained (see above), the face fabric will become saturated, reducing the breathability.
• Every porous fabric is in danger of contamination from dust, sweat salts, and body oils. Once a fabric is contaminated it will stop being breathable and might lose all future breathability. The best way to maintain your garments is regular cleaning using pure soap or non-bio delicate soap.
• When being very active, fabrics tend to not move moisture quickly enough, causing condensation to build in the garments – that doesn’t mean that garment is not breathable, it just means it needs time to “catch up.”

Ventilation designs for increased breathability

One last point about breathability is the fact that manufacturers add ventilation options to their products. Ventilation does not mean a garment is not waterproof, but that the product designer wanted to increase breathability and cooling options for the user. Ventilation is an important feature in garments that are worn all the time (such as mountain jackets or ski shells made for high precipitation areas) and in garments that are used for high impact activities where weight is less of an issue (such as mountain biking).

A lack of ventilation doesn’t mean that the fabric is not breathable, but it might be that weight is an important part of the garment’s intended use and therefore zips were excluded.

Ventilation tends to be a great help in keeping the layering system in a better state than it would be without zips. Even with very breathable fabrics, when working hard, condensation may build in the garment and ventilation might allow that extra bit of cooling and drying to quicken reaching equilibrium.

Wrap up

In this post, we have covered the two main aspects of waterproof clothing: waterproofness and breathability. Waterproofing is a result of using a combination of hydrophobic fabrics that are treated using PU coatings, DWR and sometimes laminated with waterproof membranes.

The two main tests for waterproofness are Hydrostatic Head and PSI, representing how waterproof a fabric is. Those tests are lab tests, meaning the garments will perform differently in real life depending on the conditions you are in due to wind impact, direct pressure by packs or by you, etc.

Breathability is achieved by one of 2 methods: membranes and coatings that use diffusion, and membranes that use an inner fabric ventilation method. The trade off between the two methods is in durability and waterproofness – diffusion is more durable and waterproof but less breathable than ventilation.

Breathability can be reduced due to contamination or wetting out (reduction in the DWR performance). It is measured in a variety of methods, resulting in many different ratings that do not correlate, rendering the attempt to compare garments impossible. The two main test results for breathability are RET (lower is better) or g/24 hours (higher is better).

When choosing a waterproof garment, remember that the most important factor is you, not the garment. Your intended use, the usual conditions you will encounter and the frequency of use should be the main deciding factors. There is no need to get a fully featured, 3-layer membrane jacket to put on when commuting – it is a waste of money, even if you will use it for the intended purpose from time to time. The daily use will reduce the breathability and waterproofness and you will not get the performance you expect when you really need it.