Waterproof Breathable Fabric - Explainedby Matt Fuller, Dr. Mark Taylor Apr/2012
This article has been read 76,440 times
Waterproof breathable fabrics divide opinion: some people believe a conspiracy surrounds these fabrics and that they are useless, while others claim their waterproof gear to be capable of miracles. There is also the never-ending debate of eVent versus GORE-TEX versus NeoShell .
This article hopes to expose some of the myths surrounding waterproof breathable fabrics, explain how these fabrics work and how they are tested, and give some ideas on how to get the best out of them.
Humans are constrained to operate within a narrow temperature range. The body's thermoregulatory mechanisms, such as sweating, are there primarily to protect against overheating, but in cold conditions these cooling mechanisms can be inhibited by the wearing of clothes.
During intense exercise the body's primary mechanism in preventing overheating is through evaporative heat loss. This is why breathability is so important: if evaporation cannot occur because of impermeable clothing then the wearer will suffer. Furthermore, wet and clammy skin not only feels unpleasant but also impacts on comfort by potentially leading to rapid cooling when layers are removed or exercise stops.
The Effect of Conditions
Conditions probably play the greatest part in determining the performance of your waterproof breathable clothing.
Moisture vapour tends to move from an area of high concentration (pressure) to one of low concentration. The maximum amount of moisture vapour that the air around us can hold is called the saturation vapour pressure (SVP) and is dependent on air temperature. The higher the temperature the higher the SVP will be, because there is greater energy present to encourage liquid to evaporate. Put more formally, SVP increases with temperature because, according to Le Chatelier's Principle, nature will try to maintain equilibrium. Evaporation is endothermic (absorbs heat), so an increase in temperature can be countered by increasing the amount of vapour present. When moisture vapour pressure exceeds SVP, either because more vapour is present or because the temperature drops, water droplets form as condensation.
In normal UK conditions the temperature outside our clothing is lower than inside, resulting in a higher SVP inside (ie. the maximum amount of moisture vapour the air can hold is higher). When we work hard our sweat evaporates into this microclimate raising the actual vapour pressure (AVP). Once the AVP inside is greater than that outside the garment, moisture vapour will start to diffuse through the fabric to the environment. It is worth noting that if the AVP outside the fabric is greater than that inside, such as in a tropical rainforest or when fire fighting, the moisture vapour pressure gradient is in the opposite direction and the moisture vapour will come through the fabric towards the wearer. Wearing breathable fabrics in these conditions may be a bad idea.
All this means that in windy, dry conditions, comfort is easy to maintain as the external vapour pressure is low and wind quickly evaporates away any moisture. Comfort is not too hard to maintain in dry conditions. It is difficult in wind driven rain, and very difficult to remain comfortable in still, rainy conditions, where the external vapour pressure is high. If it's warm and wet, do you really need to wear a waterproof? Your choice is either to get wet from condensed sweat or to get wet from rain. As an aside, it is worth remembering that condensation releases heat, which hinders your sweat-induced cooling.
Making a fabric rainproof is relatively easy. Making a fabric waterproof is really, really hard: you can force water through anything if you try hard enough. For our intended purposes, "waterproof" means that the fabric will withstand water pressures that it is likely to encounter during day-to-day wear. The official definition of waterproof is less simple though, and there is no universally agreed standard for it. One British Standard states that "the term 'waterproof' is a deprecated term which implies that the water penetration resistance of a coated fabric is equivalent to its hydraulic bursting strength." It also states that for a fabric to be called 'penetration resistant' then it should withstand a pressure of 10 kilopascals (kPa) when new. 10 kPa is roughly equivalent to a hydrostatic head of 1000 mm. The hydrostatic head is the height of water that can be withstood by the fabric before water penetration is observed.
This is where the disagreements really start. There's an early stumbling block, too: the near-universal term 'breathable' implies that a material is moisture vapour permeable; it does not relate to the exchange of air, as the term might imply. Moisture vapour permeability (MVP) or moisture vapour transmission (MVT), are perhaps better terms than breathability, and mean that the fabric transports water vapour from the body.
It is often-quoted that fabrics can be both waterproof and breathable because their pores are smaller than rain droplets but larger than moisture vapour. That's not really true and misses some crucial details, which will be discussed below.
Types of waterproof fabrics
Waterproof fabrics have been around for a long time. The Victorian Mackintosh, from which we get the word 'Mac', used rubber sandwiched between cloth to make an impermeable fabric that was waterproof but not breathable. Macs also reputedly smelt terrible and melted in hot bweather. Barbour and Helly Hansen's development of waxed jackets was important as, while still impermeable to moisture vapour, they didn't stink.
Burberry developed gabardine in 1879 and it was unrivalled for 40 years. It was simply tightly woven, proofed cotton. Grenfell was the next big development, being lighter than gabardine. However, both leaked under heavy rain and were superseded by Ventile, which is still used to this day.
Nowadays there are essentially four types of waterproof breathable fabric regularly used in the outdoors.
Type 1 – Tightly woven fabrics
Ventile is the best known fabric of this type. It's been up Everest, has been to both Poles, and has been around for about seventy years. It is an Egyptian cotton woven very tightly so that its pores are very small, making it hard for water to penetrate its structure. When it gets wet the cotton swells, further reducing the size of the pores to about 3 microns (a micron is a millionth of a metre).
Tightly woven fabrics are air permeable, which greatly increases their ability to transport moisture vapour. However, because water can slowly work its way through the pores, these garments can leak in prolonged wet weather and take a long time to dry. Ventile is arguably the original softshell.
Here, a mention should be made of Buffalo-type systems. Though not technically waterproof, the tightly woven outer fabric of Pertex combined with a pile liner means that water will move by capillary pressure – more on this later – to keep water away from your skin.
Type 2 – Fabrics with microporous coatings or membranes
PTFE (polytetrafluoroethylene) and PU (polyurethane) are the polymers most frequently used to make microporous materials for waterproof breathable fabrics. These microporous materials contain billions of holes per square centimetre that link together in complex pathways. As such, they act as a filter. They rely on surface tension to stop water penetrating them, and if the membrane or coating becomes contaminated then they can leak: water is a liquid with high surface tension, which is why it will bead on certain surfaces. Oils, such as those in sun-cream, food, or on skin, exhibit low surface tension which means that instead of beading they creep into the pores in the structure. Once inside, they affect the way water interacts with the micropores, potentially causing leaking.
Generation 1 Gore-Tex was a microporous PTFE membrane that was extremely breathable but leaked over time because it became contaminated by the wearer's oils and sweat. For this reason, Gore-Tex is now protected by a coating which reduces its ability to transmit water vapour but increases its durability.
eVent is also a microporous PFTE membrane. Its structure is protected from contamination by lining the pores with a hydrophobic (water-hating) and oleophobic (oil-hating) chemical. By doing this, eVent remains air permeable, which increases its ability to transmit water vapour. Its air permeability is not high, though, and water molecules cannot be simply blown through its structure! To get through its structure is like navigating through a maze.
NeoShell is made in a completely different manner to eVent or Gore-Tex and from polyurethane rather than PTFE. NeoShell is electrospun, which involves dissolving a plastic in a solvent and firing the solution at a collector until a film builds up. Polartec have publicly stated that the NeoShell membrane degrades over time, though they have also stated that the hydrostatic head will never drop below 5000 mm. Relative to the other techniques used to make microporous membranes, electrospinning is still in its infancy but has enormous potential for creating excellent materials because there are so many variables, such as solution concentrations and application temperatures, that can be controlled and changed.
PTFE or PU pores are typically 0.1-10 microns in diameter. A water vapour molecule has a diameter of 0.0004 microns. Rain droplets have a diameter of at least 100 microns. Therefore, in the case of eVent and Neoshell, it is true to say that their breathability largely results from the relative sizes of water vapour, their pores, and rain droplets.
These are made from a solid hydrophilic (water-loving) film or coating with no pores. They are impermeable to air. They are usually made of a mixture of PU and PEO (polyethylene oxide). The moisture vapour transport occurs by 'molecular wicking', which can be thought of as water molecules travelling across stepping stones: the water molecules are first adsorbed to the surface of the hydrophilic material then they move to the next molecule along. This process continues throughout the thickness of the hydrophilic. Hydrophilic materials are not necessarily the same on both sides, which can help improve durability and resistance to contamination.
Hydrophilic coatings stretch more easily than PTFE membranes, so stretch garments are much cheaper to manufacture. Their breathability tends to be slightly lower in lab tests than that of Gore-Tex or eVent PTFE membranes. However, their breathability is strongly affected by temperature: hydrophilics are developed to operate best at temperatures just above freezing, so sometimes perform poorly in laboratory tests that are conducted at skin temperature.
The North Face's Hyvent and Marmot's Precip and MemBrain Strata are all examples of PU coatings or membranes. Sympatax (well known in mainland Europe but less so in the UK) is also a hydrophilic coating, but based on polyester. One of its key advantages is that it can be easily recycled, assuming that it is allied with a polyester face fabric.
Type 4 – Bicomponent microporous and hydrophilic laminates
This is modern Gore-Tex. The Gore-Tex membrane is still PTFE, but its micropores are filled with hydrophilic polyurethane. Greater durability results, and very hydrophilic polyurethane can be used that would otherwise be vulnerable to damage. There is some evidence that an air layer exists between the PTFE and PU that provides insulation, increasing the temperature differential between the inside and outside of the fabric, and this can reduce condensation.
Modern Gore-Tex is impermeable to air. Active Shell, too, is impermeable to air, but its excellent breathability results from the thinness of its construction, which means that the water molecules have less distance to travel through its membrane.
Some garments are regarded as weatherproof without being technically waterproof. Systems such as Nikwax Analogy, most notably used by Paramo, and Keela Dual Protection are well known for providing excellent breathability and comfort despite achieving only low hydrostatic head results. Nikwax Analogy is a two-layer fabric, the inner of which mimics the way animal fur works, relying on capillary depression. Capillary pressure is how wicking works in a baselayer, and a similar effect is seen in Analogy, so it 'pumps' water out along its fibres. The outer of the fabric is a tightly-woven cloth somewhat analogous to Ventile. Keela Dual Protection works in the same manner as double glazing, and much to Keela's credit, is explained very well on their website (http://www.keela.co.uk/system-dual-protection), unlike the vast majority of these technologies. Both Nikwax Analogy and Dual Protection garments tend to be warmer and heavier than conventional shell layers, so may work best in cold conditions.
DWR, durable water repellence, is the chemical coating applied to a fabric to increase its ability to shed water. It prevents your jacket absorbing water (wetting out), which not only makes the jacket heavy and slow to dry, but can impact on breathability. In fact, in the case of microporous PU coatings, breathability ceases altogether once wetted out. A jacket that has wetted out will conduct heat away from the wearer more quickly, potentially making them feel cold.
DWR is usually provided by a fluorochemical or silicone coating. Fluorochemical coatings provide greater repellence and arguably greater durability than silicones, but this is offset by their negative environmental impact.
DWRs work by changing the interaction that occurs between the fabric and water. To understand this fully requires some maths.
When a water droplet makes contact with a surface (eg. a waterproof fabric) there are numerous interactions that are present: between the water and the surface, between the water and the air, and between the surface and the air. There is one other crucial factor: the contact angle between the water droplet and the surface. This is described by the following equation:
The DWR is not the only feature of your jacket that affects how it sheds water. As a DWR is only a very thin coating, once it has been worn out it all depends on the outer fabric, the face fabric. Porous surfaces, like meshes or the face fabric of a waterproof jacket are not flat planes, and this means that water interacts with them slightly differently to if the surface were flat and uniform. A fabric with a more open weave increases the contact angle, making a jacket shed water better. However, a more open weave allows more water in to the face fabric, which means it'll dry slower. This is an unavoidable trade-off: the face fabric must be woven tightly enough to resist water penetration but loosely enough to shed rain once the DWR is worn out. The durability, tactility and many other factors are also influenced by the weave.
Some myths about DWR: 1) washing your jacket ruins DWR; 2) application of a DWR hinders breathability; 3) home application reproofing agents don't work.
• Cleaning using a specialist cleaning product or soap flakes should not negatively affect a DWR. Detergent 'masks' a DWR but does not chemically remove it.
• A DWR will only affect breathability if you apply it with a trowel. It is an incredibly thin coating and its application does not inhibit breathability.
• Home application reproofing agents rely on the original factory DWR: this is what they stick to. That means that if you wait to reproof until the factory DWR is completely removed by abrasion then you hinder the chances of ever restoring it. Heat-activation helps restore some types of treatment by orientating the molecules to provide minimal surface energy. Therefore, if the label allows, tumble drying or ironing your garment before deciding to apply a home treatment is wise: it might be that you can restore the original coating.
Hydrostatic head testing
The waterproofness of a fabric is usually assessed by a hydrostatic head tester (British Standard EN 20811:1992). The machine measures the 'head' (pressure) of water that can be applied to a fabric before water penetration is observed. Results are reported in units of pressure or units of length. Despite the standard stating that a 1000 mm hydrostatic head is 'penetration resistant' it should really be seen as a minimum figure to provide waterproofness, as sitting or kneeling in standing water will exert greater pressures than this on the fabric. There are lots of figures stated by companies, academics and bloggers, but there is no definitive answer as to what hydrostatic head is necessary to keep out all water in normal outdoor conditions. 10,000 mm will certainly be sufficient.
Rain room testing uses an artificial shower to assess outdoor clothing. There are a few different standards for the testing, each varying in water droplet size, shape and force. The fundamental difference between rain room testing and the hydrostatic head test is that it assesses full garments, not just fabrics. As such it determines whether zips or seams leak, and whether hoods fit properly. The tests are usually carried out on a static manikin. If a person wears the garments under the shower then results can change as the wearer attempts to adopt a position which keeps water away from their face. The garments are assessed for waterproofness by electronic sensors or by a real human being, who checks for ingress during various points in the testing. Pockets, zips, and openings are all assessed.
This is the ability of a fabric to repel water and is affected by the fabric's coating and by the fabric itself. It is usually assessed using a spray tester, which sprays water droplets onto the textile at a predetermined pressure. The wetting of the surface is observed and graded from 1 to 5. 1 means that the specimen is completely wetted out; 5 means that the droplets formed are small and run-off quickly.
The relationship between real-life rain and hydrostatic head numbers is complex. Thus test results must be treated with caution as a higher hydrostatic head does not simply mean a 'better fabric'. When a fabric deteriorates through abrasion and use, its hydrostatic head will decrease. As such, a higher starting value may be beneficial to providing a durable waterproofness. A big hydrostatic head number also sounds good for marketing purposes. The trend for many years has been to increase hydrostatic head, starting with that of Ventile and progressing to that of Gore-Tex. However, Polartec have been brave enough to upset this trend with the release of NeoShell.
Lab tests for water vapour permeability
Assessing the water vapour permeability of water breathable fabrics in a laboratory is challenging because testing methods do not always reflect real-life conditions. However, lab tests remove the errors associated with human field testing, and are therefore a vital part of assessing a fabric's performance. There are many tests for breathability and the most common ones are discussed below. Some are very simple; some require multi-million pound equipment. Manufacturers will test their fabrics according to many of these methods, but they will rarely report the results of all their tests. They will instead report the results of the tests that favour their fabrics most. The tests can be divided into two broad types: Cup Methods and Complex Methods.
These methods form many international standards and use only minimal equipment. They are performed under constant temperature and humidity so don't represent the changing environments of real life, and work particularly poorly for hydrophilic-coated fabrics. These tests don't sound impressive, but provide results that are easily interpreted. In each method a cup is filled with water and the top is sealed by the test fabric. The amount of water that is lost through evaporation is measured.
These methods are usually more expensive to implement than cup methods. They generate large amounts of data, and not just related to water vapour permeability. They also look and sound impressive, which helps the marketing department out.
1) The DMPC (Dynamic Moisture Permeation Cell) is ideally suited to experiments where control over humidity and temperature is necessary. It generates results that correlate well with standard testing methods' results. In the test, a sample is held between flows of water-saturated and dry nitrogen gases. By measuring the change in the output humidity, temperature and gas velocity, a measurement of water vapour flux across the sample can be obtained. This is the method that Polartec favour for reporting NeoShell's moisture vapour permeability results.
2) Manikin testing is difficult to implement and very expensive but provides results that can be obtained through few other methods. However, manikins test clothing, not fabrics, and this complicates things considerably. Manikins are heated to body temperature, some can move, and some manikins can even sweat (see 'SAM' here: http://www.nap.edu/openbook.php?record_id=11959&page=34 for some extremely impressive engineering).
3) The sweating guarded hotplate can perform tests under dynamic and steady-state conditions. It forms two international standards and has been used extensively in research. It uses a controlled environmental chamber and a heat source over a water tank, which is covered by the sample fabric. It measures the amount of power required to keep the plate heated at a constant temperature despite water evaporation. The method is expensive, and models attempting to correlate its results with those of cheap dish methods have failed. It operates at 35 °C, modelling skin temperature. This tends to produce results that favour PTFE membranes whereas hydrophilics do not perform as favourably.
It is hard to compare results from different testing methods. No mainstream testing method has yet been found that correlates perfectly with user trials, and companies may continue to invent new tests that prove their fabrics to be the best. Julie Gretton, now at Berghaus, developed a non-isothermal (ie. changing temperatures) testing method that correlated well with most aspects of real-life testing. However, this method has not gone on to form an international standard.
Long gone are the days of bad fabrics and good fabrics. The differences between them are now small, both in the lab and in real use. More important is design, fit, price and environmental impact. It is essential to remember that we don't wear fabrics, we wear clothes. An ill-fitting jacket will never be comfortable and no one wants to pay for features they don't need, or for a jacket that is missing important elements. How many pockets do you want? Do you need a snowskirt? Do you need a helmet-compatible hood? What are the cuffs like? These are arguably more important questions than 'which fabric is the best?'
Manufacturers know that jackets made from Gore-Tex, eVent, NeoShell and all the other well-known fabrics can be sold at a premium cost. Thus, manufacturers make their best-designed jackets from these fabrics and they sell well. It is uncommon to see flagship designs from the major manufacturers made from their own cheaper fabrics, even though the differences in performance are small. Would you buy a full-featured jacket made from a less notable fabric? What if it was half the price? Asking opinions, reading reviews, and trying jackets on in shops are all vital parts of informed purchasing. A final impact on comfort is the fitness of the wearer, and the only way to get fitter is to get out there.
Getting the most from your waterproof clothing
To get the best possible performance from your waterproof gear there are some basic steps you can take:
• Only wear your waterproofs when it's raining hard. If it's windy then a decent windproof will do the job. Don't wear a hardshell when you're above the cloud line.
• Open the zips! Mountain Hardwear put out a bizarre video about a year ago that claimed that opening pit zips makes you sweatier. That's clearly not true. The argument was that fabrics breathe better when there is a high level of moisture vapour inside the jacket, and that opening zips inhibits this. That is all true. However, if the openings in the jacket are effectively getting rid of the moisture vapour, then you remain comfortable and the fabric doesn't have to work so hard.
• Check your other layers. It's so easy to worry about your waterproof one, yet the crucial ones are the ones next to your skin. That's where comfort matters, not on the inside of your shell.
• If you feel like you're too hot then take some clothing off. It is obvious, but a shell can add a lot to your insulation. Mark Twight outlines his approach in his book Extreme Alpinism and it's simple: wear little under your shell so you're cool when you stop. If you're cold while moving then climb faster! Don't leave the car park wearing four layers so that in ten minutes you melt and have to stop to take some off.
• Look after your clothing: wash it when it needs it and don't leave it to rot in the bottom of a pack. Don't wear it every day of the week then expect it to last ten years; instead save your best kit for when you need it most.
These offer excellent starting points on the subject and are quite easy to follow:
The more 'academic' books on this subject, for example those published by Woodhead, tend to be seriously expensive to buy. However, these books should be available in good libraries:
Unfortunately, journals often require paid access or a subscription to access them, but some are free. Typing 'waterproof breathable fabric' into Google scholar will give you loads of scope. Top authors on the subject are Phil Gibson, Julie Gretton, and Rob Lomax. Articles are only really for the super keen.
Many outdoor products are patented, but ensuring you are looking at the correct patent for a particular product is difficult. We can be fairly sure of the following patents, all of which can be found by using Google Scholar. Finding a patent is only half the battle; understanding one is far more difficult.
Authors: Matt Fuller, Dr. Mark Taylor
Both authors are members of Leeds University's Performance Clothing Research Group. Matt Fuller is an experienced hill walker, keen mountaineer, and terrible rock climber. Having completed a Chemistry degree at York University he moved to Leeds where he is now studying for a PhD that aims to improve the insulating materials used in the outdoor industry. Dr Mark Taylor usually prefers to go under mountains rather than over them. He has been testing outdoor clothing for the Performance Clothing Research Group since 2000.
Thanks to Mat Morrissey, Tom Hartland and Dave Williamson for their very helpful proofreading and suggestions.