Meaning: Mist –
Hasu Hooded Shell Jacket
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.
The job of a shell layer is to help keep you comfortable, which means being windproof, waterproof and breathable (allowing sweat to escape). Therefore they need to stop air (wind) and water penetrating them, while allowing water vapour through. Designing a material that does this is not trivial.
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 jacking wearing 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.
What is waterproofness?
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.
HASU use materials with a minimum of 7000mm in many cased above 15000mm
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 weather. 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.
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:
Waterproof Fabric Equation
S refers to the surface, G to the gas (air), and L to the liquid (water). The symbol gamma, γ, describes surface energy. Thus, γ(sg) is the interaction between the solid and the air. The contact angle, theta(c), determines whether the water droplet beads or spreads. A large contact angle means the surface is hydrophobic, so the fabric will shed water.
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.
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 hard shell when you’re above the cloud line.
• Open the zips! 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.