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Staying Comfortable in the Outdoors - A Guide to Hot , Cold, Wet & Dry

By Ian Maley, Australian founder of the original Wilderness Equipment range of outdoor gear.

A Very Counter-Intuitive Experience...

Consider this experiment. Dip a hand in bucket of room temperature water, remove it and give it just a quick shake. Next, put this wet hand into a thin, completely waterproof but moisture-vapor permeable overmitt or glove (and fasten the wrist closure). Now, re-submerge (but not fully!) the covered hand in the water and keep it there for, say, five minutes.

While the time is passing you could think about how good it would be to have a perfect clothing system for your Earthly adventures. In hot, humid weather, and under hard exercise it would keep you comfortably cool and dry. In the coldest or wettest conditions you would feel pleasantly warm and be perfectly dry. In fact this ideal clothing system would operate very much like climate-control air-conditioning. In addition, you would hardly notice you were wearing it. Bulk and weight would not be a hinderance. Any body movement you were capable of making, you could make freely.

We could go on with the same wishful thinking about nights spent in sleeping bags, curled up in a small tent or under a rock overhang while the wind screams, the snow swirls or the rain pours down.

Daydreaming again! The five minutes are up so, back to that hand. Remove it from the water and take off the overmitt. What's going on? The hand is dry! How can that be?

The explanations for this and less strange, everyday things that happen to make us wet, dry, hot or cold lie in a few basic facts and ideas from physical science. You will see references to them scattered around outdoor gear catalogues, magazines and technical gear guides. I think it's most useful to collect all this information together in one place and sort it out. Then we can easily take on these connected ideas and use them as 'tools for life'.

Fabrics: 'Breathable' vs 'Moisture-Vapour-Permeable'?

As a gentle lead into the nitty-gritty take a look at the popular conception of modern, waterproof but Moisture-Vapour-Permeable fabrics now almost universally used in outerwear shell layers. Here is a good example of how the marketing has lead to fuzzy thinking. These fabrics are now in widespread use in many types of clothing, not just high-end outdoor products. They have been around for well over 30 years. Gore-Tex® fabric was the first. Today it is a household word but there are many other permeable textile technologies and brands to choose from. The functional layer responsible for the waterproof and moisture-vapour-permeable fabric properties may be a coating, an impregnation or a separate layer laminated (bonded) to the main supporting fabric. Whether 'coated', 'impregnated' or 'laminated' a futher backing fabric, usually a lightweight knit, may be added to cover and protect the MVP layer.

The usual word used to describe and promote all these fabrics is 'breathable'. It is natural to think of 'breathable' fabrics as having an ability for air to flow through them yet, in fact, these outer-wear shell fabrics have a very high resistance to air flow, so much so that they have the side effect of being highly windproof. So, irrespective of its common usage, 'breathable' is actually a somewhat misleading and inappropriate term. When we read the words 'breathable fabrics' it is important to translate this to 'moisture(water)-vapour-permeable fabrics' and forget about any ideas of 'air flow'. The submerged hand experiment illustrates this point nicely. If that mitt had been air-permeable it would have filled with water!  

30 years of commercial rivalry has also lead to many claims and myths about the performance and durability of one MVP fabric versus another. Testing the comfort, performance and durability of fabrics is complicated, non-standardised science. The marketing people would have us faithfully believe everything we read about this garment or that fabric! Perhaps the only true way to compare the actual performance of any two fabrics is to construct and use garments made one side from one fabric and the other side from the other fabric. This we have done, because, as a result of our human limitations, we are finally only convinced by what we actually experience, be it in the bush, the mountains and on the seas!

If you enjoy being outdoors the information that follows will help you appreciate not just the functioning of modern clothing systems but also of tents and sleeping bags. If you have been wet, cold and exhausted you may see much of this as common sense. Some of it is not. The world according to a water molecule coming to us in a rain drop or leaving us as sweat vapour is not quite like ours. To get a working understanding, one that will prove valuable when choosing and using gear, we will sometimes need to zoom in on this sub-microscopic world. Let's make a start…..

Survival, Comfort and Activity Level

The biochemical processes in our bodies only work properly within a very narrow 'core temperature' band. Outside this we are in serious trouble because, without the intervention of companions, we are unable to recognise and compel ourselves to take the necessary steps to recover a stable operating temperature. Malfunction occurs either by hypothermia or heat exhaustion.

Another way to 'not survive' (biochemically speaking) is to allow water, the solvent in our bodies to become so lacking that, again, processes run outside normal, sustainable rates. As long as we are breathing we are losing water. Fluid replacement is a constant requirement.

Most of the time though, we are simply only concerned with maintaining our comfort. We don't allow matters of body temperature or fluid loss to get out of hand. Setting aside transient conditions like sickness and 'state-of-mind' factors, we sense our everyday physical comfort in terms of skin temperature and skin 'humidity'. What's going on heat-wise in our bodies shows up at the surface in terms of these two aspects.

When we exercise, the physical activity generates more heat than when we rest. Whatever our level of activity, regulating the heat flow away from our bodies is the primary function of our clothing, sleeping bags and the shelter we use. Depending on the temperature of the air or water surrounding us, and their movement, we will need to do things to speed up this heat flow or to slow it down.

Conduction, Convection and Radiation

It is useful to understand a bit about heat energy and how it is transferred. There are many ways we can put this knowledge to practical use, not just in the outdoors but at home as well. The principles apply everywhere we look, in cooking, heating water and insulating the house, to name a few. First it will help to recognise the two quite different forms of heat energy.

The first type of heat energy is movement of atoms or molecules (vibrating, flying about, etc). This is what we simply call heat and it is the only type our skin can directly sense. In solids, heat travels by conduction. Although metals are a special case, roughly speaking atoms or molecules fixed in a solid 'warm up' their cold neighbours by jostling and bumping them, and so on down the line. In fluid substances like water or air (where the atoms or molecules are not fixed, heat can also travel by convection. The less dense hot bits 'float' upwards through the denser cold bits so creating 'convection currents'. Of course if the fluid is moving as a whole (like white water in a rapid, or the wind screaming across the deck) heat is also being carried away from our warm bodies by forced convection. Nothing unusual, but remember to think of 'hot' as where the heat energy comes from and 'cold' as where it's going to.

Infra-red radiation is the other form of heat energy. Even cold surfaces radiate infra-red energy. Hotter surfaces radiate more. It travels at the speed of light and has to be stopped again (absorbed) before it can become useful heat in the form of atomic and molecular movement. Keep in mind that things we can see through, like air and glass, don't stop infra-red radiation. The warmth of a sun-filled room in winter illustrates these points. Nothing else quite warms us up like basking in the sun on a still winter's day. Infra-red radiation plays the leading role in the daily cycle of heating and cooling. After sunset, under a clear sky, the environment rapidly radiates its heat out into space. Only a blanket of clouds or overhead obstructions like forest canopy can slow this down, something that is easily appreciated when camping out.

In our homes and in the things we wear and use we can make great practical use of the fact that dark, rough surfaces radiate and absorb infra-red heat energy more than light-coloured or reflective, smooth ones. This second type are actually better reflectors, whether polished metallics or not.

Air and Wind

Air plays a leading role in our clothing systems, sleeping bags and even our tents. It is the basis of all lightweight thermal insulation. As a gas, it is almost entirely empty space. The molecules rarely bump into each other so air is a very poor conductor of heat. For air to be effective as an insulation the trick is to stop convective heat flow and loss. We use the term dead air. Down, synthetic fibre insulations, open and closed-cell foams all serve to trap air in tight little spaces or small compartments. This is not what happens in a cheap, simple air-bed where air can circulate freely. (Try spending a night on one, on a cold concrete floor!). In good insulation convective air currents are minimised or confined so heat can not be carried along. (In closed cell foam convection is confined within the small cells. Since the foam material forming the cell walls is chosen to be a poor conductor closed cell foam is an extremely effective insulator).

Wind needs our attention also. In contrast to dead calm conditions, as air starts to move and the wind speed picks up its potential to carry heat away increases dramatically. We speak of wind chill factor. For example, air with a temperature of 8 degrees C moving at only 30 km/hr will rob our skin of warmth as fast as still air at minus 3 degrees.

Ice, Water and Vapour

We can't ignore water, whatever phase it happens to be in: ice, water or steam/vapour. Water plays a central role in our bodies, in the environment and in life. We sweat it and breathe it out. We work and play on it and in it. And it is critical to understanding clothing. H2O is a truly remarkable substance. What other substance expands to float in its own liquid when it cools to a solid? However interesting, we will leave this diversion and its implications for 'life on Earth' to your curiosity. All we need to know now are the following few things…..

Unprotected and submerged in still water our bodies lose heat 32 times faster than in still air at the same temperature. Water, being much denser than air, can conduct and convect heat away much more effectively.

Evaporation. To get a water molecule off the surface of liquid water and begin a new life on its own as water vapour requires, as it turns out, a lot of heat energy. This is a crucial fact. It is why evaporation is so good at removing heat from wet skin and clothing (and why letting our clothing get wet can be a real hazard). It is also why steam causes nasty burns. When water vapour condenses on skin a lot of energy is released. The fact that water vapour is invisible until it condenses only adds to the hazard.

Condensation is the opposite of evaporation. Once water vapour condenses to liquid water droplets it requires a lot of heat to evaporate it again. At any particular temperature (and pressure to be precise - but ignore that for now) air can only hold so much water vapour – hotter air more, colder air less. When air is completely saturated with water vapour we speak of '100% relative humidity'. Adding more water vapour to OR cooling air that is this saturated will cause droplets to form or condense as dew, fog, mist, clouds or rain. The term we use depends on the situation. For a given water vapour concentration, the temperature at and below which condensation occurs is called the dew point. When it is very cold, it is not uncommon for dew point conditions to occur within our clothing, sleeping bag or tent layers. The first case for condensation (adding more water vapour) usually occurs because physical activity combined with being overdressed causes sweat to be produced faster than it can get through and out of our clothing layers. The second condition commonly occurs with sleeping bags and tents because, at rest, we are trying to retain body heat but, at the same time, the after-dark environment is rapidly losing its daytime heat and air temperatures are plummeting. Avoiding, or dealing with this when it occurs is very largely what all this interesting stuff is coming to!

Water droplets. When mist droplets form they are microscopic. They get bigger as stray vapour molecules bump into them and 'stick'. They stick because the droplet is surrounded by cooling air and there is little chance of getting the energy needed to leave again. It's a bit like the droplet is acting as a vapour trap. This 'holding power' of droplets is also responsible for the effect we commonly call surface tension. All the molecules in the liquid drop feel a certain attraction to their neighbours. The ones that find themselves on the outside, on the surface, with no neighbours in one direction, feel a stronger, lopsided attraction back in the direction of the droplet liquid. 'Surface Tension' gives a droplet its familiar smooth shape.

Hydrophyllic and Hydrophobic

You will have noticed water soak into or 'wet out' some surfaces and 'bead up' on others. We use the term hydrophilic to describe water loving substances. They actually cause themselves to get wet by chemically breaking through the surface of a water film or a tiny droplet's surface tension 'skin'. On the other hand water hating, hydrophobic substances do not have this ability. Water droplets, even big ones, sitting on a hydrophobic surface, stubbornly retain their integrity. There is absolutely no attraction, rather a repulsion, between the molecules of the hydrophobic substance and a water molecule. You can think of it as a chemical stand-off, if you like.

Hydrophobic chemicals like silicones and fluoro-carbons are widely applied to the surfaces of fabric fibres to make them water (and stain) repellent. The advantage of keeping a fabric dry in this way is that it stays lighter in weight and is not subject to evaporative heat loss. Do not confuse water repellency with proper, reliable waterproofness. The size of the holes or gaps between the yarn filaments in even the tightest microfibre weaves are simply not small enough for the hydrophobic surface tension effect of even the most effective fibre treatments to positively prevent water forcing its way through at everyday pressures. Also, what ever the level of initial 'water resistance' these water-repellent-treated microfibre weaves may have, they rapdily lose this resistance with depletion of the water-repellent chemicals but particularly also with contamination by every-day substances that we use or which are present in the built-up environment.

Polytetrafluoroethylene is a pure, highly hydrophobic substance. It turns out that when this polymer is stretched under certain conditions it becomes riddled with sub-microscopic holes. This "expanded" poly-tetra-fluoro-ethylene, ePTFE, is the basis of the membrane used in fabrics like Gore-Tex® and eVent®. In simplified terms, provided the holes are very tiny compared to the size of the smallest droplet that may form on the highly hydrophobic membrane, it will take pressures much higher than experienced day-to-day outdoors to overcome the droplet surface tension and squeeze the water through these holes. At the same time, these tiny holes in the membrane 'appear' huge to an individual water molecule and present little barrier to its passing travel through space.

Some modern fabrics also use highly hydrophobic substances to solidly coat and encapsulate tightly woven microfibre yarns further reducing the size of the gaps and pathways through the finished fabric. This encapsulation technology represents a significant shift up from common water-repellent treatments. Water resistance is improved while maintaining good moisture-vapour permeability and even some air permeability. These comfortable, soft, packable fabrics are a great solution for garments or shelters where 'showerproofness' is an acceptable level of water resistance. They are not a adequate when come-what-may waterproofness is the required against heavy or continuous rainfall.

Hydrophilic substances also find a place in outdoor clothing as waterproof, vapour-permeable coatings on fabrics. These are often urethane polymers and they work in a completely different way to the hydrophobics. In the same way as detergents collapse water droplets, water molecules are attracted into the coating layer. Here, within the sponge-like polymer structure there is just enough room for individual molecules (but not liquid droplets) to move along the surface of the long molecular chains. Along these chains are 'chemical handles' that loosely hold the (polar) water molecules as they diffuse through the coating. Why should they move one way or another? The next section explains the driving force that causes water molecules to move in a particular direction through any form of vapour permeable fabric, whatever type it may be.

Diffusion

Think about this. We are jogging quietly along in a tee shirt on a comfortable, temperate day. Inside the shirt, near our skin, due to our activity level, the air is somewhat warmer and more humid than the air outside. Higher humidity means there is a higher concentration of water vapour / molecules under out tee shirt than outside it. The higher temperature means these water molecules are moving about more quickly. Even though the air outside our tee shirt contains water molecules, the higher temperature and humidity inside means more water molecules are hitting the inside of our tee shirt more often than those on the outside. Now the effectively huge holes in the knit of our tee shirt look just the same to a water molecule which ever side it is on. As a result, there are more molecules escaping out through the holes in the tee shirt fabric knit than are coming in through them, although there are always some coming in through our tee shirt and some going out. Many will even do the round trip!

The situation is no different in a highly technical moisture-vapour permeable rainshell. No coating, lamination or other treatment can selectively stop outside water molecules coming back in if those inside are allowed to get out. It's always a two-way street but the net flow will always be from the more crowded, more agitated side to the emptier, calmer one. (If it wasn't, air conditioning systems for example could run without an external energy or power supply).

T-shirt or rain parka, the greater the difference in temperature and humidity from inside our clothing to the outside air, the greater will be our net rate of loss of moisture vapour (sweat) out through the fabric. You will see the word gradient used to describe these temperature and humidity differences.

Comfort and the Ideal Clothing System

We've just covered a lot of ground! In fact you are now probably better informed and able to understand the workings of your clothing and tent than most marketing and advertising people who's job it is to sell these products! Let's not slow down.

Below a certain 'comfortable humidity' our skin actively and constantly gives off water vapour to make up for the dry air in contact with it. Just as we keep our lips moist (and blink our eyes for the same reason), it is a matter of maintaining suitable 'living conditions' for our skin.

When we exercise and overheat a quite separate process starts. We sweat because our bodies evolved to use the real heat loss benefits that come from evaporating water. It's a good idea to avoid sweating unnecessarily. When liquid water is at a premium we should conserve it. A good example is high in the mountains where water can only be obtained by melting snow and stove fuel must be carried to do this. Also, the accumulation of condensed sweat in our clothing could possibly lead to dangerous heat loss after we stop to rest. The take-home message is simple - avoid overheating!

So now we can understand many of the important characteristics of a good clothing system.

The level of insulation should be variable. This may be achieved by designing for variable ventilation or by adding or removing separate clothing layers. No layer should absorb or hold water. The careful selection and treatment of textile fibres according to this function is important. Our clothing system should promote the outward movement of any excess moisture from our skin surface. It should be easily and effectively ventilated so that moisture vapour removal is maximized before it has a chance to condense. It should have a completely waterproof outer 'shell' layer so precipitation is unable to wet insulating layers. The system must include a windproof layer. Finally, all layers, including the outer shell should be as moisture-vapour-permeable as possible, particularly for the times when driving rain or blizzards prevent opening the system for maximum ventilation.

In very hot or extremely cold environments certain factors need special attention. Hot environments tend to have a lot of infra-red radiation zapping around. External reflection and ventilation become important clothing design issues. In extreme dry cold environments, like the polar regions, getting rid of excess body heat is relatively easy. Well designed insulating clothing, by being adjustable according to activity level, should minimise the risk of sweating. A vapour barrier layer can then be introduced between our skin and our insulating layers. This barrier will reduce body fluid loss due to that steady, invisible flow of background perspiration. The vapour barrier prevents the escape of skin moisturising humidity. It also eliminates the source of frost that would otherwise accumulate within our insulating layer. Although specialised, the concept of a vapour barrier is an interesting one.

Functional Clothing Combinations

Whether it's a trip to the shops or an expedition to the Himalaya we ask the same question, 'What is the minimum clothing I need?' We look at the range of weather conditions we are likely to face and the levels of activity we will undertake, from sleeping to high-aerobic exercise. We should also factor in the possibility of unplanned stops due to extreme weather, breakdown or injury.

Layering as a clothing principle is hardly remarkable. It's more like common sense. What is actually important when we are choosing clothing is to consider how a particular layer contributes or works when combined with other layers. Here are some thoughts….

Something to keep firmly in mind is the fact that, as layers are added, bulk accumulates quickly at the wrists, neck, waist and underarms. Limit elaborate, adjustable closures to one layer only. It makes most sense that this be the outer, waterproof shell layer.

Stiffness particularly, but also bulk and weight increase with the use of full-length vertical zips, commonly used for centre front openings. Successive clothing layers with this construction should be avoided, so too zip-together layer systems, for the same reasons. Touchtape closures are also bulky and stiff. Uncomplicated pullover layers, using lightweight press studs or snap buttons rather than zips to close openings, and simple, light elastic binding on cuffs and waist bands deliver the twin advantages of torso flexibility and low bulk. Keep it as simple as possible!

Ventilation openings such as neck openings, underarm zips and loosened cuffs may work well in isolation but they become ineffective unless all layers have matching features. Then the problem again is that bulky design details which may work ok in isolation, when overlapped with other layers rapidly negate any advantages. Again, keep it simple!

Now think ahead about what would happen if one or more layers got soaking wet? Double-layered cuffs, collars, waist bands and other details take a long time to dry out.  Collars and cuffs also happen to be located where body temperature can be easily regulated and where discomfort can also be particularly frustrating. Yet again, look for simple, low-bulk, quick-drying design solutions.

Fleece garments that incorporate a windproof membrane are also extremely slow to dry. Worse, they can not be comfortably combined under a rainshell layer without severly reducing the ability for perspiration to escape the garment. The combination of any two membranes makes for a very high overall resistance to evaporative transfer (Ret), or, put another way, a very low moisture-vapour transmission rate (MVTR). Windproof fleece garments are best kept for single-layer street outerwear, not the back-country. On the street they are great and the ability to substitute alternative garments when the weather changes is never far away. For back-country use the most effective, versatile windproof layer is a simple, thin, shell pullover, made from a lightweight, downproof sleeping bag nylon or similar. Worn over your fleece layer the tightly woven fabric cuts the wind but is no barrier to perspiration evaporation. You can see grey-haired ski-tourers wearing such shells. Often they are home-made!!

In the Sleeping Bag and in the Tent

All the principles and ideas explored above apply just as much to managing your comfort during hours spent living in a compact tent. If it is raining, snowing or just plain cold they will prove very useful in optimising your comfort. It maybe just for overnight, but if you are forced to stay put for days on end because it is impossible or unsafe to travel then maintaining comfortable living conditions may become critical. Choose your tent and sleeping bag once you have a good idea of what you are possibly in for weather-wise. Here are a few comments I hope may help.

The type of tent, where you set it up, how you align it, how you arrange ventilation, where you cook, how long you use the stove for and what you do with damp clothing, for example, will all impact on your comfort. The instructions for using WE tents included in this website contain much practical information regarding these matters. After basic waterproofness and strength, effective ventilation design is critical in a compact tent. It should be possible to provide flow-through ventilation whether it is raining or snowing. By the simple process of breathing an adult human exhales something like a litre of water overnight. Sources of water vapour within the tent are us humans, the stove fuel combustion process, the cooking pot and damp clothing. If this can not escape from the tent, when the relative humidity reaches 100%, and this happens very quickly in a small tent in the cold, this moisture vapour will condense and become a nuisance. Imagine taking your 2 litre water bottle and pouring it around the tent! More serious is the fact that down-filled sleeping bags in particular may take up moisture and rapidly lose their insulating effect. A down-filled bag with a standard shell fabric will lose most of its loft and insulating effect after several days in such a closed, condensing atmosphere. Then you will be looking at two layers of ripstop and some soggy clumps of down!

The other way to reduce condensation is to keep the tent as warm as possible. This can only be done by limiting heat loss from the outer surface of the tent and the way to do this is to set the tent up where it can not radiate directly to a clear overhead sky (so under a tree canopy or overhang) and at the same time it can receive infra-red radiation from these nearby objects. This is a powerful effect easily seen in a camp ground where some tents have been set up under cover and some have not.

The choice of sleeping bag, the way zip openings can be used, the possibility of moving the down filling from the upper side to the under side, whether it has a water-resistant shell fabric and the hood design will all affect comfort. These variables, possibly in combination with a choice of clothing layers, must allow for just the right level of insulation for sleeping. Over-heating will result in perspiration. This moisture will immediately be absorbed by down filling. It will only travel out from the sleeping bag if the tent atmosphere is relatively dry, which only comes with improved weather conditions. Be sure your sleeping bag is not too 'warm' for where you will be using it. Remember, you can always add clothing layers and a better night's sleep comes from sleeping cooler than warmer.

For snow camping (where the possibility of being tent bound for several days is always on the cards) there is a great advantage in using a sleeping bag with a highly moisture-vapour-permeable but waterproof shell. In temperatures around and just below freezing (common in the Australian snowfields) this shell will prevent the inevitable dampness in the tent soaking into the down filling. It is well worth the premium price. In extreme cold it makes little difference since moisture doesn't get far from its warm source before freezing. Ice crystals forming inside the tent are no hazard and can be shaken out. On the otherhand, in extreme cold, the 'freezing point temperature' contour or surface is likely to fall within the sleeping bag insulation layer. This means perspiration from sleeping too hot (or from simply using the bag for weeks on end in such conditions) will condense and freeze within the down layer, and that is a hazard. This is where a vapour-barrier sleeping bag liner comes to the rescue.

END

 
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