Below is the Warmlite article on vapor barriers with my comments in bold- John
Vapor Barrier - Why?
Ice, solid water, has very low energy. To melt ice to liquid water you must add 144 BTUs per lb. (BTU = British Thermal Units = heat energy needed to warm 1 lb. of water 1°F.) It takes 1080 BTUs to evaporate 1 lb. of water to water vapor. The amount of water vapor in the air is called humidity, expressed either as absolute lbs. of water per lb. of air, or as relative, % of the maximum that could be there at that temperature and pressure. It is common to refer to water vapor as humidity. SWEAT is the liquid water your skin exudes from sweat glands in your skin to COOL you when you are overheated. Unfortunately, that sweat also contains oils and SALT! Salt and soluble oils are moisture absorbents: depending on concentration and type of salt and oil, it can take up to 3 times the heat energy to evaporate water from such absorbents, and all that excess energy goes into chemical change. You have noticed that initial sweat seems to cool you much better than later sweat: dried salt and oil resist evaporation, and release heat to your skin from contact with new sweat (see SUMMIT Oct.’59). A fresh water rinse cools you and restores the cooling of initial sweat. NOTE that the PURPOSE of sweat is ONLY to COOL you. Obviously then, at first sign of sweat wetness you MUST remove any excess insulation (or ventilate to carry off excess HEAT.) If conditions are cool enough that you need ANY clothing, then you want to immediately STOP any sweat loss and use convection, conduction, and radiation to get rid of excess heat. Any moisture lost thru sweat MUST be replaced soon (which may be difficult or impossible at the time, so it’s best to STOP the loss when it starts!)
When water evaporates from the body surface, 0.58 Calorie of heat is lost for each gram of water that evaporates. The energy required to evaporate sweat does not change to any significant extent.
Humans have a problem which we are told other animals don’t have: the moisture IN our skin evaporates in dry air, thus losing heat and water. That moisture loss is called “insensible sweat”, which term, like “military intelligence” is an oxymoron (ie, self contradiction). That “insensible sweat” is NOT sweat, and IS sensible: you FEEL it cooling you (but don’t feel it as wetness, thus the “insensible”). Water vapor from evaporation IN your skin, with it’s high energy, diffuses rapidly thru to outer clothes where heat is lost. Usually in cold weather the outside relative humidity is near 100% so outside air can’t accept more humidity, and thus most of that moisture condenses to cold water, soaks your clothes, disables your insulation, lowers humidity again, so more chilling evaporation occurs IN your skin, repeating the cycle of chilling and soaking your clothes. Even if outer fabric is completely porous the vapor WILL condense where temperature reaches dew point in the clothes. The outer layer (“breathable” or not) keeps water IN, out of sight, so you don’t realize you’re losing insulation until later, when miserably COLD. Evaporative cooling and water loss depends only on the relative humidity of the air next to your skin, so you have no control over it. Or do you? (think for a while).
Insensible evaporation cannot be controlled for the purposes of temperature regulation and results from the continual diffusion of water molecules through the skin and respiratory surfaces regardless of body temperature. Water evaporates insensibly from the skin and lungs at a rate of about 600 ml per day. That may be elevated slightly at altitude due to increased respiratory rates. It cannot be felt cooling the body because of the slow rate of heat loss at 12 to 16 calories per hour. Relative humidity next to your skin has little to do with insensible evaporation. The body does not sweat to maintain skin surface humidity.
Heat production and loss is not uniformly distributed over our bodies. We can sweat under our arms while being too cool elsewhere. We detect changes in temperature only on our skin, but can’t determine absolute temperature of our body by what we feel on skin: get cold enough to shiver, then get into a hot tub and you’ll feel too hot while actually being too cold. As you warm, your skin gets accustomed to the warmth so you don’t feel as hot! Get out of the hot tub when sweating from overheat and you immediately feel cold! Dry off and you feel warm. We rely ONLY on wetness of sweat to warn us of overheat. If your heat loss equals production you’re comfortable. If activity then increases, overheat causes sweat, for evaporative cooling. WHEN (or IF) you notice wetness from sweat, you’ll vent or remove extra clothes, get cooling of evaporative or convective heat loss, stop sweating and you’re soon dry. Wickable underwear moves sweat from overheat away from your skin so you won’t notice it and it won’t annoy you, (which is fine for comfort indoors or for short periods). That wicking prevents cooling when and where you need it, and wets outer clothes so they won’t be warm LATER. Please note that it’s wickable and moisture absorbing fabric that aids comfort then, not just porous or so called “breathable” junk. Non wicking polyester, acrylic, Goretex and similar won’t provide any comfort, so YOU have to constantly adjust insulation or venting in response to wetness from overheat, (which can be an advantage IF you’re observant and intelligent enough to do proper adjusting). Heat stroke or heat exhaustion is caused by not being aware of and correcting for overheat. Wicking clothing makes you unaware of sweating, so can be dangerous. Instead of sweat cooling you when needed, it soaks your clothes, reduces insulation and chills you later when you need the warmth! You won’t notice overheat until soaked, so delay your normal reaction of venting or removing excess clothing, until too late. When you tire, slow down or stop, and need your insulation, you find it is wet and useless. Instead of the sweat which wicks out evaporating, humidity from within condenses, making outer clothes even wetter. That’s controlled by the temperature in outer layer(s), not whether they are porous or sealed. Before you die of hypothermia from believing false ads claiming their insulation is warm when wet, I suggest you soak your jacket, shake it out and wear it. Experience just how cold, wet insulation really is! False advertizing won’t keep you warm.
Wicking doesn't prevent cooling entirely. It does move some sweat away from the skin so flash cooling is minimized in cool/cold conditions. That is a good thing. In warm weather, loose fitting clothing is advised to take advantage of maximal evaporative cooling. Vapor barriers do help stop water from getting into insulation in winter expeditions.
Part of the idea of using wickable underwear for warmth is the insane idea that your skin continuously LEAKS, so they want to move leaked moisture away from your skin before it evaporates and cools you. Any kid old enough to talk can tell you your skin stays dry UNTIL you sweat from OVERHEAT, and then you WANT evaporative cooling AT your skin. NOTE: Just to confuse you more, several companies say their materials “wick moisture vapor”, but you know that wicking only applies to LIQUID, not vapor! Most of this isn’t a problem if you’re going outside for short periods with steady activity and not overdressed. But for someone jogging, skiing, hiking, or mountaineering it can be a very serious matter.
The skin does continually loose water and heat at the rates quoted above and that is called insensible evaporation. Insensible evaporation is not felt as wetness on the skin surface because of the slow rate of water molecule diffusion.
Obviously wicking underwear can’t stop chill of moisture evaporating from within your skin (misnamed insensible “sweat”), since that moisture is not on the surface where it can be wicked away. The ONLY way to reduce that evaporative chilling is to raise humidity next to your skin by raising humidity in surrounding air (limited to dew point in that air), or by retaining humidity with vapor barrier (VB) next to the skin. A VB that blocks 95% of evaporative heat and water loss is excellent. (Goretex will block 97%. They call that 3% loss “breathable”).
There is no chill of moisture from insensible evaporation.
If humidity next to your skin reaches 100% (meaning it can’t hold any more water vapor), evaporation stops, chilling stops, and “insensible sweat” stops. That’s why a humid day feels warmer than a drying day. (Note that it’s common to call low humidity dry when the correct term is dryING, which low humidity causes.) A wet rainy day feels colder because the rain acts as a condenser, removing humidity from the air, leading to drying condition. Often a “dry” sunny day feels extra hot due to the high humidity the sun has caused by evaporating water that fell as rain before.
When skin moisturizing can’t keep up with rapid drying, your skin gets dry, chapped, and is more likely to suffer frostbite. Evaporative chilling makes 32°F feel like 12°F.
Insensible evaporation may be slowed by 100% humidity, but that is a dangerous environment to be, in warm or cold weather.
It’s reported that you lose up to four pounds of water each night thru evaporation of “insensible sweat”, when sleeping in a porous “breathable” sleeping bag. Weighing of such bags in the morning shows 2 to 4 lbs. increase, confirming that statement, and also showing that sweat and vapor don’t make it out of those bags: sweat wicks in, and vapor condenses in the insulation, leaving the bag wet. The 4320 BTU of heat stolen from you to evaporate 4 lbs. of sweat is lost at outer surface of your bag, as that sweat condensed to soak your insulation. It takes 144 BTU to melt one pound of ice. Thus the heat to evaporate four pounds of sweat is enough to melt 30 pounds of ice! (4 x 1080/144 = 30). Would you take 30 pounds of ICE to bed with you? That’s the effect you get by not using vapor barrier interior in your sleeping bag. If you lose 4 pounds of water during 8 hours of sleep you can expect to lose much more during 16 hours you’re awake and active. That dehydration can lead to serious impairment of circulation due to thickened blood, increasing risk of frostbite (thus the good advice to drink LOTS of fluids in cold dry weather). You can create a warm humid condition around your body all day with VAPOR BARRIER (VB) clothing, and thus reduce dehydration.
Over a day you lose (from skin and respiration), as stated above, about 600 ml of water through insensible evaporation which weighs a little over one pound. That is corrected with drinking water so the change is basically not noticed. In cold conditions, some of that moisture may accumulate in a sleeping bag and freeze there. Vapor barriers would help in that situation. Vapor barriers will not reduce dehydration.
During World War II US cold weather troops used Vapor Barrier (VB) socks to totally cure frostbite and trench foot. Those led to the vapor barrier “Korean Bunny Boots”, still the standard for cold weather use. We started promoting use of VB socks (baggies, bread bags, etc) in 1957, then gloves, shirts, and in sleeping bags since 1967. Others have sold VB clothes and bag liners on and off, but the bad response to uncomfortable coated fabrics, poor education, and problems with tie in bag liners, led most to drop VB. Manufacturers and retailers want to sell what is EASY, and avoid anything that requires educating customers. Heavy promotion of “breathable” materials makes some retailers unwilling to risk big markup sales by telling customers the whole truth. Often they won’t tell you anything about things they don’t sell. The most common excuse we hear from manufacturers and sales persons for not selling VB lined bags and VB clothing is they can’t take the time to explain it to their customers. Mighty inconsiderate! If you want an honest evaluation of VB, get it from someone who uses it. If you want to avoid it, ask someone who hasn’t used it, or sells only “breathable” gear, thus avoiding getting confused by the facts!
Vapor barriers can be good for the hands and feet, but their prolonged use in keeping high moisture next to the skin can cause problems of their own.
VB in a sleeping bag gives no added warmth when vented but always protects the insulation from condensation and sweat soaking, thus it’s advisable to have VB in your bag for ALL seasons. The surface wickability of Stephensons FUZZY STUFF makes it especially desirable for summer use when you’re sure to overheat, (even if nude.)
When a VB is vented the moisture is now in your tent and may condense just like any other moisture on the outside of your sleeping bag. Using a VB in summer is not recommended for your safety.
A common argument against VB is actually excess praise FOR VB: they say VB will ALWAYS overheat you! Wouldn’t it be nice if we could get ALL needed warmth simply by controlling humidity! Physics limits us to maximum of 20° added warmth from VB. It’s the overheat DETECTION SERVICE that VB provides (by making you immediately aware of sweat when it starts) which “they” think is overheat caused by VB: don’t blame the messenger for the message!
Wearing a VB in the summer may lead to heat stroke. In winter, there are some uses for a VB. Be careful when using vapor barrier clothing while active in winter.
Will Steger used “breathable” Quallofil sleeping bags for his much advertized dog sled trip to the north pole: those 17 lb. bags (almost as thick as our 4 1/2 lb Goose Down bags) were carried loose on top of sleds “for best drying”, yet weighed over 52 lbs. in a few weeks from sweat condensing to ice. Luckily they were flown out from the pole. Meanwhile a Canadian - Soviet team cross country skied across the pole, using WARMLITE bags they had purchased, which stayed dry and warm for the whole trip. Will Steger bought FUZZY STUFF Vapor Barrier liners from us for his Quallofil (read, $500,000 support from Dupont!) bags for the much longer south pole trip and thus kept the bags dry and warm the whole trip.
Vapor barriers are good for winter expeditions in keeping moisture out of insulation as stated.
VB clothing that doesn’t wick sweat over it’s surface is likely to be uncomfortable and lead us to frequent insulation changes, or sadly mislead some into rejecting VB and the benefits it can give them. Proper comfortable use of VB requires more intelligence and awareness than some people have, but is made a lot easier with modern VB material having wicking inner surface, such as FUZZY STUFF. With VB keeping water vapor and wet sweat out of your sleeping bag and clothes, you can use ANY fabric, ANY insulation without concern for wickability, and can use ANY exterior wind breaker without concern for “breathability”.
How do users of VB react? Generally with orders for more VB clothing and sleeping bags, and recommendations to their friends. From 1967 to 1998 we sold about 9500 VB lined sleeping bags, and only about 1/2% of customers objected to having to consciously adjust insulation. But even they agree that VB is good for extra warmth and insulation protection, and most of those became best promoters of VB! We’ve found many of those people have low metabolism, need more insulation to stay warm, and thus NEED VB the most! No matter what one’s metabolism is, the extra heat produced from activity is the same, and thus the person who wears thicker clothes for warmth when inactive will sweat more when active due to those extra clothes. To stay dry they must adjust clothes more. VB underwear helps them notice the need to adjust, and keeps all outer clothes dry even if they fail to control sweating. When you are awake and active it is easy to adjust insulation to avoid overheat without venting VB clothing. When asleep the normal reaction to overheat is to push covers away, reducing the extra warmth, while VB still protects the bag from condensation and sweat. Sleeping bags rarely get wet from outside. Bags without VB ALWAYS get wet from INSIDE condensation and sweat!
Sleeping bags can get wet from inside or outside.
Most of you are aware that wind can chill you. If nude, wind reduces the insulating air boundary layer on your skin, increasing conductive heat loss thru that layer. Stop the wind, or block it with wind tight fabric, or get inside a structure, and that chilling stops. Then as you all know, adding ANY layer of even the most porous clothing makes you warmer. At some point any additional layer overheats you, which you notice only when you start to sweat and feel wet. Do a test: In a wind blocking shelter when it’s cool enough to need a warm jacket, replace the jacket with two thick bulky knit sweaters (as open a knit and thick as you can find). Soon you’ll start sweating from the overheat (note that it is only the sweat that tells you that you’re overheated!) Mere porosity or “breathability” clearly can’t keep you cool. Replace the thick sweaters with a light raincoat (after you cool down). Soon you will feel too cool, clearly proving that a simple waterproof coating is not enough to keep you warm or overheat you, but it can help. Assuming condition cold enough so you are wearing an undershirt, 1 or 2 insulating shirts, and the warm jacket: replace just the innermost shirt with a vapor barrier shirt (lacking a proper one, use a plastic bag with holes cut for head and arms). Soon you will notice sweat from overheat and will need to remove the jacket to stop overheat (if smart you’ll speed up the test by not putting the jacket back on after changing to VB shirt, and will then notice you are as warm as before and not sweating.) The VB shirt reduces loss of humidity and thus reduces evaporative cooling at your skin, much like a humid day in summer.
In each case if you carry test to point of overheat, notice that it is the wet feel of sweat that told you “you are overheated”. Our bodies are very poor at telling us how warm or cold we are, and skin senses changes more than absolutes.
I first notice a heat sensation before feeling sweating when wearing too much clothing for a given temperature. Humid days in summer are among the most dangerous due to the heat index.
VB clothing has many other benefits:
- Elimination of condensation in your tent. People who regularly over dress and rely on wickable clothing to carry away sweat, add much more humidity to a tent. If you must change your shirt in less than 3 days due to sweat odors you will also likely cause excessive condensation in any tent you use. Wearing VB helps you recognize and correct overheat and unnecessary sweating.
- VB clothing doesn't stop condensation in your tent because most of the moisture is coming from breathing. Please don't put vapor barrier clothing over your face. It will suffocate you. Wickable clothing doesn't mean more humidity in a tent. Going to bed in wet clothing does. A smelly shirt does not mean excessive condensation will occur. It means you have b.o.
- Elimination of sweat odors on clothing and yourself. It’s obvious how outer clothing is protected. Apparently quick sensing and thus avoidance of sweating, plus blocking of air circulation that causes sweat to turn rancid, reduces or eliminates sweat odors on you and the VB clothing as well.
(Polypropolene underwear is infamous for terrible sweat odors: apparently it passes sweat so well that people sweat excessively with it without realizing it. BUT it absorbs all the oils in the sweat, and those oils turn rancid, stink, and stick to the polypro.)
- VB clothing will not stop body odor as described above. It will stop moisture from getting into insulation, most useful in cold conditions. Air circulation does not cause sweat to turn rancid. Bacteria on your skin does.
- Reduces dehydration and amount of water you must obtain and drink. Dehydration is a major contributor to frostbite, hypothermia and altitude sickness. It thickens your blood, impairs circulation (thus decreases proper heat and oxygen distribution), and reduces oxygen intake. It’s especially difficult to drink enough fluids when not wearing VB clothes and ALL your water most come from melting snow! In several days the weight of fuel saved due to use of VB can greatly exceed the weight of the VB clothing.
- VB clothing will not reduce dehydration and the amount of water you must obtain and drink, to any significant extent. At least three liters of water per day should be consumed and more if you feel thirsty. Water lost through sweating is not reabsorbed to any significant extent, if at all, back through the skin.
- With 1st layer VB you can then wear any kind of material for outer layers, no matter how uncomfortable or impractical that material might be otherwise, since you’ll have no concern with it getting wet. Your outer windbreak layer can be any coated or laminated fabric, preferably NOT “breathable” so you don’t have to be concerned with dirt causing it to leak. When weight is a consideration, chose your layers for the most thickness per pound. Use coated Nylon rain wear windbreaker.
- This is old information and is outdated with todays fabrics.
- Avoiding winter “colds”: most medical writers say a “cold” is only a “cold virus infection”, (typically with symptoms of irritated nose and throat and clear fluid from your nose), which your body self cures in 3 to 7 days. But, your nasal and throat passages usually have lots of all kinds of infectious bacteria in them, which are harmless to you as long as they can’t get past mucus surfaces. Virus infection, or bad allergy attack, or dry irritated nasal passages due to excessively dry air, can ALL let those bacteria attack, resulting in what we usually know as a “cold” with greenish yellow nasal discharge, sore throat, cough. Untreated that can last a whole winter, or be stopped in 3 days with antibiotic. Wearing VB clothes at home allows you to keep air temperature about 10° cooler resulting in less drying and irritation of throat and nasal passages.
- The common cold is caused by a virus, not bacteria. VB clothing does not stop colds, allergies, or nasal dryness.
- For some of us with poor circulation to hands and feet, VB gloves and socks are essential to keep hands and feet warm enough to function (other common solution is to move to warm climate!)