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!)

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 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).

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.

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.

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. (Gore Tex will block 97%. They call that 3% loss “breathable”).

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.

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.

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 Vapor Barrier socks (baggies, bread bags, etc) in 1957, then gloves, shirts, and in sleeping bags since 1967. Others have sold Vapor Barrier 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 Vapor Barrier. 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 Vapor Barrier lined bags and Vapor Barrier clothing is they can’t take the time to explain it to their customers. Mighty inconsiderate! If you want an honest evaluation of Vapor Barrier, 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!

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 Vapor Barrier in your bag for ALL seasons. The surface wickability of Warmlite’s FUZZY STUFF makes it especially desirable for summer use when you’re sure to overheat, (even if nude.)

A common argument against Vapor Barrier is actually excess praise FOR Vapor Barrier: they say Vapor Barrier 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 Vapor Barrier. It’s the overheat DETECTION SERVICE that Vapor Barrier provides (by making you immediately aware of sweat when it starts) which “they” think is overheat caused by Vapor Barrier: don’t blame the messenger for the message!

Will Steger used “breathable” Quallofil sleeping bags for his much advertised 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 Barrier 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 Vapor Barrier and the benefits it can give them. Proper comfortable use of Vapor Barrier requires more intelligence and awareness than some people have, but is made a lot easier with modern Vapor Barrier material having wicking inner surface, such as FUZZY STUFF.

With Vapor Barrier 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 Vapor Barrier react? Generally with orders for more Vapor Barrier clothing and sleeping bags, and recommendations to their friends. From 1967 to 1998 we sold about 9500 Vapor Barrier-lined sleeping bags, and only about 1/2% of customers objected to having to consciously adjust insulation. But even they agree that Vapor Barrier is good for extra warmth and insulation protection, and most of those became best promoters of Vapor Barrier! We’ve found many of those people have low metabolism, need more insulation to stay warm, and thus NEED Vapor Barrier 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. Vapor Barrier 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 Vapor Barrier clothing. When asleep the normal reaction to overheat is to push covers away, reducing the extra warmth, while Vapor Barrier still protects the bag from condensation and sweat. Sleeping bags rarely get wet from outside. Bags without Vapor Barrier ALWAYS get wet from INSIDE condensation and sweat!

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 Vapor Barrier shirt, and will then notice you are as warm as before and not sweating.) The Vapor Barrier 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.