Stephenson's WarmliteIndex | |
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Stephenson's WarmliteIndex | |
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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 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).
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. (Goretex 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 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!
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.)
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!
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.
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!
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.