** DAVE'S VAPOR BARRIER FAQ **

CONTENTS:
1)  What's in this file?
2)  What are Vapor Barriers and what do they do?
3)  How do Vapor Barriers protect your insulation?
4)  How do Vapor Barriers stop evaporative cooling?
5)  Why won't I sweat to death? (The controversial stuff)
6)  What kinds of VBs are available?
7)  Do VBs work?
8)  When and how should I use VB sleeping bag liners?
9)  When and how should I use VB socks?
10) When and how should I use VB clothing?
11) What about VB-like clothing?



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1) What's in this file?

What follows is a hodge-podge of information and ideas about
Vapor Barriers (VBs) that I have culled from the lively discussions
on the topic in rec.backcountry combined with some of my own
personal experiences.  Note, I am not a physicist nor a
physiologist, although several of those who've helped are.  I am 
in much debt to them for the insights and information they
have provided. 

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2) What are Vapor Barriers and what do they do?

A vapor barrier (VB) is a non-breathable layer that is added 
to a clothing or sleep system with the goal of keeping you warmer. 
What they actually *do* is to prevent water vapor from passing 
from one layer to the next. Hence the name - vapor barrier. 

Stopping water vapor from moving from one layer to the next 
has 2 primary effects: 
a) It protects insulating layers from getting wet from moisture 
   produced by the body  
b) It stops the process of evaporative cooling which can continue
   to make you cold, even when you are at rest

Note, for the sake of clarity, I find it useful to 
separate what VBs do (prevent moisture transport) and
their effects (protect insulation and physiological effects).
I think the failure to make this distinction clear is at
the root of many of the heated discussions I have heard
surrounding VBs and their use.


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3) How do Vapor Barriers protect your insulation?

To stay warm, it is important to keep your insulation layers
dry.  Surprisingly, the biggest culprit for getting insulation
layers wet when winter camping is often your body's own sweat.  
Our bodies sweat even during even during times of relative
inactivity, like when sleeping.

The effects of sweat condensing and freezing in insulation
is typically cumulative; getting worse with each passing day
you are out.  In his book recounting his crossing of Antarctica, 
adventurer Will Steger noted that their synthetic sleeping bags 
became so full of ice they weighed an astonishing 20 lbs each 
and were impossible to pack.  Realistically, most of us will 
only go on winter trips measured in days, not weeks or months.
Still, if you want to prevent sweat from condensing in 
your insulation, VPs provide a solution.


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4) How do Vapor Barriers stop evaporative cooling?

Most of us understand that when sweat evaporates off of our 
skin, it cools our bodies.  Not surprisingly, the process is 
called evaporative cooling and it's good news for us when 
we're working hard and hiking up a steep hill.

What is less well known is that our bodies continue to
produce sweat, even when we are cold.  To prevent our skin
from drying up like rice paper, our bodies pump out moisture
any time the air near our skin lacks humidity.  This is 
sometimes called "insensible perspiration" because, unlike the
sweat rolling off our brow like when we're hiking, we typically
don't sense this kind of perspiration.  

What this means is that when you are shivering in your sleeping
bag, your body is almost certainly perspiring.  This is bad
news, because as that sweat is evaporating off of your skin,
the process of evaporative cooling is making you colder.

However, if you have a VB layer near your skin, the air
between your skin and the VB soon becomes super humid, 
effectively preventing sweat from evaporating from your
skin.  In short, VBs stop the process of evaporative cooling.


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5) Why won't I sweat to death? (The controversial stuff)

The claim is that VBs can, under the right conditions, 
dramatically slow down, or entirely stop, the rate of 
insensible perspiration.  Specifically, the claim is
that our bodies will only perspire for one of two reasons:
  a) our body core is hot or
  b) our skin is dry.  

So, proponents of VBs will argue that once the air inside 
of the VB gets humid enough your body will stop perspiring,
provided that you don't overheat and begin to sweat in an
attempt to cool off.
  

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6) What kinds of VBs are available?

I group VBs into 3 categories: 
  + VB liners for sleeping bags 
  + Bags with built in VBs 
  + VB clothing

SLEEEPING BAG LINERS
The cheapest VB bag liners can be easily made from a couple 
of Hefty bags, some duct tape, nylon cord and imagination.
Not very durable but, it's a great way to begin experimenting 
with the concept. 

VB liners are also made a variety of equipment manufacturers, 
and are generally out of coated nylon. The North Face has made 
them in the past, as has Marmot.


SLEEPING BAGS WITH BUILT-IN VBs
There is currently only one sleeping bag on the US market that 
I am aware of that incorporates a built-in, permanent VB; the
Stephanson Warmlite bag. Stephanson uses his own non-breathable
"Fuzzy Stuff" material to line the bag. It is so named for the 
soft, fuzzy synthetic facing on the fabric.


VB CLOTHING
With regards to VB clothing, few companies are currently producing 
them. Stephanson produces shirts, pants, and socks.  Black Diamond 
and Climb High both make VB socks.  Occasionally, you will even
see non-breathable neoprene socks and gloves being sold for
winter use.  

As with VB bag liners, you can often make your own VB clothing.  
Long thin plastic bags like those that wrap your local newspaper 
or those found in the vegetable section of the grocery store make 
workable VB socks. They fit poorly and tear easily but they are 
cheap and allow you to easily experiment. 

Light, coated nylon rain gear makes workable VB shirts and pants. 
In order to save weight, you may want to chop the hood and extra
pockets off of a rain jacket to convert it to a VB shirt.  You 
may also want to size it smaller than normal to fit *under* your
insulating layers, not over them.  And you may want to seal off 
any ventilation features on the jacket in order to use it as a 
VB shirt.

	
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7) Do VBs work?

This is perhaps the most commonly asked question
with regards to VBs. The answer depends entirely on what
is meant by 'work'? 

If 'work' means preventing water vapor from passing to
your insulation layers, then the answer is yes, up to the
waterproofness of the VB.  This is, after all, what VBs 
really *do*. 

For example, Plastic VB bag liners held together by tape are
very efficient. On the other hand, a sleeping bag with
a built in VB having a large number of unsealed seams
may work somewhat less well as water vapor may escape
through the needle holes.  Whether or not this is enough
to allow a significant amount of water vapor to enter the
insulation is something that I don't know.

In any event, this is probably not what is meant by 'work'.
More likely, what somebody wants to know is "Are VBs workable?"
or more precisely, "Will I sweat to death in a VB?" The 
short answer is that if you use a VB in the right conditions
you will stay warmer and dryer and you will not stew in your 
own sweat.  

Anticdotal evidence suggests  that is a significant minority 
of people who simply are not comfortable in VBs.  But judging 
from the discussions in rec.backcountry, it is not clear 
whether or not this is related to the individual's unique
physiology (as is often claimed) or to his or her's failure 
to use the VB correctly (as is often charged). 

Regardless, you will be more comfortable using VBs if you
understand their effects on the body's physiology and work
to adjust your insulating layers and activity to prevent
yourself from overheating, which will cause you to sweat
even if the air in the VB is humid.


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8) When and how should I use VB sleeping bag liners?

Most folks will tell you that VB liners become workable when 
the temperatures get pretty low, like down around 0F. That has 
certainly been my experience with plastic VB liners.  

Others suggest that VB liners aren't worth the hassle on short
trips.  My experience with VB liners makes me pretty sympathetic
to this opinion.  VB liners are usually just tubes of waterproof
material, with no zipper running down the side.  So, getting in
and out of them means that you need to shimmy and squirm.  If 
your VB sack is crinkly plastic, your tent is small and if your 
tentmate is the grumpy type then this may be a concern. 

In the Complete Walker III, Colin Fletcher says that he
finds it easier to use a VB liner by only pulling it up
under his arm pits and not up to his neck.  This would allow
you to adjust the humidity in the VB by moving it up and 
down your torso (which will let moisture move into your
insulation).  It would also let you wear a jacket or sweater
*over* the VB.

Still others will argue that VBs allow you to shave ounces
from your sleep system.  The argument is that since you
sleep warmer with a VB, you can get by with a lighter
sleeping bag.  While this makes sense in theory, my experience
doesn't quite confirm this.  I'm of the school of thought
that if you aren't sleeping in all of your clothing at
night, then you carried too much clothing.  That is, I assume
that if I hauled a nice warm parka up the mountain I'm
going to put it to good use by wearing it.  The problem is
that VB sacks will get every piece of clothing that it inside
of the VB wet.  What this means is that I can't wear my nice
warm parka or sweater to bed.

So for me, the question about using a VB liner to save weight
comes down to this.  Given the same sleeping bag, which will
keep me warmer:  the VB liner or my extra parka?  For me, the
answer seems to be that I am warmer if I add the extra clothing.

Now, the ultra-light types may differ from this point of view.
Some ultra-lights devotees forgo the warm sweater or parka
and plan instead on using their sleeping bag to stay warm.
I've seen these types hustling through their camp set up chores
in their trail clothes before they cool off.  Later, they
hang around camp with their sleeping bag draped around their
shoulders like a shawl, effectively (or not) replacing the
need for a sweater.  If this describes your approach, then
adding a VB liner will boost warmth of your sleep system
for very little weight.

Jack Stephanson and many folks who use his bags will tell you 
they can be used in all temperatures, including warm temps, 
without discomfort.  This is for 2 reasons.  First, since the
VB is built into the sleeping bag, the sleeping bag zippers
make is easy to regulate the heat and humidity in the bag - much
more so than what you can do with a VB liner.  Another reason
why the Warmlite bag may be more comfortable is due to 
Stephenson’s unique Fuzzy Stuff (tm) fabric, which has both a 
waterproof barrier and a brushed surface.  I can attest from
experience that the Fuzzy Stuff fabric is very comfortable
next to the skin.  Unless you are dripping with sweat, the
Fuzzy Stuff fabric feels dry next to the skin.



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9) When and how should I use VB socks?

VB socks can be worn when you are actively hiking and skiing 
with no problem.  This will keep your insulating socks and boots 
dry from sweat.  Common wisdom has it that if you wear VB socks 
during the day, you should not have your feet in a VB at night 
so that your skin can recover.  This advice may be most applicable 
to multi-day trips.

I generally use VB socks on any winter over night trip.  I find
it allows me to enjoy dry boots in the morning since they
aren't frozen up with sweat.  This also frees me from needing
to sleep with my boots.  Dry boots left in the cold are still
cold when you put them on but they warm up in a hurry.

I also use VB socks on winter day trips if the temps are
going down into the low teens or single digits (or below).
If find that it keeps my feet warmer since my boots are getting
soaked from sweat from within during the day.

I should mention that I generally use full coverage, rubber
randed gaitors over my leather boots in the winter.  I use
Shoe Goo (or its equivalent) to glue the rubber rands to the
boots, which generally keeps my boots dry.  I mention this
because the advantages of keeping your socks and boots dry
from sweat by using VBs is entirely negated if you allow your
boots to get wet from the outside.


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10) When and how should I use VB clothing?

I've seen only a few commercially manufactured VB shirts.
I know that Patagonia in the deep distant past has made them.
And I think that Marmot has made them too, but please don't
quote me on that.  Stephenson’s Warmlite still offers VB
shirts made from their unique Fuzzy Stuff (tm) material.
While a bit heavier, it has the advantage of being able
to be worn next to skin comfortably.  With the Patagonia
VB shirt I've seen, you would definitely need to wear
a synthetic shirt under it to be comfortable.

I have had pretty good luck with my Warmlite VB shirt.
I will generally pack this shirt before I will pack
the VB liner.  I simply find the VB shirt more useful.

First of all, to use  a VB liner, you must be assured that
you bag and VB liner alone are warm enough to handle the cold. 
With VB clothing, you can always add extra clothing.

Secondly, I need to get up to relieve myself every night
at 4:00 am like clockwork. With a VB liner, this would mean 
layering on extra clothing or standing outside in damp skivies! 

Finally, with VB clothing, I get the extra warmth and 
moisture protection of the VB without being relegated to my
bag. I can wear it under my down jacket while hanging around
camp while others are huddled in their bags.  I can also use
the VB shirt to help dry out the fleece that I wear while
hiking.

I normally hike in the winter wearing synthetic underwear, light
fleece and some sort of shell. This combo allows me to deal with
changing conditions and exertion levels fairly well.  However, 
it also guarantees that I arrive at camp with under layers and 
fleece that are damp from sweat or snow or both.  As soon as 
I get to camp, I put on my VB shirt over my damp under layer and 
layer my wet fleece sweater over it.  Setting up camp is not heavy 
work (I forgo the snow cave scene) and the VB shirt can immediately 
start to work as my body cools down from the hike.

Since my body heat outside of the VB is now dry heat, my fleece
top dries dry much faster than with out the VB. Once my fleece is
dry and my body begins to cool further, I then layer my down jacket
over the fleece without the worry that moisture from my hiking 
clothes will end up in my down.

Given that a VB shirt is basically a non-breathable
plastic shirt, you may be surprised that rec.backcountry
has seen considerable dispute about whether or not
VB clothing can be used when you are active. Jack Stephanson
claims that you can hike VB shirts in his literature as do 
several posters to rec.backcountry.  Others vociferously disagree,
claiming that it is just unworkable.  I've played around
with this idea a bit.  I've had enough success to not 
dismiss it outright.  But neither have I had enough 
experience with it to advocate it.  Experiment with it
and let me know!

I have also used VB pants on several occasions. Experience
with scraping frost off of the surface of my bag on several
occasions suggests to me that the groin area pumps out as much 
or more moisture as the torso and arm pits do.

I've never seen commercially manufactured VB pants.  Instead,
I just used a cheap pair of coated nylon rain pants.


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11) What about VB-like clothing?

Some folks claim that you don't have to wear full-blown
VB clothing to get some of the benefits.  Remember, one
of the claims about VBs is that they can slow down or
stop insensible perspiration by creating a humid micro-climate
near your skin.  I've seen this basic argument raised by
different people and different companies over the years.

One of the longest running products for which this claim
has been made (off and on) is the Marmot Dri-Clime Windshirt.

The Arctic and Antarctic adventurer, Will Steger, has made
similar claims about wearing windshirts near the skin as a
way of creating a moist micro-climate near the skin to reduce
insensible perspiration.  This claim was made in his book
on his Antarctic crossing and hinted at in the add copy for
a Pertex windshirt/shell that Lands' End briefly produced
with his name attached to it.

Finally, another mention of this approach can be found in
Chris Townsend’s book on Backpacking (I forget the name of
it but it is a great book).  

I often use a Pertex windshirt/shell for winter hiking
and xc skiing and have, on occasion, pressed it into service
as an under layer at night, wearing it over a light poly 
shirt.  I have also used an ultra-light pair of water-proof/
breathable rain pants in the same way.  My experience is that
the less breathable the garment is, the more effective it
is at keeping me noticeably warmer.  Still, it is an interesting
idea and one that I have not fully enough explored to make
a specific recommendation on.  

Copyright 2003 by David Mann