Re: [Paddlewise] Sleeping bag liner

<<What is a Vapor Barrier?>>

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It's that plastic sheeting put into the walls of new homes to retard
the movement of air and moisture. ;-)

A vapor barrier is also a coated nylon or plastic bag or garment used
in winter camping to retain a high level of humidity next to the skin.
It is most commonly used as a sleeping bag liner at temperatures well
below freezing, since it also keeps water vapor from your body from
condensing inside your sleeping bag and possibly reducing the
effectiveness of the insulation.

At subfreezing temperatures, a vapor barrier liner (VBL) can increase
the temperature range of a sleeping bag by 10% to 15% at a very small
cost in weight and volume. If you think about it, the water vapor that
your skin constantly exudes contains heat. If it didn't, it would be
exuded as frost! So retaining the vapor retains heat. A body inside a
VBL senses the increased humidity caused by the VBL and throttles back
the output of water vapor to maintain a constant level of humidity.
Thus, the VBL also helps to combat dehydration. (Winter air in
Minnesota and other northern locales is as dry as desert air, so it is
very important when winter camping to maintain a high level of hydration
 -- dehydration can contribute to hypothermia and frostbite, among other
things.) If you are careful not to overheat, you will feel humid inside
a VBL, but not wet. This is sometimes difficult to achieve, and I find
a VBL most comfortable at temperatures below 0 degrees F. It is also
one reason VBLs are rarely used in clothing, since varying levels of
activity make it hard to maintain a constant body temperature.

Think of it as an inverse dry suit.

Chuck Holst  
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