[at-l] Re: Vapor Barrier Hiking [Was: Warming up after hiking.]

[daveh@psknet.com (Dave Hicks)]

As I understand it, VBL was developed for arctic expeditions -- to keep
sleeping bags from turning into big, heavy chucks of ice.

Keeping you warmer and reducing dehydration while sleeping were an
unexpected benefit.   Which gets to the point of this post.  VBL when
inactive and VBL when active are quite different things.  The key is
insensible sweat vs. active sweat.

When cold air is warmed to skin temperature the relative humidity next to
your skin approaches zero. This causes rapid evaporative of the moisture
in/on the skin and cooling of your skin.  When this happens your sweat
glands produce "insensible sweat" to keep your skin moist for flexibility,
even when cold.  That's right.  You sweat as you freeze, even when inactive.
When insensible sweating cannot keep up with the excessive drying your skin
gets dry and chapped.  Think about that next time you complain about dry
winter skin.

Additionally, this insensible sweat rapid evaporation makes you cooler --
e.g., when it is 32 degrees Fahrenheit outside, evaporative cooling can more
it feel 22 degrees cooler.  Hence the use of a VBL in periods of inactivity.
Conversely, many folk find a full VBL [emphasis added] INSIDE [emphasis
added] their sleeping bag will 20, or more, degrees to the bags rating.

In the inactive case, a VBL not only keeps you warmer (by stopping the
rapidly evaporation of  insensible sweat, which would cool you down), it
also reduces dehydration through the water loss through that rapidly
evaporation of  insensible sweat.

OTOH, as OB points out >> They were used by wrestlers to lose weight,
resulting in some deaths from hyperthermia and dehydration a few years back.
<<  This is an extreme opposite example of using a VBL to produce excessive
amounts of obvious sweat for the express purpose of dehydrating oneself --
i.e., quick weight loss to make a weight limit through water loss.

The use of VBL while actively hiking falls between these extremes (sleeping
vs. sweating off the pounds), yet nearer the sleeping level of sweat.

To be safe, the active hiker needs to keep closer to the insensible sweat
level than the active sweat level.  We all know, in principle, how to do
this, by layering.  However, it takes a fair amount of practice and
experience to manage it in the field.

Bottom line is insensible sweat is just that -- you are not aware of it.  If
you have even a hint of moisture, it is time to un-layer.  OTOH,
cool-but-not-uncomfortable could signal the slippery sloop of hypothermia.

IMHO -- active VBL hiking is one of those techniques which requires mastery
with a partner (for a safety net) and under controlled conditions (e.g.,
around the neighborhood).

It works, but it can kill you if you don't get it right.

Chainsaw