[pct-l] Layering

[Matt Thyer]

I'd amend that statement a little bit Amanda,

 

Its trapped air that gets the job done.  Indeed there is air inside any
layer that will hold onto heat.  Your body heats this air, and the air
outside the layer (if it's colder) will pull this heat into that outside
body of air.  That's convective heat loss.  There is also air trapped
between layers.  Depending on the layer this air can also insulate.  

 

Consider how a wet suit works for a moment and the idea should make sense.
Wet suits trap a layer of, at first, cold water against your skin.  The
neoprene of the suit makes it difficult for the water inside the suit to
move around.  Your body is always producing heat at a higher temperature
than that of the water surrounding your body.  The water trapped in your
suit is heated by your body and because it can't leave the suit easily it
serves to insulate your body making the water feel warmer than it really is.
The wet suit slows the transfer of heat from your body to the environment
around you.  Because the U value of two 4 mm wet suits is the same as one 8
mm they won't act to give you any more insulation from the cold water
outside.  However, if you sandwich a layer of water between two 4 mm layers
you've incidentally created a third layer of insulation.

 

You can calculate the rate of heat loss if you know a couple of constants
and have the measurements for some common environmental variables.

 

q = UAΔT = (1/R) AΔT

 

q is the rate of heat loss (what we'd like to know)

U is the proportionality constant thermal conductance of a homogenous
material

ΔT is the temperature difference (Ti - Ta) or the difference between the
inside and the outside

R is the inverse of U, also known as the conductive thermal resistance of a
homogenous material

 

I'd suggest that knowing the actual q for any layering combination of
clothing is less valuable than understanding how heat is lost and thus what
we can do to stay warm.  A couple of key concerns in this equation are the
difference in the ambient temperature inside and outside as well as the rate
that heat is transferred.  

 

Understanding that you don't have to insulate equally because you don't heat
equally may help reduce pack weight.  Start by understanding where you lose
the most heat and look for ways to increase the amount of insulation around
those areas (not always where you feel coldest either).  Personally, a hood
for me makes the biggest difference in cold weather.  A necker (wool) can
contribute to exceptional warmth as well.  The hood acts to channel heat
from your core to your head and face (where you lose a lot of heat) and the
necker is useful for keeping that heat inside your protective layers.  The
same technique can be used around your waist, hands, and feet when you
purposefully create baffles that won't allow the heat you've already created
from escaping easily.

 

Adding layers inside a sleeping bag probably won't help too much.  Looking
for ways to keep that heat inside the sleeping bag is what you're really
after.  Again the necker/neck-collar and hood on your mummy sack or quilt
become indispensable.  A sealable tent is just another layer of trapped air
which your body will heat. 

 

MT

 

-----Original Message-----
From: pct-l-bounces at backcountry.net [mailto:pct-l-bounces at backcountry.net]
On Behalf Of Amanda L Silvestri
Sent: Tuesday, December 22, 2009 10:37 AM
To: pct-l at backcountry.net
Subject: [pct-l] Layering

 

Thanks Bill.  I had misunderstood that concept. "It is the air trapped in
the layers, more than the air between the layers, that provide insulation."

 

Shepherd

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