[pct-l] Insulation above, insulation below

[Brick Robbins <brick@fastpack.com>]

At 06:39 AM 11/09/1999 +0000, Brett Tucker wrote:

>I find that most of the insulation needs to come from above the reposing 
>hiker, since this is where it does the most good. Insulation above the 
>body traps the body's heat, which rises. Insulation beneath the hiker 
>traps less heat,

You are leaving out one VERY important factor: Thermal conductivity. The 
ground below you conducts heat away from your body much better than the air 
around you does. Still air is a very poor thermal conductor (ie good 
insulator), which is why 70F air seems warm, but 70F water (a very good 
thermal conductor) seems cold. Moving air is a good conductor, because your 
body has to continually heat different air molecules.

Most of what a down bag does is trap a layer of still air around your body, 
just like double pane glass traps a layer of still "air" (poor conductor) 
between two layers of glass (good conductors).

That is why an insulating pad is so important. Even if the ground is warmer 
than the air, one can lose much more heat down into the ground. This, of 
course, depends on the thermal conductivity of the ground which depends on 
the soil type and moisture content. As "Ray" says, if you can find a bed of 
nice insulating pine duff, you don't need a pad. Unfortunatly, there just 
aren't that many good pine duff beds between Mexico and Canada.

Crushed compressable insulation below you (such as down) doesn't do you any 
good, since it relies on its loft (which is gone when crushed) to trap 
still air to provide the insulation. The "air" in a insulating pad is held 
in place by a more rigid structure so it doesn't compress (much) under body 
weight, and still provides insulation. That is why a quilt is such a good 
idea. No down that is not doing its job.

-Brick
  Posting as list member, not list admin







* From the Pacific Crest Trail Email List |  http://www.backcountry.net   *

==============================================================================