Insulation Basics

 

 

 

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By Bob Amick 

I don't know of any videos that specifically make reference to how wet cotton absorbs body heat (through evaporative cooling), although they probably do exist. You might check with local mountaineering stores or do a web search for topics on hypothermia to see if there are any videos or similar reference materials. 

There used to be a film produced by the Seattle Mountaineers and Safeco Insurance entitled "Hypothermia: Killer of the Unprepared" but I don't know if it is still around; it had a good explanation of how hypothermia occurs insidiously on various trips; but it was produced about 30 years ago so may be out of print.   

However, if you want a good demonstration, take a pair of cotton jeans (and cotton "long johns," socks/tee shirt if you like) weigh them dry, and then soak them in water and weigh them again, after they are wet to show how much water they absorb when wet. Then do the same thing with a polyester fleece (and polypropylene or cool-max synthetic "long johns," Thorlo socks, etc.), which absorbs virtually no moisture. 

We have found it to be very convincing for for folks to see that difference. Also point out how quickly the damp synthetics dry and insulate even when damp, unlike cotton. Have students do the "feel test" by placing a hand inside a wet cotton garment and then a damp synthetic garment to see which feels warmer even when wet. 

An additionally convincing demonstration is to then take the wet cotton garments and the damp synthetic fabric garments and place conventional or electronic thermometers on the wet fabric of the cotton and synthetic garments. Place them all in front of a powerful fan (to simulate wind) and watch the temperature drop from evaporative cooling on the cotton garments and change very little on the synthetic fabrics. 

A slightly more elaborate demonstration would be to take two aluminum water bottles filled with water heated to about 98 degrees F. (to simulate human core body temperature) and place a thermometer in the opening of each bottle; then place a bottle inside a wet cotton garment and another inside the wet synthetic garment with the fan blowing on both to show the relative drop in water temperature due to evaporative cooling.   

 

By Charles Feller

The best demonstration we found to impress boys (and their parents) is to take three T-shirts: 100% cotton, 50% cotton/50% polyester (or similar poly-cotton blend), and 100% polyester (Coolmax or Thermax).  Soak each in cool water, thoroughly wring out, and hang outdoors to dry at 60F to 70F. 

You can demonstrate this during an autumn campout. The results are

bulletPolyester - 20 to 30 minutes 
bullet 50-50 Polycotton - 2 to 3 hours 
bullet 100% Cotton - 4 to 6 hours 

These are typical results can vary depending on outdoor temperature, humidity and wind. For more dramatic results, include 100% cotton denim trousers (blue jeans) and a 100% cotton sweat shirt. Drying times for these heavier garments can go out to 8 hours. We had one campout where jeans were still damp the next morning after hanging all night, yet our Cordura polyester trousers were dry within the first hour! 

Marc Godbout Adds:

Drying time is only part of the equation. For example, a 100% wool sweater will probably take as long, or even longer than cotton to dry, but it will insulate while wet, where cotton will not.  

Charles Feller Replies:

Natick Research Laboratories testing has evolved into the three Battle Dress Uniform (BDUs) fabrics deployed into today's service (regardless of color dying and printed camouflage patterns) : 

bullet Cold weather (<50F) = a polyester-wool blend rip stop fabric.
bulletTemperate (50-80F) = a 65%/35% polyester/cotton blend rip stop fabric.
bulletTropical climates (80F+) = a 100% cotton rip stop fabric. 

The cotton fabrics are also Sanfordized (preshrunk). Some fabrics are pretreated with fire retardants, odor absorbers, water repellents and/or insect/arthropod repellents.

Wool is natural, and has good insulation value when dampened or wet, but 

bulletIt is a more expensive commodity 
bulletIt can weigh more
bulletIt is slow drying compared to modern-day synthetics
bulletA higher percentage of people have either discomfort and/or allergies to wearing wool. 

As a part of Scout demonstrations, we have not tested wool for its drying characteristics. Thank you for bringing wool back to our attention. We will hang some out to dry in our next comparison study. 

 

By Jonathan Dixon 

Probably the best I've seen (and one that makes quite an impression) is a demo from the Okpik course that the Denver Area Council puts on. I modified it slightly to fit the materials I have at hand. 

Take three empty soup cans; weight them with a rock; put them inside the end of the leg of each of a pair of jeans, a pair of wool pants, and a pair of fleece pants; put into a dishpan so that about 1" on each leg is sitting in the water. 

Then wait 15 minutes. (do this at the beginning of a presentation, then come back to it at the end). 

By this time, the jeans will be wet to about the knees, the wool pants will be wet for that inch that was in the water, and the fleece pants will be almost dry as soon as you take them out of the water. Usually gets the point across in spades. 

Heath & Safety

 

 

   

 

 


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Heat Loss Bear Song ] COLD Key Bear Song ] [ Insulation Basics ] Frostbite ] Hypothermia ] Breaking Through Ice ] Responsibility to Group ] Women's Hygiene ] Permission Forms ]

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Last modified: July 03, 2013.