Staying Warm is About a Lot More than Test Results.
Insulation in the classic sense is the sum of two parts: dead air space and some kind of substance that creates little voids where motionless air rests. The dead air space is important because, next to a total vacuum, still and dry air is as close to a perfect lightweight insulation as the natural world gets.
The apparel industry measures insulation values with a benchmark Clo Test. It was originally intended to measure the amount of clothing necessary for an adult human to sit inactive in a nice, warm, dry and draft-free room. A Clo measurement of zero is the equivalent of being naked and a Clo of one equals the clothing necessary to be comfortable while resting at room temperature. The test results are linear and additive and mostly useless in relation to the outside world, however, they are the only test numbers we have.
When we wear insulation outdoors the conditions often are not nearly as nice as those in the lab. Outdoor outerwear has to deal with three things in the real world that don’t factor into insulation testing at all — humidity, wind and cold air.
Water conducts heat pretty well and a humid atmosphere, such as air with a lot of water vapor in it, supercharges the effect of the outside temperature on the human body. When it is really hot and humid the conditions become muggy and miserable. The heat is oppressive because the hot, moist air is literally touching you. In cold and wet environments the same thing happens in reverse; body heat is conducted away from you by cold humid air. Great examples of this are riding outside on the deck of a Puget Sound ferry in the winter, or walking across downtown Chicago on any given night in January.
Cold air has a physical limitation that greatly impacts staying comfortable over time. Super cold air has a very low absolute humidity. That means the amount of water vapor the air can hold is tiny and no matter what you do it isn’t going to hold anymore. The warm toasty environment inside your outerwear has trouble “breathing” water vapor into the surrounding atmosphere in these circumstances. When the humidity from the body reaches the outer shell of the apparel system the vapor is forced to immediately condense. The insulation gets wet and the shell gets frosty. Cold weather campers know this phenomenon all too well.
Now throw in the evaporative cooling that comes with windy conditions, and cold, wet weather becomes a recipe for serious discomfort. Water has this weird physical trait where it can absorb huge amounts of energy without changing from a liquid to a gas. When water evaporates it takes this latent heat energy with it. What that means to you and me is that when wind blows across wet skin, or wet apparel, it takes massive amounts of heat with it. Cold wind is the biggest heat thief out there.
There’s one other thing I should bring up. Humans are made mostly out of water. We breathe out water vapor, our skin is moist and we sweat when we get too hot. Like when we exercise in insulated garments. We can’t do much about the first two, but avoiding the third is the key to cold weather comfort. Water is what makes insulation systems fail. Windy, cold, and humid environments, combined with damp clothing and sweat soaked skin, will easily create conditions that conduct heat away from the body faster than it can be replaced. The Greeks have a fancy word for this, they call it hypothermia.
Disclaimer: Mr. Gray sleeps with a couple of fur covered heat thieves and he theorizes that they are in some way powered by this stolen energy source. Textile Insight’s Publisher may not share in his opinions or choice of bedtime companions.