We all learned in science class that there are three ways by which heat can be transmitted – conduction, convection and radiation. So to keep heat in or out, we humans have designed various means to block conduction, convection and radiation. A jacket keeps us warm by preventing heat loss from our bodies through conduction. Generally speaking, the thicker the material or the more air it traps, the better it is able to insulate our bodies from heat loss through conduction. Down jackets are very effective because of the many pockets of air that can insulate the body.
Heat loss through convection is the principle behind cooling fans and wind chill factor. Assisted by wind, the temperature does not have to be very low to cause hypothermia. Windproof shell jackets (often quite thin) are designed to block out wind and prevent heat loss through convection. If it’s not too cold and wind is the main factor, you don’t need a very thick jacket to protect yourself – just something that can “break” or “cheat” the wind.
What about radiation? The mirrored glass inside our thermos flasks prevents heat loss through radiation. Even though there is a vacuum in between the layers of a thermos flask, heat can still escape through radiation. The mirrored surface reflects heat back into the flask, keeping the heat in. How does this apply to outdoor clothing? The answer is pretty obvious and I’ve often wondered why designers of winter clothing had not extensively used shiny surfaces. In fact, shiny surfaces have already been used – just not so common in jackets.
You may have seen one of these emergency blankets carried by rescue workers. They are easily carried and though they lack thickness to provide insulation, the reflective nature of the material causes the body heat to reflect back on itself. While it may make a difference, it does not work very well under extreme conditions. Still it’s better than nothing. But would you deliberately silver the inner surface of a jacket to increase its heat-retaining property?
The Columbia Omni Heat Vest has such an interesting design. The type of outdoor clothing material with the best insulation properties is down (feathers). This vest does not have a lot of down, but it has a reflective inner surface that works in combination. I bought it for a recent trip and I confess that prior to this, I have not worn any jacket with a reflective inner surface. Why a vest and not a full jacket? Maybe I have budget issues. But seriously, I prefer down vests because they do not restrict my arm movement. The thin down also means that it takes up less space in my backpack.
So how warm is it? In camp at night, it has kept me reasonably warm, probably as warm as a thicker down without the thermos effect. What about setting off before dawn when it’s still freezing cold? I’ve worn it under my shell jacket while making the summit push on Gunung Agung. After a few minutes, the vest was trapping so much heat that I was perspiring. If I had worn a normal down vest without the reflective lining, it would have been OK. The lining also renders the vest less breathable than it should be. Hence, moisture accumulates.
So while the reflective surface does help prevent heat loss, it is also extremely effective in trapping heat generated by your body when you’re on the move. My advice is to wear it only when you are resting in camp. It’s good to have but be prepared to take it off when you’re on the move.