If you’ve done any backpacking, you should be familiar with the practice of layering your clothing. Layering is the foundation of lightweight thermoregulation: you take layers of clothing off when you get too hot and start to sweat and you put them back on again when you start to get cold. Layering lets the sweat you generate evaporate. Evaporation is the process where warm water molecules turn from a liquid form into a gaseous form. When this occurs, we feel cooler because the warm molecules leave the surface of our skin, leaving the cooler ones behind.

Vapor Barrier clothing and gear completely prevents the sweat your body produces from cooling your skin. It prevents the evaporative process, also called wicking, by wrapping your body with a layer of fabric that is completely non-breathable. This is of course completely contrary to everything you’ve ever learned about layering, which makes vapor barrier clothing and gear such an interesting topic.

The advantage of vapor barrier clothing is that it can significantly lighten the amount of clothing or insulation you need to wear or carry, particularly in the winter. The trade-off is that your skin may feel wet and clammy, particularly if your get too hot. Normally, you wear the varpor barrier directly next to your skin and it is relatively common for winter backpackers and mountain climbers to wear vapor barrier socks under wool socks or to line their winter sleeping bags with a vapor barrier liner. You definitely want to avoid wearing the vapor barrier over an insulating layer because it will quickly become soaking wet.