If you’ve done any backpacking, you should be familiar with the practice of layering your clothing. Layering is the foundation of lightweight thermo-regulation: 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 completely contrary to everything you’ve ever learned about layering, which makes vapor barrier clothing and gear such an interesting topic.
Advantages of Vapor Barriers
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 vapor 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.
Very few mainstream gear manufacturers produce vapor barrier clothing or gear. Western Mountaineering, the Rolls Royce of down sleeping bags, makes a vapor barrier sleeping bag liner called the Hot Sack VBL which is a very useful piece of gear for backpackers who have down sleeping bags on extended winter trips.
The problem with down bags is that they don’t breathe very well and when you sweat in them in winter, your sweat gets trapped in the down fill and compromises its insulating properties. This problem can become quite severe and add literally pounds of ice to the weight of your sleeping bag. The Hot Sack VBL prevents this by keeping your sweat from evaporating into the down. The Hot Sack weighs a mere 5.5oz is much sturdier than a mylar emergency bivy (3 oz.), which will rip apart rather easily when you use it as a sleeping bag liner.
The two manufacturers who major in vapor barrier clothing and gear are RBH Designs and Stephenson’s Warmlite. RBH manufactures a full line of vapor barrier mitten liners, socks, vests, pants, shells, hats and sleeping bag liners. It’s pretty high tech stuff. Stephenson’s Warmlite also carriers a full line of vapor barrier clothing. The WarmLite Catalog (1MB PDF) is a bit eccentric, but don’t be put-off by the fact that all the models are naturists and pose in the buff. The Stephenson’s know their stuff.
Written 2008. Updated 2015.
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