Vapor Barrier and Radiant Barrier Liners
Winter backpackers can add a lot of warmth to their sleeping bags by using a vapor-barrier liner: a six-foot-long bag made of a waterproof, non-breathable coated material. A VB liner stops the heat loss caused by evaporation of insensible perspiration, the water you constantly lose through your skin just because your skin is not watertight like a plastic bag. A VB liner also stops the evaporation of sensible perspiration, the kind you produce when you're overheating, so you need to regulate your temperature carefully by shedding clothing if you start to sweat. Used properly, a VB liner can allow you to sleep comfortably in temperatures 10 or 15 degrees lower than you could without a liner. Used improperly, a VB liner will awaken you with the feeling you've encamped in tropical Borneo.

VB liners provide a crucial additional benefit: They help keep your sleeping bag dry. In severe cold, without a VB liner, the moisture that escapes from your body will condense inside your insulation, whether it's down or a synthetic, reducing its effectiveness. During my second expedition to Alaska, in 1980, the down bags used by my two companions collapsed completely as moisture built up during our 13-day epic ascent of the south face of Mt. Hunter. Peter Metcalf said later that his bag became so useless he would simply have thrown it away if it hadn't cost so much. During Will Steger's dog-sled expedition to the North Pole, the team's synthetic sleeping bags accumulated 35 pounds of ice through condensation because the team wasn't using VB liners. In 1982, when both Peter and I used VB liners inside our bags during our ascent of Reality Ridge on McKinley, both of our bags stayed dry and lofty, in large part because of the liners, but also because we took every possible opportunity to dry our bags.

While preparing for an Alaskan expedition in 1983, I took the vapor-barrier concept into the realm of fanaticism and decided I needed to back up the coated nylon VB liner I normally used with a giant plastic bag. Unfortunately, the requisite size of bag was only available in 100-bag rolls. Fortunately, I was able to persuade a friend to go in with me on the purchase of a roll. Even more fortunately, my friend remained my friend after this rather shameless imposition in an obsessive cause. A normal coated nylon vapor barrier is all you really need, even in severe cold. Summer backpackers need not concern themselves with vapor barriers at all.

For a time in the early 1980s, manufacturers experimented with a different kind of liner, one designed to block the loss of heat in the form of infrared radiation. Texolite was the most common brand name. The material did indeed prove its worth in the synthetic sleeping bags of the day, where adding the weight of the liner provided more additional warmth than adding an equivalent weight of insulation. The same was not true of good down bags, however, where a user needing additional warmth was better off adding more down than adding Texolite.

Texolite's problem, at least in the minds of summer users, was that the material was also a pretty effective vapor barrier. People complained that Texolite bags had too limited a comfort range: They found themselves overheating too easily. Partisans of vapor barriers like equipment designer Jack Stephenson would argue that all these complainers wanted was the license to sweat and soak their insulation, and that the better solution would have been for overheated users to take off some clothing. Despite such cogent arguments, however, the consumer rules in our society, and Texolite and its competitors have gone the way of the dodo bird, at least for now.

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