Did you know that you sweat and exhale approximately one full cup of water during the night? If you’re out camping, a lot of that moisture goes straight into your sleeping bag. During the summer, this is seldom a problem. A few minutes in the morning sun will quickly dry it out. But during the winter, when temperatures may not rise above the freezing point, all that moisture penetrates into the sleeping bag insulation and then stays there, compromising the bag’s warmth.
Moisture build-up in your sleeping bag is not generally a problem if you’re just out for a single winter night. But on multi-night winter camping adventures, the cumulative moisture can significantly compromise your nighttime warmth and comfort. The solution? A vapor barrier liner.
A vapor barrier liner, or VBL, is essentially a waterproof, non-breathable sack that you place inside your sleeping bag. It blocks all of your body’s moisture from reaching the sleeping bag insulation and thus keeps the bag completely dry, no matter how many nights you sleep in it.
VBLs also make the sleeping bag warmer, even though the material is whisper thin. How? By stopping your body from sweating. First, a quick physiology lesson. Whether you realize it or not, you are constantly perspiring. Your body constantly adds tiny amounts of moisture to your skin to prevent it from drying out and cracking. As this moisture evaporates from your skin’s surface, it sucks away a small amount of body heat. (The phenomenon is known as “insensible perspiration” because you are generally unaware that it’s happening.) The process is amplified in the cold; the air is drier and your skin needs more moisture to stay healthy and supple.
Now back to the VBL. When you sleep inside a VBL, the micro-environment around your body rapidly reaches a humid state. Because of this, your body no longer needs to add moisture to your skin and the rate of insensible perspiration dramatically drops off. As a result, you lose less body heat to the process—and thus stay warmer. In my experience—and I’ve spent dozens of nights testing this out—a VBL adds roughly five to ten degrees to a sleeping bag’s warmth.
So what’s it like sleeping inside a humid, non-breathable sack? Quite comfortable, in my experience. Though it does get subtly moist, the inside of the VBL never really gets unpleasantly damp for the same reason that it provides extra warmth—it stops insensible perspiration and thus maintains the VBL environment at a constant, comfortable level of humidity.
I recommend wearing a thin base layer inside the VBL, which keeps your skin from cloying to the moist VBL fabric, and provides an additional layer of warmth. Do not wear extra layers inside the VBL—they will absorb a considerable amount of moisture and can actually lead to a colder night’s sleep.
VBLs can be challenging to find. Relatively few manufacturers produce them and buying one online may be your only option.
Western Mountaineering offers its ultralight HotSac VBL (4.5 ounces, $95), which incorporates a reflective heat barrier for a little extra warmth (pictured above).
“Equipped” is an AMC Outdoors blog, written by Matt Heid.