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Vapor Barrier Clothing and Sleeping Bag Liners

Western Mountaineering Hot Sack Vapor Barrier Liner
Western Mountaineering Hot Sack Vapor Barrier Liner

If you’ve done any backpacking, you should be familiar with the practice of layering your clothing: 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.

Vapor Barrier Gear and Clothing

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 carries a full line of vapor barrier clothing.

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  1. I thought the reason a down bag will gradually accumulate moisture over time during a winter trip was more related to water vapor (sweat) cooling as it passes through the down layer into the colder air outside the bag. What I’ve read is that once the water vapor reaches the dew point it turns to liquid and further if the temperature of the outside fabric of the bag is below freezing, the water from your sweat will freeze into ice. The VBL keeps it from passing into the down. I don’t know that the issue is related to down bags not being “breathable”.

  2. While the VBL idea may seem odd and counterintuitive (“you’ll be soaked in your own sweat”), my experiences using one have been mostly positive.
    A negative was having my arms inside a cinched up VBL which was a little claustrophobic and a hassle to adjust the sleeping bag of zipper. (Instead of having arms inside, I now cinch the VBL under my arm pits so most of my core and legs get the benefit of the VBL). I wear a light synthetic layer and socks inside the VBL.

    My first time trial was in an igloo with inside igloo temps at or around 32F. Initially I thought my skin felt wet in the VBL but when I physically touched my skin it was just fine. Now I don’t notice my skin feeling wet unless the night temps are too high ( and your body may be trying to cool itself??)
    I have been curious what the temp needs to be for a VBL to no longer feel comfortable. Last summer I ran a little experiment and used the VBL a few nights. For me the temp seem to be somehere i above 40F.. Below that temp, VBL use was fine
    I think the VBL is an often overlooked tool to get more warmth from your sleeping as well to help keep the down dry from moisture build-up.

    • I use oven roasting bags in winter when I want to keep my socks from soaking my boot insulation or liners. I’ve found that VBL comfortable and useful below 15 degrees. Much warmer and I sweat too much, but that’s possibly because I’m more active (hiking with a pack) than you when you’re lying still in a bag.

  3. I have been using RBH Design VB clothing for winter backpacking and it’s fantastic. I have used this very light weight VB clothing hiking in temps as low as -20 and was kept very warm. When night temps drop below 15 degrees I put on my VB clothing and cover myself with my 15 degree quilt. I’m very comfortable and warm at temps as low as -25 degrees.

    VB clothing is always in my winter backpack.

  4. I am a cold sleeper and my VBL adds about 15 degrees to my quilt. I made the liner myself, so I could use less fabric and save weight. Essential item in my pack.

  5. I have read a number of explanations of how the VBL works. Too often a pseudo-scientific explanation is given that contains errors, or at best is a poorly founded generalization. A supposed authority on the subject said that insensitive perspiration stops when humidity gets high. That might be true, but it is highly unlikely. There is a great deal of information on this in the medical literature and none of it i could find says insensitive perspiration ever stops. Until someone goes out in the field with instruments and makes measurements and reports the data and supports the conclusions I doubt we will know exactly why it works. The most favorable subjective reports, I have observed, come from people dealing with very cold (like zero F and less) temperatures.

  6. Very interesting! Definitely planning to try this.

    Just one comment. “…the warm molecules leave the surface of our skin, leaving the cooler ones behind…” is highly inaccurate. Evaporation requires energy. This energy is taken from our body surface which lets us feel cooler. The other way condensation releases energy. That’s why it feels so frickin hot when you hold your hands over a pot with boiling water and steam condensates at your hand.

  7. What do you think of using non-breathable rain gear as a VBL? For example, compare:

    * RBH Lightning Bug ($230, more if you want pit zips, etc.)
    * LightHeart Gear Rain Jacket ($99)

    The rain jacket is made out of silnylon, as is the RBH garment…. so, like, is there really any substantial difference here other than price and that one is a “rain jacket” and the other a “VBL”?

  8. Well, it’s been almost two years since anyone else commented here.
    But the comment of James Johnston got me thinking.
    Use a silpoly rainjacket combined with an Exped Schnozzel pumpbag. I like getting as many uses as possible out of a piece of equipment. I am going to try this.

  9. I,ve used plastic vapour barrier when out at -40c. Works fine. Except needed to get use to feeling clammy and that funny sound when moving around.

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