I was asked by ChrisH to make up a simple illustration about vapor barrier liners (VBL) in an effort to help explain how they work and when to use them. I've experimented with VBL's in the past with good results (and some real disasters!), but the real experts are those who've successfully used them in the field. I contacted sclittlefield, one resident expert on VBL's in the forums, who helped frame a lot of my thoughts. I also read and researched several articles, but the one article I found that did the best job is from the long-distance hiking guru, Andrew Skurka. Skurka's article "Vapor Barrier Liners: What they are, how they work, and when to use them" was perfect, and I think his real-world experience is a must-read if you are interested in VBL's.
My illustration and notes hopefully explain some of the basics, and perhaps add a visual reference to other, more thorough articles like Skurka's.
The main points I've gleaned from personal experience and from what I've read is this:
- VBL's work best when it is cold (<= 40°F/5°C).
- A VBL should be close to and completely surround your skin to effectively create the micro-climate your body needs to stop perspiring and maintain its temperature. For comfort (e.g., to eliminate a clammy feel), you can wear a base layer before the VBL.
- VBL's are critical for long-term winter camping, but can be effective for overnight trips as well
From my reading, VBL's receive criticism because of excessive moisture build-up. Proponents argue that the presence of moisture build-up is an indication that the VBL was used incorrectly.
A VBL can effectively "trap" the insensible moisture around your body to create a micro-climate where body's hydration and temperature are then regulated and stabilized. Once the micro-climate is stabilized, the body stops transmitting insensible moisture.
That's Great, But How Does That Apply To Hammocks?
A VBL can be used in a hammock very easily, as either a bag or quilt liner, or as Skurka recommends, as part of a layered clothing system.
VBL's should not be used on the outside of insulation (either under quilts or top quilts) because moisture will eventually build up inside the quilts and will collapse the insulation. Underquilts are less of an issue since water vapor tends to rise upward.
Some pads (e.g., closed-cell foam) create VBL's since they don't breathe, however, they only provide a VBL on one side of your body, which can create irregularities with your body's thermoregulation.
You can use any non-breathable material to create a VBL: plastic bags, sil-nylon, coated nylon or polyester, etc. The real "trick" to a VBL is to make sure your body temperature is properly regulated. Skurka used VBL's while hiking, backpacking, and snowshoeing, proving that VBL's can be used during moderate exertions.
Anyway, I hope the illustration helps. Again, I would refer you to Skurka's article, which I think does a much better job in explaining VBL's.