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  1. #1
    Senior Member Shewie's Avatar
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    Can somebody explain vapour barriers

    Another newbie question sorry

    Am I right to assume that a vapour barrier is to stop moisture passing through the hammock and into a down underquilt ?
    If the moisture is blocked then does is not just wet out the bottom of the hammock, leading to damp clothes or sleeping bag ?

    I have played around with a foil survival blanket before but shyed away from using one in the field as they're not breathable. Am I missing something ?

  2. #2
    Senior Member oldgringo's Avatar
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    No.

    And yes. There's more to it than that.

    I don't understand it very well, myself, so I won't try to take it any farther and hurt myself.

    Someone that does will come along shortly, though.
    Dave

    "Loneliness is the poverty of self; solitude is the richness of self."~~~May Sarton

  3. #3
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    I use vapor barriers regularly at temps into the mid 30s but as low as about zero. They really help with painful numbness I get from poor circulation in my extremities. They can get somewhat uncomfortable after a time, but they add a lot of warmth for negligible weight.

    By the way, you'll see lots of people warning about how you will get trenchfoot with vapor barriers. That's a misunderstanding. Vapor barrier socks were actually developed and used by 20th century militaries as protection against trenchfoot, which was caused by feet being in unsanitary wet cold muck all the time. If you've ever had a limb encased in a cast, you might have a fair idea of how a vapor barrier works and feels. It gets hot and your skin gets clammy, but you don't get trenchfoot from a cast and you won't from a vapor barrier either.

    There's a good discussion about this on the website of a gear manufacturer: http://www.warmlite.com/vb.htm

    Jeff did a beautiful job of summarizing vapor barriers several years ago:

    You want to sweat, but the sweat isn't really what keeps you warm. Look at it this way - your skin is always perspiring even if you can't feel it, b/c your skin needs a certain level of moisture. Without a VB, your body creates that level of moisture, then it evaporates and your body must create more, which then evaporates, so you create more, and on and on. And every time it evaporates, it takes some heat with it. (That's how sweat cools you - it isn't just wetting your skin, but it takes heat with it as it evaporates because phase change takes energy.)

    So you put on a VB. Your body creates the level of moisture your skin needs, but it can't evaporate. Since it can't evaporate, you stop losing the heat that you normally lose through evaporation. If you manage your heat production (insulation, exertion, etc) to match your heat loss (to the cold air/water) then your body will stop sweating and remain at the proper level of moisture. You'll get a little clammy and it'll dry quickly when you take off the VB. In practice, you have to pay close attention to avoid overheating, sweating too much, etc. This is more of an issue for shirts and pants than it is for socks, I think.

    But the big thing is that the sweat doesn't make you warm, but the VB stops the evaporative heat loss that normally takes place...and sweat is a necessary part of the process.
    .. truly to enjoy bodily warmth, some small part of you must be cold, for there is no quality in this world that is not what it is merely by contrast. Nothing exists in itself. If you flatter yourself that you are all over comfortable, and have been so a long time, then you cannot be said to be comfortable any more. - Herman Melville

  4. #4
    Senior Member Shewie's Avatar
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    Thanks vitamaltz I get that I think.

    So are we saying it's okay to block the perspiration and cause it to become trapped against the hammock ? A scenario I'm thinking about would be with a down underquilt with a VB between that and the hammock.

  5. #5
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    Well, there's the rub. An effective vapor barrier traps your perspiration and holds it against your body and anything between the VB and your body. That would include the hammock fabric if you have a space blanket underneath. I actually don't use a space blanket in that configuration, but a lot of people do, particularly those with Hennessy Super Shelters, I think. Some people report a lot of dampness on the hammock, others not so much. I think those that don't get much may not have a fully effective barrier or they just don't notice. Generally, the closer the barrier is to your skin, the better.

    When I'm sleeping, I find that the absolute warmest configuration for my feet is plastic bags against the skin with a thick sock overtop. The next warmest is a thin polyester dress sock and then the bag, then the heavy insulating sock. I've tried VB socks before over a medium-weight hiking sock and I got cold. I think it's a combination of wetting everything between the skin and the vapor barrier as well as creating an airspace between you and the barrier. Since space blankets and plastic bags are so cheap, it's worth it to experiment at different temps and find out what works for you. Let us know! Rest assured that your skin won't turn green and fall off.
    .. truly to enjoy bodily warmth, some small part of you must be cold, for there is no quality in this world that is not what it is merely by contrast. Nothing exists in itself. If you flatter yourself that you are all over comfortable, and have been so a long time, then you cannot be said to be comfortable any more. - Herman Melville

  6. #6
    Senior Member Just Jeff's Avatar
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    Haha - I was going to type a response but vitamaltz says I already did! Thanks for looking that up.

    To be truly effective, the VB should be completely sealed to your body b/c it has to STOP the perspiration from forming. If the perspiration has a chance to escape, it will evaporate (taking heat with it) and your body will create more perspiration to replace it. This is how you begin to sweat, rather than just creating the insensible perspiration.

    So to be most effective, you could wear a VB suit. But lots of folks say putting a VB only on one side (bottom, between UQ and hammock) works well for them. I think there are a few variables on how this works for each individual, but the principles are the same. If you sweat a lot, you may create too much moisture and feel uncomfortable...wetting out your hammock, and possibly creating extra moisture in your top quilt. People's bodies are different and some folks just produce more sweat than others...if you're completely sealed, this shouldn't matter; if you have a way for the moisture to escape, it may be enough to make you uncomfortable. That's why some folks are comfortable with pads and others make too much sweat. Of course, temperature, insulation and hydration level will impact this as well.

    I've had good results with VBs on my feet - I put a thin liner sock (nylon dress sock), the free veggie bags from the grocery store, then a thick winter hiking sock. I've had pretty poor results with pads in the hammock b/c of sweaty back syndrome. I'm gonna start experimenting with VBs between my hammock and underquilt soon though...got the Yeti with reflective liner on the way, so we'll see how it works. And my next homemade underquilt will probably have silnylon for the layer against the hammock.

    What does all that mean? Try it and see if you like it. You may discover something new that helps the rest of us!
    “Republics are created by the virtue, public spirit, and intelligence of the citizens. They fall when the wise are banished from the public councils because they dare to be honest, and the profligate are rewarded because they flatter the people, in order to betray them.” ~Judge Joseph Story

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