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19072My Thoughts on Vapor Barriers... again

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  • Dave Womble
    May 13, 2008
      Vapor barriers aren't that easy to use if you don't understand what is
      going on and they aren't that hard to use if you do. Does that make
      any sense? If it does, you don't need to read any further. If it
      doesn't, then read on and I will see if I can make it make sense for you.

      I use plastic or silnylon as a vapor barrier between my hammock and my
      SnugFit Underquilt. The suspension system on the SnugFit helps keep
      the vapor barrier against the fabric of the hammock to minimize air
      gaps that would encourage insensible perspiration to condense and
      pool. With the vapor barrier held in place against the underside of
      the hammock fabric, the hammock fabric wicks away any slight buildup
      of insensible perspiration where it can more easily evaporate and I
      usually don't even notice it. I pick when I do that and understand
      how to use it. It is been very effective for me at extending the
      lower temperature range that I can use my underquilt. I would not use
      a vapor barrier when I was using the underquilt in warmer conditions
      because then I would be dealing with sweat and there would be larger
      amounts of sweat/moisture to deal with as the vapor barrier would
      cause me to overheat even more and prevent moisture from sweat from
      passing through the breathable underquilt.

      In general, breathable insulation works best when your insulation is
      getting too warm for you and less breathable insulation works best
      when your insulation is not quite warm enough for you. That has
      everything to do with how, when, and why your body produces sweat (or
      sensible perspiration) and insensible perspiration.

      Your body produces sweat to help cool off at the outer surface of your
      skin with evaporative cooling when you overheat. Your body does not
      produce sweat when you are not overheating... you don't just leak
      water through your skin all the time. When you are not sweating, your
      body can produce insensible perspiration to keep your skin from drying
      out. If your skin is moist enough, or not too dry, your body doesn't
      produce insensible perspiration because it senses that it doesn't need
      to.

      It takes energy for your body to produce insensible perspiration.
      When you are not overheating and your skin is not producing sweat, a
      vapor barrier will cause your skin to quit producing insensible
      perspiration after some period of time. Your skin quits producing
      insensible perspiration because the vapor barrier creates a high
      humidity environment by trapping the moisture from your previous
      insensible perspiration. When this happens your body does not use
      energy to produce that insensible perspiration anymore and can use
      that energy to help keep you warmer. It a sense, your body becomes a
      more efficient furnace.

      A vapor barrier is not so good when used at the wrong time or when
      used incorrectly. When you are overheating and using a vapor barrier,
      your skin continually produces sweat as a means of cooling off via
      evaporative cooling. The vapor barrier prevents the evaporative
      cooling because the sweat is trapped by the vapor barrier. You just
      keep sweating and moisture can build up. You need to do something to
      keep from overheating because what you body is doing isn't working
      because of the vapor barrier. You need to remove the vapor barrier,
      vent, or remove insulation.

      When you use a vapor barrier with breathable insulation between it and
      your skin, that breathable insulation is subject to getting moist or
      even wet from insensible perspiration. The insensible perspiration
      will initially pass through the breathable insulation and stop when it
      hits the vapor barrier. This continues until the humidity builds up
      enough for skin to quit producing insensible perspiration. But until
      that happens, that breathable insulation is going to be getting moist
      too. What you want is a thin wickable sheet of fabric between you and
      the vapor barrier such that it can wick any slight moisture buildup
      away where it can evaporate into the surrounding air. Of course it
      helps for the surrounding air to be able to absorb that moisture
      because if it can't, it won't and you will be clammy.

      And of course, if you use breathable insulation between you and a
      vapor barrier while you are overheating, you will soak that breathable
      insulation with sweat (sensible perspiration). That is bad and that
      happens when people don't understand how and when to use a vapor
      barrier.

      Vapor barriers work well for people that know when and how to use them
      and are often problems or even disasters for people that don't.

      Dave Womble
      aka Youngblood AT2000
      designer of the Speer Segmented Pad Extender, SnugFit Underquilt, and
      WinterTarp
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