Well, it is that time of year again and I thought I would repost my revised spill on vapor barriers.
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 see if this helps.
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 your 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.
When closed cell foam pads are used with breathable underquilts in hammocks, we get a situation where the overall insulation is greater than the sum of the various components making up the insulation. That is because closed cell foam pads are also vapor barriers. The vapor barrier stops your body from producing insensible perspiration and causes the breathable underquilt to provide more apparent insulation (although it isnít doing anything different) because your body is now operating more efficiently at keeping you warm. If you use a ccf pad that provides 20F worth of insulation with a breathable underquilt that provides 40F worth of insulation by itself, you will end up with more than 20 + 40 = 60F worth of insulation. You might end up with 70F worth of insulation or more.
You can also wear a vapor barrier to help stay warmer. You can do it selectively for parts of your body or you can do it for your whole body (but donít forget you need to breathe). You can use vapor barrier liners for sleeping bags or plastic bags over your feet, head (as a hat), torso, etc or rain suits. I havenít tried a vapor barrier liner for a sleeping bag but have used a rain suit for a vapor barrier (even breathable ones work) and used plastic bags over feet, head, and torso with success. I always have used thin liner material under them with the understanding they would likely get slightly moist from insensible perspiration.
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.
[I am a little conflicted about this particular paragraph. On one hand I feel it is important if this write up is going to be used as somewhat of a reference for vapor barriers. On the other hand I donít want to scare folks about a problem they arenít likely to encounterÖ but here goes.] Understanding vapor barriers, how to use them, when to use them, and when not to use them becomes more important the colder it gets. I donít have experience in extremely cold temperatures but my understanding is that if you use breathable insulation at extremely cold temperatures you risk insensible perspiration condensing inside the insulation as the dew point temperature moves inside the bag itself. Vapor condensing inside the bag creates moisture inside the bag and is just as troubling as sweat or any other moisture getting inside the bag. When this happens the insulation may not dry out at the temperatures you find yourself in and can accumulate over the days you are exposed to extremely cold temperatures. This can be a troubling phenomenon if it occurs as your bag can become increasingly heavier and less efficient. For most of us this isnít an issue as I am referring to extremely cold conditions and we shouldnít be concerned about the dew point moving inside the bag itself at the more moderate cold conditions that most of us camp or backpack in. I only mention it because it should be of interest to the few individuals that are camping or backpacking in extremely cold temperatures.
aka Youngblood AT2000
designer of the Speer Segmented Pad Extender, SnugFit Underquilt, and
May 13, 2008
Revised December 1, 2008