My Thoughts on Vapor Barriers
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
Thanks for the repost on this Youngblood. It's apropos for something I've been thinking about.
I'm queuing up for my next project, a differential cut underquilt for a bridge hammock to be used in cold (but not arctic) conditions. I've been thinking to make the layer next to the hammock from silnylon, and seal the seams. This would build in to the quilt what you're doing with a separate VB.
Biggest drawback here will be sewing the baffles, because of the slipperiness of sil. Before launching into said project I'm going to experiment with using tissue between the sil and the feeder dogs.
Ditto everything you said, YB. I have been using a VB from my first days as a hanger, as part of my SS. I have also routinely used it in my PeaPod at temps between 10 and low 30s.
The only thing is, I have not personally had any problems so far using the VB/SB even at the warmer temps, hanging below me in a hammock environment. Though I do see the risk, and some people will have those problems. And I might in the PeaPod if I tried to use a VB much above the low 30s. But so far for me, as long as I don't allow myself to overheat and adjust on top as needed, there is no significant moisture in my hammock, quilt or even on top of the SB itself. There are often a few small drops on the SB. However, a recent SS user ( was that Hugger?) was having moisture problems that never did get resolved. So I guess folks vary.
In fact, even in the high 40s to low 50s, the only trouble I ever had with condensation was the one time I used my SS without the SB. The next morning the foot of my OCF pad was quite wet. That was, of course, because the SS UC is also a VB, thus I had insulation between that VB and myself. And that insulation got wet, and the foot of my bag did also a bit. But I was warm anyway.
Oddly, there have been times at even warmer temps when I have been able to use only the UC without a pad or SB, and have not gotten any condensation. But I suspect if it had been a few degrees cooler, I would have been wet.
... in cold weather i've been wearing frogg toggs OVER several layers of clothing in my sleeping bag in my hammock over ccf pads without any noticeable condensation -- when cold i must be perspiring little enough, and the frogg toggs must breath well enough to avoid a problem... it seems your argument would encourage me to wear the frogg toggs closer to my skin -- say just above a wicking layer... with my other clothes on the outside of the frogg toggs to encourage the slight moisture buildup to take place where it will do the most good -- at my skin. i am starting to think/understand that avoiding moisture buildup is only one goal -- and encouraging insensible perspiration at the skin is another. Is it your experience that vapor barrier clothes reduce the issues over condensation at ccf pads?
I have a 20 year old pair of Patagonia VB socks. They have on occasion made a world of dif, whether in the sleeping bag or in boots. Keeps socks and boots and quilt/bag foot sections bone dry, assuming moisture is prevented from entering from the outside. They do feel a bit strange and unpleasant, though, what with moisture accumulating under the VB. And boy will your feet get a blast of cold if you pull them off. But they don't feel as unpleasant as cold feet!
BillyBod... I could have written the same post you did. I have used GoreTex over socks, Seal Skins (with built in cotton liner... what where they thinking), and finally NeoSocks. Warm and wet beats cold and wet but they do feel funny. I haven't used the NeoSocks very much so I don't have a feel for them yet, I figure they will be warm and less likely to hold moisture in.
Lazarus... I have slept in Frogg Toggs over all my clothes before and did okay. The argument is that if you stop the moisture close to your skin and have your insulation on the other side, then no moisture can get to your insulation. It is hard to argue with that as dry insulation should work better than wet, damp or slightly moist insulation. But slightly moist insulation might work better than no insulation, if that is your choice. I think Frogg Toggs would be difficult to wear under other insulation. They are cut big and are kind of bulky and a little bit delicate.
Grizz... I think Brandon has done that with his underquilts and they worked well for him. I think about doing that at times but it is hard to give up breathable, at some point you might to want to use it at the high end of its temperature range. But it is all about tradeoffs and what you want to do with it.
yb, good thread, now it will be easy for folks to find. question, how many degrees does a vb (non-reflective) add to the low temp of your uq?
another point, is that i have used a sil shelled synthetic uq several times in several different conditions, and had similar results, maybe even better (no moisture build-up) i think there is a HUGE difference between laying directly on raw ccf and having a vb between the uq and the hammock. it's almost as if it breathes just enough to avoid moisture build-up.
grizz, here's an idea i've been throwing around for a down uq with a sil shell.
outer layer shell: sil, inner layer shell:netting or breathable ripstop, second inner layer shell sil sewn only on 3 sides and left open on one end to flip inside-out leaving the breathable shell is exposed for drying. this way, when inside out it has one breathable side, but when right side out, it's sil on both sides. so you have a built in vb, and a sil outer shell for more water resistence.
i've noticed a difference in full sil uq's and full breathable uq's. if i'm exposed (small tarp) i can feel a gust of cold wind somehow make the uq cold for a second, like it is penetrateing or sucking the heat out. this was not the case when using the sil uq. a good winter tarp probably stops this from happening as well which most will have in cold conditions. i could tell a difference though. it may have been really breathable ripstop i was using, the stuff i have now is parachute 1.1 so it has a very low cfm (air permeability) so i'll have to test it specifically for that and see if it's similar or not, but the sil outer shell would def stop wind well so it might be worth doing, if ysing a winter tarp though, both wind and water resistence may no longer be an issue though.
I used Frogg Toggs on occasion; usually when sleeping in clouds in the cold, they worked pretty good and I never seemed to suffer any 'excessive' moisture build-up.
I used the sil shell WBG is talking about and never had any moisture build-up in it the next morning; expected to, but didn't. However, I never really noticed a difference in temps using the shell. The only exception to that would be in high winds under a non-winter tarp when you can actually feel the breeze thru the quilt; the shell almost eliminated that and kept me much warmer than without.
Good post, Dave - made this thread a sticky.
Brandon - I see the benefit of what you're talking about, but what if you just made the section next to the hammock from sil and all the rest from DWR? Then just use a Garlington Taco when you need it. For very little extra weight, you get the flexibility of a half-quilt, or use the taco only, and get an extra tarp/poncho, groundcloth, etc.
I've slept with a fleece inside my breathable rain jacket and woken up with a very damp fleece. The next night, in almost the same conditions, I wore a t-shirt then rain jacket then fleece on the outside. No dampness on the fleece and I was much warmer.
My opinion is that there isn't an additive temperature improvement by adding a vapor barrier to breathable insulation. I think it is a multiplier that you apply. There are a lot things going on to get the 20F per inch of breathable down insulation guideline that we often use. There is certainly the R-value of the insulation (or clo-value or tog-value) of the insulation itself but that number is applied to the metabolic rate (or something, it has been a while since I looked at all that so that might not be the correct term) of the individual. It is that metabolic rate that you are modifying with a vapor barrier placed between you and the breathable insulation. So that 20F per inch might become something like 25F per inch for breathable down insulation with a vapor barrier in place.
So with that, if you add just a vapor barrier to a hammock you might only get a degree or so of improvement. If you add it with 1 inch of breathable down underneath you would get a 5 degree improvement or 10 degrees with 2 inches, etc.
But this is just the way I have it figured based on what I have seen, experienced, and surmised. I don't have references to back it up. I haven't seen it characterized this way, at least I don't recall seeing it.
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