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Originally Posted by
Owl
So...newb question on the subject......................
Seems to me that you'd want to have some kind of wicking agent if possible closest to your body...then your insulation layer(s), and on the farthest outside your vapor barrier. So I'd assume that your vapor barrier/windbreak would be OUTSIDE of your UQ, not between the UQ and hammock.
Newbie questions welcome! And certainly things concerning vapor barriers are high on the list when it comes to confusion. And using vapor barriers while not understanding them will probably leave you wet and cold. It seems to me the system you are describing (A true VB over (outside) all of the insulation but NO skin level VB) would end with a cold and wet person. I'm fairly sure the system you are describing – probably one you have been using – does not have a "vapor barrier" for the outer layer. (Correct me if I'm wrong) it probably has what is known as a waterproof breathable (WPB) outer layer like some form of Gore-Tex. Now people argue about how breathable these WPB's really are, but I can guarantee you that they are no match for something like Sil-Nylon when it comes to blocking vapor. These WPB's are meant (designed) to hopefully block liquid water from the outside (not necessarily outside vapor) and to allow water in vapor form on the warm inside to pass through. A true vapor barrier is both waterproof (blocking the passage of sweat or rain) as well as blocking water in vapor form. If your body's vapor (insensible perspiration) cannot get past the vapor barrier, then it cannot reach the outer layers of cold insulation and condense in them.
Want to know for sure if you have an actual vapor barrier for your outer layers? Take your jacket (again probably a WPB) and put it on next to your skin or over a T-shirt and sit down in your easy chair. Or maybe walk around at a mild level of exertion on a cool day. Most likely, you're pretty comfortable as well as dry. Now take that jacket off and put on a large garbage bag to which you had added holes for your head and arms. Feel the difference? Even just sitting still in your easy chair, it probably won't take but a few minutes until you feel that dampness. Sit outside on a cool day, when it's very unlikely that you will sweat, and you will still feel the dampness with that garbage bag. That is a vapor barrier. If you put such a thing only on the outside of your insulation, it won't be long before that insulation is wet even if you are sitting perfectly still and feeling on the cool side. No sweat is necessary for this wet insulation, only enough time.
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This is assuming that the vapor you're trying to avoid is coming from outside moisture, not sweat. Sweating would be avoided by temperature regulation inside the hammock/sleeping bag/TQ and clothing, I'd think.
Again...I'm a newb...asking a newb question.
Where am I wrong, and why? Thanks!
The moisture we are trying to avoid is not coming from the outside, and it is probably not even from sweat. It is your body's insensible perspiration. A very thin layer of extremely high humidity that your body produces to help keep your skin from drying out for one thing. This layer of high humidity is pretty much always present whether or not you are approaching overheating and sweating. It's like the vapor that comes out of your mouth every time you exhale. Even if you on the verge of hypothermia, if you breathe on a very cold surface that is very close to your mouth, your breath (vapor) is going to condense into liquid when it hits a cold enough surface. And that cold surface can be the outer layers of your insulation or rain garment. Sweat is an entirely different matter. Sweat is not necessary for you to get condensation in your sleep system, just like you can get condensation on the inside of your tarp even if you are nowhere near overheating.
However, if you do manage to sweat a bit inside your vapor barrier, that moisture should also be prevented from reaching your insulation layers. But it will obviously be better and more comfortable to learn to recognize the "overheat" situation and vent as needed.
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Originally Posted by
NickJ
I'm not convinced by this (or possibly just not getting it). If you are sweating then your body is too hot, yes?
Yes!
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in that situation the vapour needs to go somewhere.
Sweat is not vapor, it is liquid. Your body produces this liquid which then is supposed to evaporate ( go from liquid to vapor ) and cool you down by the process of evaporative cooling. But even if you do sweat, then it does need to go somewhere, but hopefully not into your insulation. Hopefully it will be contained inside the vapor barrier.
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If it doesnt, it either condenses somewhere and causes a problem, or you just get hotter. Surely it has to be a very fine line where a VB would actually work, or rather a very narrow band of temperature/microclimate. Would it not be better to ensure that you're not sweating in the first place and so prevent the problem?
It's certainly better to ensure that you're not sweating, but that might be easier said than done when you're asleep. However, you don't have to sweat for your body's insensible perspiration (vapor) to cause mischief in your sleep system or other insulation items.
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At the weekend I noticed that the inside of my tarp was very moist. It was a good away away from the hammock, and the hammock has a breathable top shield.
Exactly. And there was no sweating required from you in order for that tarp to get wet, right? All that was needed from you (or from some other source) was some vapor floating through the air which could then condense on the cold tarp surface (vapor barrier).
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The wind had been strong in the night, though the tarp (WB superfly) had both doors closed and about 8 inches of space to the floor. It had rained very hard during the night, and I figured that the condensation on the tarp was simply the result of the temperture difference between the inside and the outside of the tarp, and the fact that the air was very moist. The ambient was around 4 degrees C at a guess. I was warm and not sweating with the combination of north face blue kazoo bag (used as a TQ) and an HG winter burrow UQ. I was wearing Woolpower thermal top and longjohns.
Plus, especially with those doors closed, any warm vapor that you were contributing from your body and (mostly) your breath could not rapidly escape from inside the tarp, and naturally condensed when it hit the cold tarp surface – a vapor barrier. No doubt there was plenty of humidity adding to the problem, but the vapor that you were producing inside the tarp was probably also a factor.
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I've read the text and looked at the diagram, and it's not making sense to me (yet). I can't understand why this is neccessary when if you're sweating too much you need to either vent something or wear less clothes, not use a VB, a VB is going to make you sweat more UNLESS you are cold to begin with and as you warm up the VB helps that process, but ultimately you're going to overheat / create a pile of condensation on the VB unless you vent.
Sweat is not the issue, insensible perspiration and vapor is. Although, if you sweat without a vapor barrier, then sweat is also part of the problem. Even the effect of sweat on your insulation will be greatly reduced by a vapor barrier. You are right, you want to avoid sweat with or without a vapor barrier being close to your skin. You will not sweat inside a VB unless you over heat, but you will feel damp. Once you reach 100% humidity at skin level, your body stops producing more vapor. But it feels like sweat, but in my experience it only goes so far and then gets no damper. But if you over heat- with or without a VB- you can keep right on producing sweat as your body tries to cool itself.
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You want vapour to be dispersed yet the air between you and your insulation to be warm, you dont want any risk of moisture creation inside the hammock, and the best way to avoid that is to have windproof/breathable material. Isn't it?
Condensation can occur even with breathable materials. Over the years many a person has reported how their breathable insulation often gets heavier and heavier on multi-day trips in very cold weather. I have a friend who has reported problems with that even with temps only in the 30s at the coldest and on other trips with lows no more than the higher 40s. This was all with breathable gear. However, condensation is virtually guaranteed with a true vapor barrier. So you must make sure that if you have a true vapor barrier (a waterproof shell) outside of your insulation layers, that there is another vapor barrier between you and your insulation. What ever insulation is between you and any vapor barrier is most likely going to get wet. The only exception I know of to this is when I use a space blanket directly under my hammock but on top of my HHSS insulation, or under my hammock inside of a Peapod. ( though there is not much insulation between me and that VB, only what I am wearing) There is still possible condensation under these circumstances, and I have from time to time seen a couple of drops on top of the space blanket, but not much. Supposedly that is because being that close to my skin( though not tight against it), and surrounded by the outer insulation, the space blanket (VB) is kept close to body temperature. So condensation against a cold surface should be minimal, and apparently it is. And apparently since it is only under my back and not surrounding my body, most of my insensible perspiration manages to remain vapor, and work its way out of the breathable topside. That's how it is work for me, anyway, both warmer AND dry.
I would say that VB theory is pretty solid and practical in the real world. Though it will make matters worse if used incorrectly. So anybody using it should make sure they understand the theory, and hopefully do some experimentation under very safe circumstances. Do a search here for Wisenber's fairly recent posts on VB use. Quite impressive indeed! It seems he added at least 20*F to his insulation's abilities, plus kept it dry. (Maybe some of his posts are in this thread?) Because a VB does not just keep your insulation dry, it halts a major form of heat loss: evaporative cooling.