Bivy Condensation

A couple weeks ago I used an RBH Designs VBL shirt (against the skin) for the entire weekend outing in West Virginia with about five inches of snow on the ground and with day temps in the 30s and night temps in low 20s/high teens. There was no precipitation called for both nights, so I skipped the tarp and just used an MLD Alpine bivy (eVent) for the evening, along with an Exped 9 and JRB High Sierra Sniveller quilt. I didn't get a bit of condensation either night, and liked the VBL for what it's worth, though I was a little damp on the inside for parts of the weekend. It didn't bother me much and kept me warm and my insulation dry and happy (including down quilt and bivy).

I've used the RBH VBL socks for quite some time to help reduce condensation in the foot of my bivy, and it's worked wonderfully. Bought an RBH VBL shirt and used it a couple of months ago in sub-freezing temps, and it worked wonderfully. Did not sweat all night, had no condensation in my bivy, and stayed warm all night.

I'm really looking forward to Thom Darrah's cuben quilt liner to use as an easy, light VBL to see how that works.

As you can tell, I'm a big proponent of VBL. I wear the insulated RBH VBL socks in the winter alone as my hiking sock and it keeps my feet quite warm all day, and even during short breaks. Love 'em.

I've used a vapor barrier (actually my non-breathable silnylon rain gear) when it's below freezing and have had zero problems with condensation within or on the surface of my sleeping bag. I have always been quite comfortable. For me, at least, the theory that the skin stops putting out moisture, once a high humidity level is reached, seems to be valid. My skin feels just comfortably moist and I don't at all have the feeling I'm in a sauna. I wear my base layer (Capilene 2) inside the VBL, which makes things far less clammy (no way would I want plastic or silnylon against my skin!). I also only do this when I am pretty sure the night is going to be below freezing.

Different people I know who've tried this have different reactions to using a vapor barrier. It doesn't seem to work for everyone. Some are like me; for others it needs to get a lot colder (teens or lower) before they are comfortable (which perhaps might be due to too warm a sleeping bag–maybe a little venting would help?).

I suggest you try using a VBL in below-freezing weather before you disparage it. If it works for you, it will keep your sleeping bag insulation a lot dryer in cold weather!

I've used a garbage bag VBL top – punch out holes for neck and arms.

Cheap way to get an idea whether you like it

You can also try wearing your rain gear to bed. Works well as a VB.

I just assumed that I would be soaked from the moisture that was trapped inside the vapor barrier, so I always ignored the concept.

Great advice/reminder about the advantages.

So called "insensible" perspiration is a result of your skin leaking water vapor b/c a humidity gradient exists across your skin. I like to think of my skin as a wpb membrain (relatively high on wb and low on b, i guess). If you put a non breathable layer next to your skin, this humidity gradient approches zero and the driving mechanism for this evaporation disappears. This process is governed by physics of mass transfer or (diffusion).

This is significantly different than the physiological responce to the body being too hot. This "sensible" perspiration is a process directly controlled by your central nervous system.

James

I don't believe you can stop insensible perspiration with a vapor barrier. You won't find that in any human physiology textbook. Please post your findings to the contrary.

"I don't believe you can stop insensible perspiration with a vapor barrier. You won't find that in any human physiology textbook. Please post your findings to the contrary"

That's what it says on Stephenson Warmlite site, but they don't give any references

http://warmlite.com/vapor-barrier

John,

As I recall you are a MD and this topic is outside my domain of expertise. So, my statement is really a solicitation to be educated by the more knowledgeable forum contributors such as you. By absorbing moisture through the skin isn't the net inflow effectively the same as saying we are stopping insensible perspiration?

The vapor pressure of water in the wet internal mass of a living mammal is about 44 mm. Hg. However, the vapor pressure of water within, or just beneath the cutaneous water transpiration barrier is not exactly known but one can say that it is functionally about 23 mm. Hg because with water vapor pressures in the ambient air below 23 mm. Hg, water vapor passes from the epidermis to the air and with pressures of water vapor in air higher than 23, water vapor enters the epidermis from the air. Shouldn’t a vapor barrier liner create vapor pressure above 23 mm. Hg on the skin’s outer surface?

The following medical references all discuss that the net intake of moisture to the outer skin from the environment(typical VBL environment) above 23mm. Hg on the skins outer surface:

-Hale, F. C., R. A. Westland and C. L. Taylor:
Barometric and Vapor Pressure Influences on
Weight Loss. J. Applied Physiol., 12:20,
1958.

-Hardy, J. P.: Physiology of Temperature Control.
Physiolog. Reviews, 41:521, 1961.

– Taylor, E. A. and K. Buettner: Influence of
Evaporative Forces on Skin Temperature
Dependency of Human Perspiration. J. Applied
Physiol., 6:229, 1953-1954.

Hi Richard. A couple of other interesting articles would be (I may have them at home on another computer)

Henane R, Buguet A, Roussel B, and Bittel J. Variations in evaporation and body temperatures during sleep in man. J Appl Physiol 42: 50–55, 1977.

Newburgh L, Johnston M. The insensible loss of water. Physiological Reviews 1942; 22(1):1-18.

and

http://advan.physiology.org/content/29/4/213.full

Average water losses per day are about 1200 ml urine, 200 ml stool, 900 ml insensible (600 ml skin; 300 ml breathing). Older texts put, IIRC, insensible at 600 ml for both skin and breathing. Insensible water loss is affected by environmental temperature and humidity, altitude, volume of air inspired, air currents, clothing, blood circulation through skin, and water content of the body.

Skin loss can be decreased by high humidity, but I don't think you can stop it. One research article said high humidity can lower by 40% the water loss in infant skin. Infant skin is more permeable to water movement than adult skin. If 600 ml moves through skin per day, maybe 300 ml or less moves at night if in winter tent for 12 hours. If it takes several hours to achieve high humidity to decrease skin insensible water loss, then an insignificant amount of water is "conserved".

My 2006 response to the Warmlite article is at

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=4821&nid=37855

Steven,

I'll try most things once and I have tried a vapor barrier for sleeping. I don't plan on repeating the experiment. My body gives off too much moisture, even while sleeping, for this thing to work.

Moisture built up within the vapor barrier to the point where I and my sleeping clothes were very wet. In the middle of the night I couldn't take it anymore plus I was getting colder and colder from the wetness. I got up, stripped everything off, put on dry clothes, ditched the vapor barrier and got back in the bag.

This was extremely unpleasant! Temps were in the 20s.

I bought and returned a Stephenson Warmlite tent in the 70s and also tried to apply most of the advice he included in the catalog. Almost none of it worked for me the way he said it should. The tent, for example, was dripping with condensation shortly after entering it on a cold snowy evening. Then it froze and for the rest of the night it was like I was sleeping inside a freezer. Every time I brushed part of the tent it would drop frost on me which would then melt and wet my sleeping bag.

Daryl

Hi David

> I thought there was a section (not MYOG) to place this info.

Yes indeed: Gear Deals. You would be welcome there (with disclosure).

Cheers

John S.,

Thanks allot for adding your expertise to this discussion!

The baby information was, for me, the biggest missing piece of the puzzle. As a result of you filling in some missing pieces I think I finally GET IT.

Hi Richard

Interesting. My wife reckons on very wet days she hardly needs to drink. If water does go in through the skin, would it be picked up by the circulatory systems?

Cheers

This has been a great discussion on Vapor Barrier theory. I noticed a couple of responses where they liked the idea of a VBL and both happened to be in very cold climates. This seems to be consistent with other things I have heard. For example, some people who have traveled above the arctic circle said the ONLY way they could keep their sleeping bags from becoming a block of ice over many days was to use a VBL and they would not go out there w/o it. I too looked at some scholarly studies done on the subject (in the medical field, and one on infants) and there didn't seem to be any evidence that insensible sweat stops at a wide range of humidity or temperatures studied. So the paradox remains: Some people seem to believe in the stuff and others don't. Perhaps individual differences or perhaps there are too many factors to control for in a practical experiment. Many thanks for the input. It is about the best I have seen on the subject.

I can't say I "believe" in any theories about "stopping insensible perspiration." Since it appears that this concept doesn't have any scientific backup, it may be pure folklore! All I know is that my vapor barrier layer keeps the moisture from my body from going into my sleeping bag and condensing inside the sleeping bag in below-freezing weather. Under these conditions, I find the VBL quite comfortable.

I'm well aware that a VBL doesn't work for everyone! I'm sure that some folks put out a lot more "insensible" perspiration than others. I have a few friends who find a VBL a sauna even in zero (F) weather. Many others, like me, swear by a VBL in below-freezing weather.

My question is, simply, why reject the concept without trying it?

http://www.backpackinglight.com/cgi-bin/backpackinglight/vapor_barrier_liners.html

"Old" Skurka article on this topic. Not a doctor…but a lot of miles.

John,

I have never peeked in a physiology textbook but that's not where I would go to try to understand this subject…

It appears the medical definition for insensible perspiration is: "perspiration which evaporates before it is percieved as moisture on the skin". Similar to the definition for insensible water loss — water loss that a human isn't explicitly aware (so not urination or spitting etc) of and thus can't readily be accounted for.

I think its pretty straitforward to conclude that when wearing vbl clothing the relative humidity next to the skin surface would be nearly 100% w/ the air temp btw vbl and skin at nearly skin temp.

Evaporation under these conditions is nearly impossible. This is why high temperature/high humidity enviromental conditions can be so dangerous (as you have mentioned) b/c sensible perspiration dosn't evaporate (and thus cool) nearly as effectively as high temp/low humidy or lower temp/high humidity.

Whether you believe insensible evaporation is function of relative humidy near skin or not a few things are reasonable to conclude:

– A vb will greatly help keep moisture out of your sleeping insulation (if of course its worn under the insulation).

– A vb will help keep you warm under nearly all condition you would be camping in (effectiveness at suplementing warmth decreases with increasing environemental temp/humidity).

– A vb will keep you skin (and anything inbetween it and the vb) wetter than it would otherwise be under nearly all camping conditions. Effect on wetness would be a strong function of how well your insulation matches the enviromental demand.

James

> A vb will greatly help keep moisture out of your sleeping insulation
That is the whole point of VBL

> A vb will help keep you warm under nearly all condition you would be camping in
Not really! Do NOT rely on this for warmth!!!! You still need every bit of that quilt or sleeping bag!!!!
But do not overdo the warmth bit: try to sleep slightly cool: that will reduce/minimise the sweat and water inside the VBL.

> vb will keep you skin (and anything in between it and the vb) wetter than it would otherwise be
Yes. By sleeping cool you will however minimise this.

Cheers

My experience, too, Roger–it might affect the warmth a little, but not enough to be significant. It does keep the insulation drier so the insulation can do its work.

Comment one: glad we agree

Comment two: note I never tried to quatify how much. I think it is reasonable to say that additional warmth is obtained as you are evaporating less water. Quantifying the warmth gain is something I meant to aviod. On a 30F night I would expect relative warmth gain to be significantly less than my down bag (but how knows how they compare when considering eaches weight…). I won't be heading out alone with my vbl and mylar blanket :)

Comment three: definatly agree original post reflects that though

thanks,
James

James, I agree with all of your conclusions. My statements only mean that a VBL does not stop insensible water loss. A VBL does stop evaporation. Skurka's article is very good.

Roger,

http://www.ehow.com/video_4399547_skin-absorbs-water.html

http://ajph.aphapublications.org/cgi/content/abstract/74/5/479

It appears so but the proof looks a little sketchy.