I have been wondering if the average wateterproof/breathable rain gear might be usable as a vapor barrier for sleeping. My theory is that if you use this stiff inside your sleeping bag their would be no noticeable temperature difference between the two different sides of the fabric therefore it would not pass any moisture. What do people think, could this be an effective strategy for keeping your sleeping bag dry in cold condtions?

WPB will not function as a vapor barrier. The primary factor that makes WPB "breathable" is the humidity difference, not a temp difference.

See the following article for more details

http://web.archive.org/web/20040719025505/http:/asc2004.com/23rdASC/summaries/g/GP-21.pdf

Douglas - I have used raingear as a vapor barrier.
I put on synthetic briefs and then PVC (non-breathable) rain pants and hooded rain jacket. I left this on for 2 days of snow camping and energetic backcountry skiing in the Sierra in March. I did take off the hood and un-zip the jacket in the heat of the day. When it got cool I would pull some light capilene over the rain jacket and pull a fleece cap over the PVC hood. At night I added fleece over the capilene and slid fleece pants over the rain pants. If it had been colder or windy I would have added parka and pants shell over the insulation to trap air. As I was wearing VBL, breathable or non-breathable outer shell would have performed the same. There would be no moisture to pass through.

This test proved the theory that far less insulation is needed when evaporative heat loss is defeated by a VBL. I was truly amazed by how little insulation I was wearing while sedentary in camp at night. Also, insensible perspiration is shut down by the humidity inside a VBL so it did not turn into a slippery swamp.

The lack of "fuzz" (which Stephensons provides on their VBL clothing) on the inside of my makeshift VBL suit and the non-stretch loose fit meant that when I moved, the folds of material allowed to chill while away from my body felt very cold when I moved and they came back into contact with my skin.

Oh, and after 2 days I smelled like a cheese factory.

Solution: Pony up for Stephenson VBL pants and shirt. Perhaps wear them only in camp and not while exercising heavily on warm days.

Regardless of the problems that I had, the theory was definitely proven. If you can get VBLs to work and learn when and how to use them, they are SUL.

Al, insensible perspiration is not shut down by a VBL. The medical literature does not support that myth IMO.

Jack Stephenson had to make up some good junk to combat the onslaught of gore-tex in the 60's, and he failed. (my imaginative theory)

Edited by jshann on 03/05/2008 07:26:43 MST.

It may not shut down insensible perspiration but it dose keep that perspiration from collecting in your sleeping bag.

Vapor Barrier clothing isn't intended to stop you from sweating. Instead it's intended to trap the heat you lose from insensible perspiration. Trapping this heat and moisture will lower your body’s amount of insensible perspiration a great deal but it will not stop it completely.

VBL's purpose is not to trap moisture or heat otherwise lost from insensible perspiration.

If it trapped the moisture, it would fill with fluid in a surprisingly short time. Our skin puts out quite alot, even at rest. Fortunately, persperation decreases dramatically when the skin becomes saturated. It's the body's way of not wasting water when it's too humid to evaporate and cool the organism.

The heat is not trapped because it is not used in the first place to evaporate the sweat. It can't because it is sealed between the skin and VBL in a 100% relative humidity chamber. No evaporation, therefore no evaporative heat loss.

Al: VBL's purpose is not to trap moisture or heat otherwise lost from insensible perspiration.

John: A vbl's purpose (while sleeping) is to prevent moisture from entering and affecting insulation loft.


Al: If it trapped the moisture, it would fill with fluid in a surprisingly short time.

John: No it wouldn't when only about 600 ml is lost per day by skin AND lungs. Yeah, the respiratory losses go up with exercise and extreme altitude, but that is respiratory and not skin losses. If a vbl didn't trap moisture, how is it going to stop evaporative heat loss Al?


Al: Fortunately, persperation decreases dramatically when the skin becomes saturated. It's the body's way of not wasting water when it's too humid to evaporate and cool the organism.

John: No insensible water loss doesn't decrease when the skin becomes saturated. It just doesn't evaporate.


Al: The heat is not trapped because it is not used in the first place to evaporate the sweat. It can't because it is sealed between the skin and VBL in a 100% relative humidity chamber. No evaporation, therefore no evaporative heat loss.

John: There can still be conductive and radiative heat loss going on though.


Bottom line: The outdoor industry has propagated the wrong physiology for decades about the sweating mechanism especially in how insensible evaporation occurs.

References:
1. Guyton's Textbook of Medical Physiology
2. Skylark Medical Clinic

Edited by jshann on 04/29/2008 23:07:14 MDT.

John,

John: A vbl's purpose (while sleeping) is to prevent moisture from entering and affecting insulation loft.

Thanks for the clarification. You are half right.I was thinking about VBL clothing's use to retard heat loss, but inside both a sleeping bag and insulative clothing VBLs can also keep the body's moisture from invading the insulation. I was talking about one benefit of VBLs and you have pointed out the other.

Whether sleeping or active, VBLs allow us to use significantly less insulation due to both not wetting our insulation and shutting down Evaporative Heat Loss (EHL). My use of raingear as VBL demonstrated a dramatic decrease in insulation needed to remain comfortable. With or without the VBL, my insulation (both down and synthetic) felt dry and lofted well in the dry Sierra air.

If a vbl didn't trap moisture, how is it going to stop evaporative heat loss Al?

A VBL does trap moisture. That is how it shuts down EHL. See my post above.

John: There can still be conductive and radiative heat loss going on though.

Heat loss from the human body through radiation is negligible. If you look at the formula for heat transmission from conduction, convection and radiation (which I have posted elsewhere on this forum) it is immediately apparent that at low temperatures (98.6 degrees), conduction is the primary heat transmission agent and radiation has almost no effect. That is why aluminized mylar space blanket marketing is pure hyperbole.

Conduction is present with or without VBLs.Fortunately, when we take EHL out of the equation, this conductive heat loss becomes much more manageable.

If VBLs did not dramatically decrease perspiration, as you posit, the ensuing moisture build up would, among other problems, increase conductive heat loss greatly through the puddle of liquid trapped in the clothing and flooding into gloves and socks.

Fortunately, I have observed no such occurrence.

Thanks for keeping me honest.

For what seems to be an effective full-body VB suit, try a soft tyvek coverall. Mine weighs 150 g (< 6 oz) and cost $12. It can be worn against the skin.

Tyvek is a breathable fabric, especially the grades used for jumpsuits, so I doubt it would make an effective vapor barrier fabric. I did experiment once with a painter's jumpsuit, thinking that the fabric might be only slightly breathable, but it still allowed moisture to enter my sleeping bag, based on interior condensation against the inside of the Pertex shell.

From what I have read, soft tyvek is about 4 times less breathable than Gore-tex, so I suppose some condensation is still possible and the VB effect is not absolute.

A bit about Tyvek from the point of an architect. . .

Tyvek is intended to be used as part of a waterproofing system in buildings.

Tyvek is not waterproof on its own.

Tyvek is designed to allow interior water vapor to escape while resisting the passage of water from the exterior.

When using Tyvek in a backpacking application to resist the passage of water it is important to place the Tyvek surface with the writing on the wet side of your system (ie twords the wet ground when use as a groundsheet). Failure to do so will accelerate water passing through the Tyvek.

Edited by chadnsc on 05/13/2008 10:15:15 MDT.