I understand that a VBL traps the sweat between your baselayer (or skin) and the VBL. How does this trapped sweat evaporate?

Say I am wearing a thin sock, a VBL sock, an outer thicker sock, and my boots. The VBL keeps the sweat between the thin sock and the VBL, but isn't my thin sock totally drenched? Since it's a boot, how does one dump heat? Is the idea that the baselayer might be sopping wet but the outer layers are not and so I am still warm?

Does one need to ever dry a VBL? Does a VBL stink?

If you sweat while sleeping without a VBL and wake up steaming, wouldn't you sweat even more with a VBL on under your bag? So, you're uncomfortable and wet but your bag is dry?

Is the moral of the story that I need to pack multiple baselayers and socks and whatever else that's kept under a VBL on a several day winter backpacking trip?

First of all, vapor barriers work best when it is intensely cold. The theory is that the vapor barrier traps a certain amount of warm/humid air next to the skin. Then, after a while, your skin notices that it is pretty humid next to the skin. At that point, the skin drastically reduces the amount of insensible perspiration. That means that your body will begin the loss of less water during that period, usually night. In the morning, the thin layers beneath the vapor barrier will be warm and humid. However, since they are thin, they can generally be aired out quickly. Your sleeping bag has less overnight vapor in it, so it is quicker to dry out in the morning.

Does a vapor barrier stink? Sometimes it isn't noticeable over the first night, but by the second or third, it can become bad.

Some called him a vagabond.
Some called him a vagrant.
But they all knew when he was around
Because he was so fragrant.

--B.G.--

How does this trapped sweat evaporate?

With a perfect VBL, it doesn't.

Say I am wearing a thin sock, a VBL sock, an outer thicker sock, and my boots. The VBL keeps the sweat between the thin sock and the VBL, but isn't my thin sock totally drenched?

Perhaps.

Since it's a boot, how does one dump heat?

You still lose it by all the normal methods of heat transfer, that is, convection, conduction and radiation. The only difference is that evaporation doesn't amplify convection or conduction. In convection, the air moves around allowing more sweat to evaporate (which loses heat) instead of becoming saturated. In conduction, your wet socks would allow heat to flow directly into the cooler surfaces of your shoe. A VBL doesn't necessarily stop convection, so you can still have fresh air flowing into your thick socks. Since your socks are not perfect non conductors, you'll still have some conduction going on.


Is the idea that the baselayer might be sopping wet but the outer layers are not and so I am still warm?

You got the idea.

Does one need to ever dry a VBL? Does a VBL stink?

If it's wet, I would want to dry it before putting it back into my pack. If it's below freezing, drying it may be as simple as laying it out for a few minutes and then shaking the ice out of it. Stink? Every piece of fabric that touches me stinks...

If you sweat while sleeping without a VBL and wake up steaming, wouldn't you sweat even more with a VBL on under your bag? So, you're uncomfortable and wet but your bag is dry?

This takes getting used to. Ideally you'll find the perfect temperature at which your sweat is at equilibrium. I always let myself get too hot, so I sweat all night. I usually find the sweat pooled up under my pad if I'm using a bivy. Once it's under my pad it won't evaporate again, so it's no longer a problem until the morning when I need to shake out the sweat.

A note about vbl ...

You should reduce yr activity and insulation level to a point where you are not sweating profusely

It doesnt help you if you are sweating alot in winter no matter what you wear

Bpl has a good article on it

If you are sweating in yr bag ... It is simply too warm ...

Edited by bearbreeder on 11/29/2011 19:32:25 MST.

VBL's are a tactic for very cold conditions (i.e. consistently below freezing), like winter camping in the north.

In such conditions it is cold enough outside that the dew point is reached inside your sleeping bag, meaning your perspiration (without VBL use) will condense inside the bag and begin to wet the insulation.

After a few days of winter camping (again without VBL use), and with no opportunity to meaningfully dry the bag during the cold short days, you begin to suffer a collapse of the insulation, which is pretty dang uncomfortable and even dangerous.

So to counter that problem, you suffer through the damp and clammy conditions imposed by the VBL instead. The perspiration stays close to you, and there is less of it because your body creates less in such conditions, so your insulation stays dry (at least from perspiration).

It's not ideal from a comfort standpoint, but what else are you going to do? Heated tents are not UL!

Here is a really good article on VBL's by Andrew Skurka (must be a member to read)
http://www.backpackinglight.com/cgi-bin/backpackinglight/vapor_barrier_liners.html

I remember when VBL were popular and then people were misusing VBL and most manufactures said to quit using them. Western mountaineering still sells one I think Stephenson warm lite but the rest of the industry threw them by the way side because they did more harm than good for backpackers.
Terry

Edited by socal-nomad on 11/30/2011 09:48:19 MST.

In below constantly freezing situations a VBL will keep your insulation layer dry. i.e. A felt liner in a feltpac boot or a foam liner from a ski boot will stay totally dry from your perspiration and thus keep you warm all day.

I use a thin neoprene diver's sock as a VBL with my feltpacs and Scarpa T3 ski boots.

AT BEDTIME IN CAMP:
1. liners come out of boots and into my sleeping bag (size long, for dtorage at foot)
2. remove VBLs, turn them inside out and dry them in the tent
3. remove thin poly liner socks & put them in laundry bag
4. put on new liner sox and a thick sleeping sock
5. before going to sleep put the inside-out, mostly dry VBLs into my sleeping bag

For longer (over 2 days) winter trips it's advisable to use either a VBL suit or liner bag to keep your sleeping bag from losing insulation AND gaining pounds of weight in frozen perspiration.

I have used VBL sleeping bag liners up to about freezing. When I get too hot, I throw back
the top part of the sleeping bag and pump a little fresh air into the VBL. Just like
I do with the quilt on my bed at home. Not so hard to understand.

I also once wore some VBL socks (neoprene) all by themselves with no liner, for a week
in the summer when my other socks wore out. Worked OK. Feet felt moist all day. No blisters, dried my feet at night. Smelled worse than wool socks, but no worse than synthetic socks.

All good points already stated, but one thing important to remember is that it "may" take practise getting your VBL to work for you the way it should - trying it once and saying you got "soaked/froze so it doesn't work" is like buying a hammer, hitting your thumb and saying the hammer doesn't work. A VBL is a tool, you need to learn to use it. I had to learn when to vent, how long to vent for, the different feeling between being wet/hot/humid, the temps I could use it with, etc. Unfortunately, not everyone has the chance to do this and then they miss out. What I can say is that when you do have it dialed in correctly, it is a fantastic piece of gear.

For the last 2 years, I've been using a VB on literaly every trip I go on, summer and winter. Good luck!

Edit: OK, after reading David's post above (I was still typing), I feel like I made a VB sound more complicated than it is. I like his summary "When I get too hot, I throw back the top part of the sleeping bag and pump a little fresh air into the VBL. Just like I do with the quilt on my bed at home. Not so hard to understand."

Edited by Steve_Evans on 11/30/2011 10:34:47 MST.