vapor barrier inside sleeping bag

4:51 p.m. on March 9, 2009 (EDT)
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Here is a new topic (or maybe just new to me)...

Has anyone tried using a vapor barrier inside their sleeping bag to help control moisture loss and condinsation inside their tent and/or bivy? This sounds like an idea that might work for me during the winter. I have had problems with condensation only when there are extream temperature fluctuations over a very short period of time (40 deg. to 0 deg in a couple of hours). Sometimes when this happens I find there is a lot of condensation inside my bivy in the morning.

Typically the condinsation is due to perspiration and trying to dry out my wet gloves or sox. I do not breath inside my bivy, so I know this is not the problem. You know, you dress to warm when you go to bed knowing the temperature is going to drop. You wake up in the morning and your a little damp from condensation. Or my favorit, you brought your -25 deg bag and it only gets down to 10 deg. Its just a little damp, but if you are out for a few days, this could be a problem.

Would a VB help?

5:07 p.m. on March 9, 2009 (EDT)
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Use a lighter sleeping bag so you dont over heat or sweet you dont need to be that warm/hot to stay comfotable.

5:46 p.m. on March 9, 2009 (EDT)
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Mike Horn, in "Conquering the Impossible: My 12,000 Mile Journey Around The Arctic Circle" said he had to use a vapor barrior in his sleeping bag to prevent it gaining substantial weight each night from his body moisture. It was frequently 30 - 70 degrees below zero and there was no practical way to dry it our.

This is obviously an unusual and extreme situation.

7:13 p.m. on March 9, 2009 (EDT)
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Vapor barriers are an oft-discussed topic on backpacking websites. I've seen discussions on them for years, including here. Do a Google or Yahoo search and you will find plenty of articles pro and con that explain the science behind the VBL.

Here is Stephenson's argument in favor of VBL use. I have no idea how accurate it is, but it makes interesting reading.

8:44 p.m. on March 9, 2009 (EDT)
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As I have discussed on Trailspace at least a few times before, there are certain circumstances where I use one in my sleeping bag, and certain circumstances where I use VBL socks. I have never found VBL clothing (other than socks) to work for me.

First, the basic fact is that you perspire continuously, losing about a half liter of water during an 8-hour sleep (plus about another half liter breathing during the 8 hours). Presumably, you will breathe through the face opening in your bag and not have to worry about that moisture getting into your bag's insulation. The half-liter your perspire assumes you are comfortably warm, not overly warm, and it assumes you did not get into the bag with damp clothes or socks. That half-liter will pass through your bag's insulation. Whether it passes all the way through or condenses (or freezes) in the insulation or on the inner surface of your bag's outer shell depends on the outside air temperature and humidity.

I almost always use a VBL in very cold conditions or snow camping in humid conditions (like the Sierra). In very cold conditions, even very low humidity, your perspiration will condense or freeze in part in the insulation. If you have ever noticed dampness or frost on the outside of your bag while sleeping in a tent in winter, that's your perspiration, not condensation from the air. A VBL in the bag prevents that moisture from getting into your bag's insulation.

In theory (Stephenson's anyway, and a few other people who have made studies), your body senses when the local humidity is above a certain level and stops perspiring (not sweating, which is the body trying to cool when it is excessively hot or you are exercising hard). For some people (me for example), this seems to work. I am sleeping in wicking long johns in those super cold conditions anyway, so the perspiration is wicked out through the face hole in the VBL, hence out of the sleeping bag. I do get frost around the face hole, but none on the bag at the -20 and lower temperatures.

When Steiger made his first foot trip to the North Pole, his party found that their sleeping bags were gaining roughly a pound a day, so that 60 days later, when they reached the Pole, their bags weighed something like 70 pounds, just from the accumulation of frozen perspiration.

The idea of VBL socks is similar, but with a small difference - you wear an inner wicking sock, put the VBL sock on next, then your wool insulating sock on the outside. The inner sock gets damp and wicks the moisture up and out of the boot and VBL sock (you need a long liner sock and long VBL for this to work). The outer wool insulating sock stays dry (assuming you are not in conditions that soak your boots and get the wool socks wet from the outside). So your feet stay warm. At the end of the day, you wring out your liner socks and change into a dry pair, while allowing the damp liner socks to dry out overnight and over the next day.

1:51 a.m. on March 10, 2009 (EDT)
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Stephenson (not sure if Jack wrote that piece on the site long ago or his son or someone else at the company) seems to pooh pooh the idea of wearing a wicking base layer, yet mentions insensible persperation, so it seems a bit of a contradiction to me.

I've never used a VBL even in the Sierra in winter. I haven't noticed my bag getting wetted out at all either. Maybe I just wasn't in cold enough weather. I don't seem to sweat much in my bag, so maybe that helps. My bag is down and the shell is made of some kind of synthetic material, not sure what, but definitely not Goretex or other similar fabric. I sleep in Capilene, including socks and on this last trip, wore a fleece balaclava which was a little wet around my nose when I woke up, most likely from breathing on it.

I read Steger's account about their bags on his trip, so I'd consider a VBL in really cold weather if I was out for a long time, but that is fairly unlikely.

10:33 a.m. on March 10, 2009 (EDT)
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I've done brief experiments with vbls and so far the results were positive. I hadn't thought of the liner sock needing to be longer as Bill mentioned so I'll pay more attention to that next time I'm out. Going forward, on anything longer than a weekend trip in the winter, I'm definately going to use a vbl in my bag to keep the down dry.

2:57 p.m. on March 10, 2009 (EDT)
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Thanks for all th useful input. Sounds like I will have to try a VBL. Any suggestions on where to buy one?

3:49 p.m. on March 10, 2009 (EDT)
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Western Mountaineering or Integral Designs are the only two I know of. Any more out there?

8:30 p.m. on March 10, 2009 (EDT)
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Stephenson has vbl clothes, not sure if they sell liners.

9:51 a.m. on March 11, 2009 (EDT)
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Campmor may still sell a vbl, but it's heavier, coated nylon. I bought one once and returned it due to it's weight. I think the ID liner is silnylon and I'm not certain what the WM liner is made of, but both will be much lighter than coated nylon. A liner would be easy to sew, but for lack of that pesky thing called spare time.

11:14 a.m. on March 11, 2009 (EDT)
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What I know is its best to sleep naked in your bag to help reduce the trapped moisture evaperating from your body. When I used to winter camp in Yosemite in the early 80's I had a -30 bag and was never cold, at least untill I woke in the morning and had to crawl out to put on my clothes. I kept them in my bag to keep them warm. To bad I could not put them on before getting out of it.

I used to use a cotton sheet sewed together on the long side and bottom for a vaporbarrier. Plus it keeps your bag cleaner on the inside.

9:35 p.m. on March 11, 2009 (EDT)
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You can make a VBL with 2 1/2 yards of silnylon if your slim.

I have to use one in Quebec because of the high humidity and temp difference, from -30c to + 5c in the same week. Just keeping your goretex top and bottom in bed will acheive roughly the same result as a VBL, that's what I did at first.

The difference with a down bag is huge after a week. it's the only way to keep it dry in winter and can save your but if the temps rise above freezing druing the prevents your bag from melting because it is still dry.

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