I need to actually agree with Jeff here. Not quite sure what a vapor barrier is, or if it goes over or inside of the sleeping bag?

I was thinking of doing something similar to the US system: I have an REI sub-kilo bag (20 F rated) and a (leaky) bivy bag. Rather than drop a few hundred on a system, could I just buy a cheapie as the second bag? Something like:

http://www.sportsmansguide.com/net/cb/cb.aspx?a=245534 OR
http://www.sportsmansguide.com/net/cb/cb.aspx?a=78532

yeah, they're heavy, but they're definetely a weather-specific item. the other option would be to buy a fleece sleeping bag and put that inside of the REI bag.

Ideas on which would be more "bang for the buck?" I don't camp out as often as I'd like, so I'm leaning towards the cheaper/lighter fleece. But... would that really increase the temp span that much?

Gladly...and I'll try to keep it short and sweet.

All it is is something that goes inside your bag and holds in the moist air which comes off your body as you sweat (It can be as simple as an oversized garbage bag or a specialized $70 sil-nylon model). It basically shuts down 'insensible perspiration' (the moisture which evaporates from INSIDE your skin, cooling you). If I'm not mistaken this happens because the air around your skin is so fully saturated with moisture, the water in your skin can't evaporate. The bad side is that the 'sensible' perspiration is increased because it won't evaporate either and that can give you a sweaty, clammy night if your gear is TOO warm.

My theory behind why the AMK thermo-lite bivy does so well is because it also reflects radiant heat.

In winter there are additional benefits (which is good because you generally wouldn't use a vapor barrier in the summer anyhow). The moist air doesn't get get into your sleeping bag. when the moist air in the bag hits the cold air outside your bag it freezes. That means a stiff heavy bag with next to no working insulation.

If you want to learn more there's information on the net in spades. One of my favorite explanations can be found here:
http://www.netbackpacking.com/vapor-barrier-and-radiant-barrier-liners.html

Here's the picture I mentioned. You can conveniently see the rating label on the sleeping bag.

Thanks for the info.
Does your wife think you're crazy for doing these experiments like mine does?

Nice goatee. It looks great. (don't take that the wrong way)

She's actually pretty OK with sort of stuff. At first she thought I was joking, then she thought I was crazy...then I explained to her that the alternative was to walk out into the bush and sleep under the stars without testing it...then she encouraged it.

A vapor barrier does not shut down any perspiration. It stops insensible perspiration (involuntary water diffusion through skin) from evaporating. If used in warmer weather it could cause more sweating. Without a wicking layer, or even with one, your skin may feel clammy from the moisture. The main purpose of a vapor barrier is to stop moisture from freezing inside sleeping bag insulation in winter. Expeditions of the past have failed due to this.

I've never used a vapor barrier liner, but have always been intrigued by the idea, worth a test run at least. Some people swear by them and some hate them. My thoughts are that a vbl will keep you bag's insulation dry as your sweat won't collect inside the bag. With a gore tex bivy sack the bag should stay dry from the outside and the combintation should keep one warm and dry and likely extend the warmth of the bag by itself. Good in theory anyway.

I read that during Will Stegar's north pole trip the bags they used gained so much internal moisture buildup the bags wound up weighing 40+ pounds each. The duration of a trip surely plays a large role here since each night an incremental amount of sweat will accumulate in the bag and freeze. On a weekend trip it may not matter.

A few web links on vbl's:
http://www.verber.com/mark/outdoors/gear/clothing.html#VaporBarrier A good write up and some additional links.

http://www.warmlite.com/vb.htm Very opinionated, take it all with a grain of salt.

High end commerical vbl's:
http://www.westernmountaineering.com/ind...mp;ContentId=44

http://www.integraldesigns.com/product_d...producttypeid=5

But if you are a heavy sweater or have a lot moisture in your exhalation (I can breath on a fish and keep it's gills going, I swear), you want to skin down before you get into a VB in cold weather or anything warm you take in there with you is going to be soaked by morning. That is the downside to them that I've discovered, but the Thermolite can be opened so...

I sweat and that's why I wear lightweight wool long underwear. Wicks the moisture away so I can't feel it so much. In an ideal world (and by 'vapor liner' theory, if you're sweating that much, your gear is too warm.

While Mark does have good stuff on his site, the vbl stuff needs work, because some of the physiology described is wrong. The warmlite stuff is junk.

A rebuttal to the warmlite article:

http://tinyurl.com/3b9dr4


Here is a good article on thermoregulation:

http://www.backpackinglight.com/cgi-bin/backpackinglight/00184.html