Jack Stephenson is a man on a mission, a mission started 30 years ago and now taken over by his son, William. The mission: convince the outdoor world that vapor barrier clothing and equipment is the way to go. Stephenson started a company, WarmLite, that specializes in vapor barrier clothing, with their vapor barrior sleeping bag being the only one on the market that I know of. I didn't find a patent on it, but I wouldn't be surprised if there was one.
The idea is that in conventional sleeping bags, moisture from your body passes from your body through the sleeping bag, making the insulation moist. If the temperature is cold enough, the moisture can freeze in the insulation, and seriously degrade the insulation of the bag. On a multiday winter trip, the sleeping bag just gets more and more damp.
With a vapor barrier bag, there is a vapor barrier next to your body, and your body moisture never enters the insulation of the bag. Without a vapor barrier, arctic explorers come back from extended expeditions with sleeping bags that weight 80 pounds from the built up ice inside the insulation. With a vapor barrier, they can be out forever, and ice won't build up in the insulation. I have certainly observed that that when you are out winter camping for more than one night, your sleeping bag gets wetter and wetter, and its impossible to get it dry. For a person doing a lot of longer trips, I would sure think about a WarmLite bag. They are not cheap, but no lifetime equipment is. They include a vapor barrier liner, and a down filled air mattress, with a built in pump. Their products are at WarmLite, where they are not bashful about the Stephenson's family's belief in Naturism.
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