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Posted by Bill S on December 8, 2007 at 1:31:18 am EST
In reply to Sleeping bag liner posted by Spion on December 7, 2007 at 11:56:29 pm EST

Neither. This is the situation where you need a vapor barrier liner. With fleece or silk, the vapor absorbed eventually gets transferred (due to your body heat) on outward through the insulation (down in your case). If the outer shell of your bag is microfiber, the vapor can get transferred on outward, though if the outside air is cold enough (in the tent, assuming you are sleeping in a tent), you will see frost and/or condensation on the outer shell. If you have a waterproof/breathable outer shell (Goretex or its more vapor-passing cousin Driloft), a fair amount will condense on the inside surface of the shell and get absorbed into the insulation. This is also one of those cases where the wicking of a good synthetic fill, like Primaloft, helps, since the moisture continues to get wicked outward.

A VBL keeps the moisture from getting into the insulation in the first place. It works for some people, but other people find it clammy. Wearing wicking long johns inside the VBL helps wick the moisture up and out of the face hole. Top quality winter and expedition bags use a completely waterproof inner shell around the face hole to prevent your breath from getting absorbed back into the insulation, but that means you have to make sure you keep your nose and mouth in the opening, not buried down into the interior of the bag (which a lot of people seem to do when cold).

Since you mention your bag absorbing moisture from your breath, try to observe how you are placing your nose and mouth at the face hole - is all your exhaled breath going outside, or are you breathing into the interior of the bag? Roughly speaking, you lose about a half liter of water from your body's perspiration over night, and a half-liter to a liter through your mouth and nose from your breathing overnight. So by making sure you breath out the face hole, you cut the vapor to be absorbed by half to 2/3.

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• Re: Sleeping bag liner - calamity 02:00:37 12/09/2007

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