Its an atmospheric thing. Your bags insulation means that while youre warm and toasty inside (at least, I assume you are), the shell of the bag is at ambient temperatureprobably 35 degrees or so. Meanwhile, youre exhaling warm, moist air. That air floats around the tent, hits the shell of the bag, and condenses out. You probably have a little moisture on the inside wall of the tent, also. I suspect as well that theres something going on with humidity and the dew point in Idaho and Montana in mid-May. When you camp outside, that has to be plain ol dew.
Mont-Bell’s U.L. Sleeping Bag Cover
U.L. Sleeping Bag Cover
I dont think a bivy sack would make much difference, at least inside the tent. The warm, moist air that your body gives off would eventually hit the bivy sack and condense there. So youd still have a damp bag in the morning. ϳԹ, however, a bivy sack might help. Try one such as Mont-Bells U.L. Sleeping Bag Cover ($160; www.montbell.com), which is made with two-ply Gore-Tex.
Whether or not you decide to take action depends on how much of a problem the moisture is causing. Obviously, a wet bag loses insulation, so thats an issue. Plus you have to take time to dry the bag out each morning before stuffing it away. But, unless the moisture is more than a nuisance, Id be inclined to just let it go.
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