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May 22, 2006

Can I stop condensation from forming on my sleeping bag?

outdoor gear question

U.L. Sleeping Bag Cover
(courtesy, Mont-Bell)

For the last four years I have gone camping in Idaho and Montana in mid-May. In the mornings, my sleeping bag’s covered with condensation whether I sleep in the tent or under the stars. Would a bivy sack eliminate this problem, or should I just live with it?

— Richard
Erie, Colorado

Do you have a question of your own?

It’s an atmospheric thing. Your bag’s insulation means that while you’re warm and toasty inside (at least, I assume you are), the shell of the bag is at ambient temperature—probably 35 degrees or so. Meanwhile, you’re 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 there’s 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.

I don’t 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 you’d still have a damp bag in the morning. Outside, however, a bivy sack might help. Try one such as Mont-Bell’s 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 that’s 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, I’d be inclined to just let it go.

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