It聮s an atmospheric thing. Your bag聮s insulation means that while you聮re warm and toasty inside (at least, I assume you are), the shell of the bag is at ambient temperature聴probably 35 degrees or so. Meanwhile, you聮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聮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聮 dew.
Mont-Bell’s U.L. Sleeping Bag Cover
U.L. Sleeping Bag Cover
I don聮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聮d still have a damp bag in the morning. 国产吃瓜黑料, however, a bivy sack might help. Try one such as Mont-Bell聮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聮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聮d be inclined to just let it go.
Pick up a copy of the 2006 国产吃瓜黑料 Buyer聮s Guide, on newsstands now, for a look at the best sleeping bags and 396 other torture-tested products.