The Minimalist
The Minimalist

Why does my sleeping bag always collect condensation?

I find that whenever I sleep outside with friends, I wake up with by far the most condensation on my bag. I have a four-year-old 15-degree Marmot down bag. It is probably near the less-warm end of the spectrum of bags me and my friends use, but not outrageously different from everyone else's. Why do only I wake up freezing cold and drenched? Charlotte Duluth, MN

The Minimalist

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Well, that’s odd. Condensation, for the most part, is a product of external conditions. Air is high in humidity, the surface of the bag is cold, and the moisture in the air condenses out. If your friends do not have the same condensation problem, it would make an interesting physics problem to see why not. Maybe your bag is more insulated so the surface is cooler than the others, so more moisture condenses out.

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Some moisture also comes from inside the bag, however. You give off several pints of water vapor at night, and that can also condense on the surface. Maybe you just give off more moisture than the other guys.

Either way, what to do? Be sure to wear long underwear as that will help absorb some body moisture (wool would be especially good). To keep off external moisture, a bivy bag would be a help, such as REI‘s Minimalist ($89), which uses a proprietary laminate that sheds water well and allows the bag to breathe. To contain interior moisture, a vapor barrier liner (Rab Vapor Barrier Liner, which run $50) will contain the moisture you give off. You’ll be damp in the morning, but warm, and your bag will be dry.

A simple light plastic tarp over the bag might work as well. And stay out of the open—even sleeping under a tree will cut condensation.

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