January 19, 2001


Should I get a bivy sack to keep my down sleeping bag dry?

I recently went on my first winter backpacking trip in the White Mountains of New Hampshire. The temperature was 0 degrees Fahrenheit at night. When I woke up in the morning, my down sleeping bag was wet on top. The tent was vented a little bit. Do I need to use a cover on my down bag? Luckily we were headed out the next day, so I didn't have to deal with a wet down sleeping bag at those temperatures. What do you suggest?

— Nancy Donohue High Falls, New York

What you had was a little condensation. A couple of things could have led to this. One is warm water vapor from your breath, which hit the cold nylon shell and condensed. It's also possible that moisture from your body (sleeping campers give off about a pint during the night) worked its way to the bag shell, then condensed when it hit the cold air.

A cover might help keep the bag drier if it was moisture from your breath, but not if it was coming from inside the bag. Either way, I don't think it's anything to worry about. If you can just leave the bag in the tent for a while, it likely will evaporate or freeze so it can be brushed off. And for trips of up to five to six days, it's unlikely that a down bag will get wet enough to lose much insulating ability.

There are things you can buy that are designed to help keep bags dry, but I'm not wildly enthusiastic about them because they're expensive, add weight, and may in some conditions exacerbate the problem rather than solve it. Still, Outdoor Research makes a DryLoft bag cover that is designed to keep light moisture off the bag. It sells for $120. Mountain Hardwear also makes a cover called the Conduit SL Bivy that goes for $85.