Whether sleeping in a tent or outside under the open sky, moisture builds up inside and outside a sleeping bag at night. Condensation can soak a sleeping bag, creating damp and chilly and potentially dangerous conditions depending on the weather and environment. Some tips to keep your sleeping bag free of condensation may make the difference between a cold soggy night and a warm cozy sleep in the great outdoors.
Ventilation
The human body gives off several pints of moisture vapor at night through breath and sweat, according to Outside Magazine. If this moisture has nowhere else to go, it will condense on your sleeping bag. Use a double-walled tent, recommends adventure writer Naomi Judd at Trails.com. Air circulates between the tent wall and the outer fly, while the fly keeps off additional moisture from dew accumulating from the night air. Even when conditions are cold, keep at least a small area of the tent door and windows or vent open to allow moisture to exit. Make sure that rain fly is stretched out off of the tent wall, advises the guides of Source to Sea, a Mississippi River paddling trip to benefit the Audobon Society. This will allow dew to run off away from the tent wall, while ensuring that the necessary protective air space remains between the tent and fly.
Cover
Moisture in the air at night condenses on all cool surfaces, creating that sheen of dew on the lawn, and on your sleeping bag, in the morning. In colder temperatures, this freezes to a glare of ice. Staying under any overhanging cover will help minimize environmental condensation, just as there is noticeably less dew on the grass under a shade tree on a summer's morning. If a tree or rock overhang is not handy, Outside magazine suggests a light plastic tarp laid over the bag can help. Even better would be to tie the tarp out as a free-floating fly, or stake one edge to the ground and another several feet up to create a lean-to, allowing adequate ventilation while providing cover protection against environmental condensation.
Barriers
Barriers between your body and your sleeping bag, and between the bag and the environment, can reduce condensation soaking into the bag. Long underwear will help absorb sweat through the night, keeping it from being soaked up by your bag. A vapor barrier or other liner between you and the bag may make you feel sweaty by precluding moisture evaporation, but it will keep you warm and your bag dry. A bivvy sack outside the bag will protect the bag from exterior condensation, and keep you warmer as well. In the morning, find a bit of sun and ensure that your clothes and bags dry out before packing them in stuff sacks and backpacks. If the morning schedule does not allow complete drying of condensation, be sure to stop for lunch in a dry sunny location and spread any wet gear out so that you don't crawl into an already-wet bag at night.
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