A comfortable sleeping system is critical for winter trips, when the climber may spend the greater part of each day in the sack. It's colder than summer, so you need more insulation. This can come from a heavier sleeping bag or an overbag with additional insulation. An overbag must be large enough that it does not restrict the loft of the insulation in the inner bag. A non-insulating over-bag, such as a bivouac sack, provides some extra warmth and protects the main bag from spills, condensation, and snow. If you don't use any kind of overbag, it may be helpful to have a sleeping bag with a cover made of material such as Gore-Tex to help keep the bag dry and windproof.
The type of insulation you want in your sleeping bag can depend on how wet the climate is. In the mild damp cold of coastal areas, synthetic bags work well as they absorb little or no moisture. As the temperature drops, there is less moisture in the air and the high loft of down clearly excels.
A vapor-barrier liner inside the sleeping bag adds to the sleeping system's warmth, especially in cold, dry environments. As with vapor-barrier clothing, the liner adds warmth by halting evaporative heat loss from your body. Less evaporation means less water vapor condensing on the sleeping bag and, therefore, a drier bag. The liner reduces your perspiration during the night and the subsequent necessary drinking the next moming to replace the lost fluid. The liner may feel clammy next to your skin so wear a single layer of synthetic underwear, which retains very little moisture and dries quickly in the morning. As an alternative to the liner, you can wear a vapor-barrier suit (shirt and bottom). The suit provides advantages similar to the liner, but it can also be worn when you're not in the sleeping bag.
An insulated pad is a critical component of the sleeping system, as the snow is virtually an infinite heat sink held at a constant 32 degrees. Inflatable foam pads such as the Therm-a-Rest are an excellent choice. You can add a closed-cell foam pad beneath the inflatable pad for added comfort, warmth, and reliability. Two foam pads or an extra-thick foam pad are good but somewhat bulky. (Take a look at Chapter 3 for additional general information on sleeping systems.)
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