Sleeping bag
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In camping and other outdoor activities, a sleeping bag is a protective "bag" for sleeping, analogous to a bed and blanket. Its primary purpose is to provide warmth and insulation. It also protects, to some extent, against wind, precipitation, and exposure to view, but a tent performs those functions better. The bottom surface also provides some cushioning, but a sleeping pad is usually used for that purpose. A bivouac sack (bivy) is a waterproof cover for a sleeping bag that may be used in place of a tent, if rain or very cold temperatures are not expected.
Design types
A basic sleeping bag is simply a square blanket, filled with cotton or other material, and fitted with a zipper on three sides allowing it to be folded in half and secured in this position. A sleeping bag of this type is packed by being folded in thirds, rolled up, and bound with straps or cords. The basic design works well for most camping needs, but is inadequate under more demanding circumstances.
The second major type of sleeping bag, sometimes called a mummy bag due to its shape, is different in a number of important ways.
- It is tapered, with the foot end narrower than the head end. This reduces both volume and surface area, improving its overall heat retention properties.
- It usually only unzips part of the way, so that the foot end is permanently cylindrical in shape. The zipper is a weak point in any sleeping bag's insulating qualities. Together with the tapered shape, this design feature helps protect the feet, which are more vulnerable to heat loss than other parts of the body.
- It usually has a drawstring mechanism at the head end, to help prevent the escape of warm air.
- The most important difference is the insulating material. "Mummy" bags are filled with synthetic, hollow fibers, or down feathers (see below).
- A mummy bag often cannot be rolled like a rectangular bag. Instead, it is simply stuffed into a stuff sack or compression sack.
Fill
Backpackers have debated for years about the relative merits of synthetic and down-filled bags. Synthetic fill does not readily absorb water, dries easily, and provides some warmth even when completely wet. These properties may be a matter of life and death if, for example, the sleeping bag is accidentally dropped into water on a cold day. Also, the synthetic material is firm and resistant to compression. Any insulation is compromised when it is compressed, so this makes the sleeping bag warmer on the bottom.
Down fill, most often used by backpackers, is lighter and retains heat better, but costs much more. Down must be kept dry; a soaked down sleeping bag may provide even less insulation than no sleeping bag at all, leading to hypothermia.
Other materials, notably cotton and wool, have also been used for sleeping bags. Wool is highly water-resistant, due to its natural greasiness, and quite resistant to compression, but also much heavier than any alternative. Cotton suffers from high water retention and significant weight, but persists in applications like stationary camping where these drawbacks are of little consequence and its low cost is attractive.
External links
Manufacturers of sleeping bags include:
- Big Agnes (http://www.bigagnes.com/str_bag_home.php)
- Coleman (http://www.coleman.com/coleman/ColemanCom/category_main.asp?CategoryID=9000)
- Kelty (http://www.kelty.com/Kelty/index.cfm?fuseaction=SleepingBags.Home)
- Marmot (http://www.marmot.com/products/cat.php?cat=bags)
- Mountain Hardwear (http://www.mountainhardwear.com/action/catalog/DisplayNode?id=690)
- The North Face (http://www.thenorthface.com/na/gear-shop-category-7.html)
- Sierra Designs (http://www.sierradesigns.com/bags.bags.php)