Temperature ratings are made so you can tell what the warmth of the bag is. The rating basically tells what the comfort range of the bag is. If the rating is 20 degrees (Fahrenheit), then you should be comfortable from about 20-40 degrees. But there is one major problem, sleeping bag companies have not set universal guidelines for ratings. Each company places the rating which they feel is correct for their bag. So this causes a little problem for comparing bags. For general 3 season backpacking, a 20 degree bag is the best place to start. This will keep you warm on cold days, but you won’t burn up on hot summer nights.
Now that you know how warm a bag is, don’t you want to know what makes it warm? There are two major materials, down and synthetics. Down is what has been used since sleeping bags were made. This is probably the warmest material for it’s weight and it compacts very nicely in a stuff sack. But if you ever get down wet, be warned, it loses all it’s warmth. This is where synthetics come in. These new materials are almost as warm as down and compact almost as much as down. The great thing about synthetics though is that if it gets wet, then it will still be warm.
Shape makes a big difference in sleeping bags. The two shapes, mummy and rectangular each have their own good parts. If you are going to go backpacking, then by all means buy a mummy bag. It is called a mummy bag because it is more form fitting and looks like a wrapped mummy. This close shape keeps you warmer since there is less air to warm up inside the bag. Rectangular bags are usually big, bulky and heavy. They are not as warm as mummies since they are not form fitting. I would only recommend this type if you are going car camping because it can double as a blanket.
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