In general there are two types of sleeping bags; down mummy bags and synthetic-fill sleeping bags. These are distinguised by the type of fiber that is used for the fill. The fill is the most important part of the sleeping bag. It’s what keeps all the heat inside. The fill should do a good job of trapping air, providing you protection from the cold outside air.
Down-filled Sleeping Bags
Down is the most efficient fill for mummy bags, warmth for weight, and traps air better than synthetic bags. This warmth for weight efficiency makes it ideal for mountaineering.
Down-filled Sleeping Bags compress better than any available synthetics and so packs down to a smaller size. When opened up, a down sleeping bags fluffs back to its open state very quickly and more completely. It also acts almost fluid like as it readily fills up the spaces of the sleeping bags.
Good quality down originates from China. The argument has been made in the past whether white goose down are better than gray goose down but there’s really not much difference.
Down filled sleeping bags are marked as 600-fill, 700-fill or 800-fill. This is called the fill power. Fill power is the number of cubic inches that one ounce of down will occupy.
The higher the number of fill power results in less amount of down required to fill the same space which makes the sleeping bag lighter. This is accomplished by using fluffier and longer-tendrilled down.
The American Society for Testing and Materials (ASTM) ordained standards for down content and fill power. By law, down must be at least 80 percent down i.e. cluster and fiber rather than quills, beaks, etc.
A good down sleeping bag will bounce back when you poke it with your finger. When properly taken care of, a down filled bag can last for a long time. One spirited hiker I personally know used his for more than 20 quite a while.
But down as a fill is not perfect. You need to avoid getting it wet or it will stop keeping you warm and it’s almost impossible to dry on the field.
Down is eye poppingly expensive and demands rather more care in maintenance. In some cases, rare as it is, may cause an allergic reaction.
Synthetic Fill Bags
Synthetic fills have been around for quite a while. They come in are distinguisedent name like Primaloft, Thermolite Extra, Hollofil I & II and more. The names change and the manufacturers will likely come up with more technologically advanced sounding names.
Synthetic fills are categorized into three forms. The fibers can be lubricated with silicon or other liquids to make them feel more silky, more like down.
Yet down is still the best fill for sleeping bags, synthetics are not far behind. One major advantage of using synthetics is that, unlike down, they still retain their insulating properties even when wet and they dry more readily.
And because synthetics are a lot cheaper than down and a further improvement will take down into extinction in sleeping bags and even apparel.