There are two types of sleeping bags; goose down sleeping bags and synthetic filled mummy bags. The two are distinguised by the material 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 with insulation against 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 are less bulky when packed. When unpacked, a goose down sleeping bags goes 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 both are good with minor differences.
Down sleeping bags are marked as 600-fill, 700-fill or 800-fill. This is referred to as the fill power. It is the amount of space that one oz of down occupies.
The higher the number of fill power results in less amount of down required to fill the same space which makes for a lighter sleeping bag. This is achieved 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 avid hiker I personally know used his for more than 20 quite some time.
But down as a fill is not perfect. You need to avoid getting it wet or it will stop keeping you warm and you’ll have better luck drying mud on a rainy day than drying a down sleeping bag on your trip.
Down is also very expensive and demands rather more care in maintenance. In some cases, rare as it is, may cause an allergic reaction.
Synthetic Filled Sleeping Bags
Synthetic fills have been around for quite some time. They come in are distinguisedent name like Polarguard, Thermolite Extra, Hollofil I & II and more. The names tend to change and the makers will probably come up with more technologically advanced sounding names.
Synthetic fills are categorized into three forms. All the three forms of synthetics can be lubricated with silicon or other liquids to make them feel more silky, more down-like.
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.
The biggest advantage, however is that they are a lot cheaper than down and a further improvement will take down into extinction in sleeping bags and even apparel.
