There are two types of sleeping bags; down filled mummy bags and synthetic filled bags. The two are differentiated by the type of material that is used for the fill. The fill is the most important part of the sleeping bag. It’s what prevents you from losing heat. A good fill is one that traps air well, providing you protection from the cold outside air.
Down Filled Bags
Down is the most efficient fill for mummy bags, because it’s very light and traps air better than synthetic bags. This warmth for weight efficiency makes it ideal for hiking.
Down Filled Bags compress better than any available synthetics and so are less bulky when packed. When unpacked, a down sleeping bags goes back to its open state more readily and quickly. It has almost fluid like properties as it readily fills up the spaces of the sleeping bags.
Down usually comes 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 filled sleeping bags are marked as 600-fill, 700-fill or 800-fill. This is referred to as the fill power. Fill power is the amount of space that one ounce of down occupies.
The bigger 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 reportedly used his for more than 20 ages.
But down as a fill is not perfect. If you get it wet, it will stop keeping you warm and it’s almost impossible to dry on the field.
Down is also very expensive and demands rather more care in maintenance. In some cases, rare as it is, an allergic reaction may occur.
Synthetic Fill Bags
Synthetic fills have been around for ages. They come in are differentiatedent name like Polarguard, Thermolite Extra, Hollofil I & II and more. The names change and the producers will likely come up with more technologically advanced sounding names.
There are three forms of synthetics characterized by the length of fiber. All the three forms of synthetics can be lubricated with silicon or other liquids to make them feel more silky, more like down.
Even though down is still the best fill for sleeping bags, synthetics are not far behind. The advantage of synthetics is that, they can keep you warm even when wet and drying them is very easy.
And because synthetics are a lot cheaper than down and a further improvement will take down into extinction in sleeping bags and even apparel.
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