In general there are two types of sleeping bags; down filled mummy bags and synthetic fill bags. These differ by the type of material that is used for the fill. The fill is the most critical part of the sleeping bag. It’s what prevents you from losing heat. The fill needs to be able to trap air very well, providing you with insulation against the cold outside air.

Goose Down Sleeping Bags

Down is the most efficient fill for sleeping bags, warmth for weight, and traps air better than synthetic bags. This warmth for weight efficiency makes it ideal for mountaineering.

Goose Down Sleeping Bags compress better than any other known synthetics and so are less bulky when packed. When opened up, a down mummy bags goes back to its open state very quickly and more completely. It has almost fluid like properties as it readily fills up the void compartments of the sleeping bags.

Good quality down originates from China. A debate has been made in the past whether white goose down are better than gray goose down but there’s really not much difference.

Down sleeping bags are marked as 600-fill, 700-fill or 800-fill. This is called the fill power. It is the number of cubic inches that 1 ounce of down will occupy.

The higher the number of fill power means that for the same amount of space, less amount of down is used which makes for a lighter sleeping bag. 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 ages.

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 a rather costly material and requires much more care in maintenance. In some cases, unusual as it is, an allergic reaction may occur.

Synthetic Filled Bags

Synthetic fills have been around for ages. They come in different name like Polarguard, Thermolite Micro, Hollofil I & II and more. The names frequenly change and the manufacturers will probably 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 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.