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The insulation or "fill" inside a sleeping bag largely determines a sleeping bag's:

• Weight (and thus its "warmth-for-weight" ratio)
• Compressibility
• Durability

 

Down

Down is the wispy, fluffy undercoating found just beneath the outer feathers of geese and ducks. This natural fiber is an extraordinary insulator. Goose down is preferred to down from ducks, prized because it is believed its plumes offer a higher "fillpower" (explained below).

Down's positives include:

 

• It offers tremendous warmth for surprisingly little weight (thus offering a superior "warmth-to-weight" ratio).
• It can be compacted into very small sizes.
• Its effectiveness outperforms synthetic insulation by years—even decades.

 

Down, though, does have a downside:

 

• If it gets wet, it is of no value until it dries—and in the field, that can take a long time.
• It is more expensive (keep in mind, though, that its resistance to deterioration makes it an outstanding long-term value).
• Down is graded according to fill power—meaning the number of cubic inches one ounce of down will displace. The higher the number, the better the insulation.

 

Synthetic Materials 

 

Synthetic materials are basically plastic threads (extruded polymers, to be technical). The threads are most commonly a continuous filament (a long, single strand). They can also be arranged in short "staples" up to four inches long. Usually the threads are hollow, reducing their weight and enabling them to trap more air.

 

The advantages of synthetic fill include:

 

• It still provides some insulation when wet; plus it dries fairly quickly. It's less expensive than down. It's non-allergenic. The shortcomings of synthetic fill are:
  • It's bulkier than down (so it takes up more space when you're carrying it).
  • It's heavier (it takes more weight to get the same warmth down provides).
  • The filaments gradually degrade over time.
  • The insulating "batts" of filaments are stiffer than down and do not drape over the contours of your body as effectively.

   

  From REI.com

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