How to Choose Your Insulation
There are two primary kinds of insulation: down (usually made from a duck or goose’s plumage, or underfeathers) and synthetic fibers. Down is lightweight, breathable, easy to compress, and excels in cold, dry conditions. Once down gets wet, however, it provides almost zero warmth and can take a very long time to dry—which can be a liability if you’re planning a trip where wet weather is a concern. Synthetic insulation insulates when wet, dries very quickly, is hypoallergenic, and is often cheaper. But it’s also heavier, less durable, and more bulky.
When you’re looking at down bags, the first step is to understand the concept of fill power. You’ve probably seen it on product labels: 700-fill down, 800-fill down, etc. But do you know what those numbers actually mean?
There’s a common misconception that fill power is the amount of down in a bag, but it’s actually a reference to the loft, or fluffiness—and therefore the quality—of the down that is used as insulation in the sleeping bag or other garment. If you take one ounce of 700-fill down, it will hypothetically take up 700 cubic inches; one ounce of 800-fill down will take up 800 cubic inches, etc. Higher-grade down (which is usually made from more mature birds) is more expensive, but it will trap more air next to your body—and the better your bag’s warmth-to-weight ratio will be.
Synthetic insulation is usually made of polyester. Most bags use one of two technologies: short-staple fills or continuous-filament fills.
Short-staple fills use short strands of thin filaments that are densely packed, which makes sleeping bags flexible, soft, and compressible—though not quite as compressible as a down bag of similar warmth. Continuous-filament insulation uses longer, thicker filaments that are less compressible than short-staple insulation, but more durable. All synthetic bags dry relatively quickly, and most are significantly less expensive than down. Most importantly, they’ll still insulate when wet.
Most people know that down is ideal in cold, dry climates, where synthetic insulation performs better in wet environments. But there are other considerations, too. Down is compressible, where synthetics are usually bulkier. Down is more breathable, but synthetic sleeping bags are easier to wash. And weight is a huge factor: if you’re planning a through-hike of the PCT, those extra ounces matter much more than if you’re car camping.
For more help, ask an expert—and remember, no matter what kind of insulation you choose, always store your sleeping bags uncompressed, which maintains loft and warmth.