After several years of taking a back seat to versatile synthetic insulations, down has been recently making a strong comeback into the hearts and packs of weekend adventurers. Nature's ultimate insulation hasn't changed one bit, so why the sudden resurgence in popularity? The simple answer is that when it comes to maximum warmth with minimal weight, nothing is more efficient than down. And as more backpackers embrace the concept of fast-and-light hiking and more weekend hikers take on multiday adventures, the need to save weight is driving more and more explorers to the power of down.
What are its uses?
Lightweight, compressible, and extremely warm, down is used as an insulator mostly in jackets and sleeping bags.
How does it work?
Look at geese flying in formation. It's probably pretty cold up there, and of course the wind chill must be fierce, yet those birds are perfectly comfortable. That's because between the bird's body and thick outer feathers is a layer of fluffy down that traps air molecules in small pockets, creating an effective thermal barrier. And we're talking really small pockets of air created by an amazing cluster of tiny hairs—up to 2 million filaments for every ounce of down—that crisscross every which way. It's a superwarm system that we've yet to duplicate in synthetic insulations.
What is "fill power"?
The 3D structure of the down clusters creates "loft" that traps air. The greater the loft, the warmer the insulation. "Fill power" is a measurement of that loft. This is accomplished by measuring how many cubic inches an ounce of down displaces when allowed to expand to its fullest. If an ounce of down takes up 500 cubic inches of space, then it has a 500 fill power; 700 cubic inches equals 700 fill power; and so on.
So, will a 20° sleeping bag with 700 fill be warmer than a 20° bag with 500 fill? Well, not exactly. They're both rated to 20°, but the 700-fill bag will have used less down insulation to get there. So, with all other things being equal, that bag will be lighter than the one with 500 fill.
What are the different types of down?
Goose and duck down are the major kinds. Goose down is loftier (higher fill power) than duck down, but also more expensive. Goose down is the preferred choice for people looking for lightweight products.
When researching a sleeping bag or down jacket, you might also come across the term "Eastern European goose down." This insulation is known for its extremely large down clusters that create very high loft, sometimes as much as 800 fill.
What are the advantages of down insulation?
Compressibility: Compared to synthetic insulations, down is typically more compressible, meaning that it can be packed into a very small space. This is great if you need to stow your down jacket or sleeping bag in a backpack.
Warmth-to-Weight Ratio: Ounce for ounce, goose down is warmer than most synthetic insulations.
Longevity: Down is very resilient and will retain its loft and ability to insulate for a lifetime if you care for it properly. Of all the reasons to choose down, this might be the strongest one.
What are the disadvantages of down insulation?
Loft Factor: Down is a very poor insulator when compressed, which is why you absolutely need a good sleeping pad underneath.
Performance When Wet: Down provides zero insulation when wet. Try sleeping in a wet down bag, and you'll have a miserable night. Having said that, the natural oils in goose down do provide some water resistance. After all, geese and ducks swim in near-freezing water all the time, and they do okay. As long as you take care to keep the bag dry, the occasional rainy night shouldn't be a problem. However, if you know that it's going to rain continuously for days, synthetic insulation that keeps you warm even if wet is a safer choice.
Price: High-loft down is usually more expensive than synthetic insulation, though if you take into account down's longer life span, it actually makes more economic sense over the long term.
What else should I consider?
Whether it's for a garment or sleeping bag, the shell fabric needs to be "downproof." Down feathers can poke through fabric unless it's tightly woven. Take a look at the shell to make sure the down isn't leaking.
Also, the shell fabric should feature a DWR (durable water repellent) coating or in some other way protect the insulation from moisture.