Learning more about the "how" of down insulation helps you make smart shopping decisions, like why you might choose down over synthetic insulation. And, quite simply, it's just more fun to know how things work.
What is Down? The Biology of Feathers
Most basic Biology classes don't spend much time on the details of feather function, structure and evolution. So, I needed to do some major digging to familiarize myself with the biological reasons for down being unequivocally the best natural insulator available.
Right away, I learned that down is the fluffy stuff under the feathers of waterfowl like geese and ducks. It helps birds regulate their body temperature and wards off the loss of body heat. After a little more research, I found out that down is a type of feather called "Plumulaceous" -- which is different from the stiff, quilled feathers, or "Pennaceous" feathers, that cover most birds. Instead of being smooth and straight like Pennaceous feathers, down feathers are soft and fluffy tufts, or clusters.
Pennaceous feathers have a central shaft, or Rachis, that runs up and down the length of the feather. Plumulaceous down feathers do not have a Rachis, making it more of a tuft of feathery filaments than what most would think of as a "feather." This unique structure allows for the compressible, breathable and heat-trapping performance that down insulation is known for.
Why is Down Nature's Best Insulator?
The high loft, or fluffiness of down, is what lends it its warmth-capturing power. The natural loft and heat-trapping capability of down is created by the multiple loose, curving strands that form a single tuft of down. This multi-filament structure produces air pockets that trap body heat and effectively protect birds (and humans) against heat loss without adding excess weight. The trapped air in the down under waterfowl's feathers also aids their buoyancy on the water.
When down is used to insulate winter jackets, vests and sleeping bags, it works exactly the same way it does under a bird's feathers; and when used in a high quantity, down has the added perk of compressible performance. The same air pockets that allow for breathable, heat-trapping and lightweight warmth are what allow down insulation to pack down small -- making down-insulated sleeping bags and jackets great for backpacking and backcountry camping.
Where Does Down Come From?
Down is an animal product, so it is not vegan friendly, but the inhumane way of plucking down from live birds is becoming more and more rare. "Live pluck" operations are illegal in the USA and most European countries, and the majority of down insulation on the market today is a by-product of the food industry. Marmot, for instance, is known for its superior down-insulated sleeping bags and jackets and uses down from geese that have been harvested for food. They require all of their down vendors to comply with their animal rights policy. If you're concerned about where the down insulation in a jacket comes from, take a look at the vendor's website. With a little bit of digging, you'll probably be able to find information about their down sources and/or animal rights policy.
How does Fill Power Work?
Finally, we come to the question of fill power, which is quite possibly the most important aspect to understand about down insulation when you're in the hunt for down-insulated gear.
Fill power is essentially a way to rank the quality of down used in your down-insulated gear. The higher the fill power (900 fill power is the highest), the higher the quality of down. Fill power is measured by the number of cubic inches one ounce of down will occupy. The loftier the down, the higher the fill power, and the fewer ounces of down needed to provide the same amount of warmth. So, an 800 fill power down jacket will weigh less and be less bulky than a 500 fill power down jacket, but provide the same amount of warmth.
Down fill power is further explained in the video below, as well as in our down vs. synthetic buying guide.
That wraps up this installment of Sierra Trading Post Explores. I hope you learned a thing or two. Join me next month for another look into the lesser known aspects of your outdoor adventures and gear!