Down insulation is natural. It comes from the lofty plumage underneath goose and duck wings. The benefit here is that down has traveled through Darwin’s survival of the fittest and developed into a very efficient insulator. It works by trapping body heat and reflecting it back to the source (you!). Premium goose down is the most expensive because it comes from mature geese, while the cheaper, but still effective, is duck down.
Down compresses very small and retains almost all of its loft, making it extremely packable. Don’t let its ridiculously light weight fool you; it’s weight to warmth ratio is as good as it gets. You’ll be sure to stay nice and toasty inside the bag despite the blizzard outside your tent. No man made insulation (synthetic) can match down’s superior and natural weight to warmth ratio.
Most people won’t buy down because of its steeper price. However, if treated with care, down insulated gear will last substantially much longer than synthetic. The natural plumage can be preserved when washed carefully. So even though you are paying more, you’ll more than likely get much more usage out of it.
‘Fill Power’ is a measurement that determines down’s insulating effectiveness. The fill power number refers to how many cubic inches one ounce of down will occupy. If you have a bag with a 900 fill power, it probably doesn’t weigh more than 1 ½ lbs, because it takes less down to fill the entire area.
Synthetic insulation is man's attempt to imitate downs natural abilities. It is not as efficient as down in its weight to warmth ratio, but it answers downs major pitfall: wet and humid conditions. Synthetic is able to keep you warm even when wet. This insulation is made from polyester threading that intertwines to work like down. When it gets wet, the moisture is trapped between the fibers, rather than in them, making it much easier to dry.
One major reason people choose synthetic over down is for the lower price. Synthetic does a good job of having down’s same qualities and leaving you with more money buried in the backyard. However, the lower priced synthetic does come with sacrifices. Synthetic will never be able to match down’s weight to warmth ratio, meaning your equipment will be heavier the warmer you want to be. Down is able to pack extremely small, while synthetic remains bulkier, making it more difficult for minimalist backpackers. The last downfall of synthetic; it won’t last as long as down. The synthetic fibers slowly break down, despite the loving care you put into preserving them.
WATER-RESISTANT DOWN (HYDROPHOBIC)
So by now it probably seems easier to make your decision between down or synthetic. But now there is a third option. Experts in the outdoor industry have developed water-resistant down. What used to be down’s major pitfall is now considered to be almost on an equal level compared to synthetic. Down plumage is light and almost completely defenseless against the power of moisture. Now the technology has been developed to allow each piece of down plumage to be coated with a water-resistant (known as hydrophobic) coating before it is stuffed into the jacket or sleeping bag. This type of attention in the manufacturing process does make hydrophobic down insulation come at a higher price. But it is able to retain its light weight and ability to loft even after being treated.