Down Insulation seems to be a really confusing topic. I routinely hear people refer to down insulation and its properties incorrectly. This article aims to end this confusion.
Down: the Story
The down of a bird is a layer of fine feathers and clusters underneath the tougher exterior feathers. Down used for insulation is made up of a combination of down feathers and clusters. The down feather has a short or vestigial rachis (shaft), few barbs, and barbules that lack hooks; it is the simplest of all feather types. Down clusters are tiny, fine, fluffy feathers from the birds’ undercoat, which has no quill shaft. Higher quality down has a higher ratio of clusters to feathers. A product is required to contain at least 75 percent down clusters to be labeled as down.
Down insulation has been used for insulation since the early 1600’s. Down has excellent thermal properties and offers good lofting characteristics. Almost all down commercially available is a secondary product of geese raised for consumption. It would be prohibitively expensive to raise geese for just down alone. The geese that are the source for lower quality down are about 4 months old when they are “harvested” for food. Down from these geese can be carefully sorted, washed, and blended, but will never loft like really mature down. The 700+ down fill comes from more mature birds that are kept for breeding purposes. A small number of these are raised and they molt (shed) naturally in the spring. While their down is loose it is then collected by hand. This down is very rare and therefore expensive. The larger individual clusters from mature down are what gives the greater loft. The only way to get down of this quality is by careful hand selection which is the major factor in its scarcity.
The Measurement of Down: Fill Power
Down insulation is rated by fill power, which is the number of cubic inches displaced by a given ounce of down (in3/oz). To measure fill power an ounce of down is put into a graduated cylinder, and a small weight is placed on top, the volume below the weight is the fill power. The test requires controlled temperature, humidity, and preparation of the sample. A fill power of 400-450 is considered medium quality, 450-550 is considered good quality, 550-750 is considered very good, and 750+ is considered excellent. In the outdoor industry you do not see fill powers below 550 very often.
The higher the fill power the higher the loft and therefore the more insulating air pockets the down has and the better the insulating ability. It provides manufacturers the ability to use less down to create the same amount of loft and therefore a lighter, more compressible, and warmer piece as compared to a lower fill piece.
The loose structure of down feathers traps air, which helps insulate the user. In the same way that other insulating fabrics work, these air pockets create a barrier between the wearer and the environment. Therefore warmth is a combination of the amount of air pockets which is determined by the loft of the down and the thickness or amount of loft. At the same thickness, higher rated down will have more air pockets and will be warmer. But fill power alone does not determine the warmth of down insulation, which is the most often misunderstood characteristic of down. A thicker layer of down provides more warmth than a thinner layer regardless of the fill power.
Loft & Compression
The loft of down directly impacts the compressibility of the down. The more air pockets the down possesses the more it is able to compress. Therefore the higher the fill power rating the more compressible the down is as well. Down can compress much smaller than its synthetic competition. Also, the higher the quality the down the more resilient it is to being compressed meaning it will retain loft longer.
Down is more durable than synthetic insulation. The more mature the down the larger the clusters and the more resilient they are. These larger clusters can withstand more compressive cycles than less mature down can. With proper care, down can last much longer than synthetic pieces.
The fill power of down tends to diminish as down gets dirty and mats. The fill power of a down piece can be restored by washing it in a washing machine with a mild or no detergent. It then can be dried in a dryer and it is good to add tennis balls to help restore the fluff to the piece. An alternative method to washing is putting the down piece in the dryer with a damp rag for 10-20 minutes. The idea is to open up the down clusters with warm air and moisture. As with all items I would suggest reading the manufacturers recommendation for care as they may differ from manufacturer to manufacturer.
Drawback of Down Insulation
Down insulation can indirectly cause allergies if not taken care of. The same air pockets that insulate the body can also accumulate dust so cleanings are important to remove this dust as well.
The main drawback of down is that it loses all insulating properties when wet. It also takes a long time to dry. With these negatives considered, down is best for dryer environments and moderate to low activity levels. Keeping down pieces dry is of grand importance while out in the wilderness.