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 Down is heralded as the lightest weight, most compressible insulation on earth (you can learn more about Insulation Comparisons here). But not all down fits that bill. Understanding down quality is key to helping you get the performance you need out of your down gear






What is Down
The down insulation in your sleeping bag or jacket began as the insulation on a goose or duck.  Waterfowl have down clusters under their waterproof feathers to keep them warm in the cold, wet conditions in which they live. Unlike feathers, which have a stiff shaft with barbs sticking out on either side, down clusters have a soft, fine stalk crowned with a puff of very fine fibers at the top.  High quality down is so light that if you closed your eyes and someone dropped a pile of it into your hand, you wouldn’t even feel it - until it started to get warm





Low quality down, like that found in some comforters and pillows, is usually mixed with a certain percentage of feathers.  It is difficult to separate down from feathers completely, but most of the down used in outdoor gear is of sufficient quality that there are very few feathers to be found






 What is this Fill Power number
Even within the range of high quality down, there is quality variation. This has to do with the type and age of the animals the down comes from. Duck down tends to be lower quality than goose down, with the exception of the down from the arctic eider duck, which is very high quality. In general, older animals tend to produce bigger, loftier clusters, which make for higher quality down.  Hungarian goose down is generally seen as the best source of fine down. Down quality is measured in Fill Power

The goal of insulation is to trap the maximum amount of air (for warmth) with the minimum amount of material (for lightweight and packability). The Fill Power measurement is a direct assessment of this efficiency. Fill power is the volume in cubic inches that one ounce of down can fill up. So, if the fill power of the down in your jacket is 600, it means that one ounce of that down would fill 600 cubic inches. The same weight of 800 fill-power down fills up 800 cubic inches, creating 33% more loft. The official tests are done under lab conditions, with a particular protocol to ensure reliable results. For outdoor gear, lower-end fill power ratings start at 500-550, and the top end possible (this is open for debate) is around 900. Anything over 750 fill power can be considered expedition-ready, top-quality down

Is it Worth the Extra Money
If performance really matters, it is. A 750 fill power sleeping bag will be about 15% lighter than a 650 fill bag of the same temperature rating. It will also pack up smaller. Moving up to even higher qualities of down will increase the benefit. High quality down is also easier to pack up and tends to last longer than lower quality varieties

SOURCE : http://www.shop-denali.com/down_fill_power.aspx