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There's many different kinds of "down" filling: "100% down" must legally be only down, while "down" can be any mixture containing down.

Then there's "fill power" ranging from about 400 to 1200, which basically measures how resistent the feathers are to crushing by weight. Higher "fill power" provides more insulation and is significantly more expensive.

Is there any quantitative data about just how much the thermal insulation ability VARIES from the "worst" down to the "best" down? In other words, is it possible that spending 10X as much $$$ on high-end down can make a significant difference in insulation, or is it negligible?

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2 Answers 2

Fill power doesn't measure crush resistance. It measures the inverse of density, in units of cubic centimeters per gram or cubic inches per ounce.

Insulation doesn't depend on fill power. Insulation simply depends on thickness. However, a higher fill power allows you to achieve a greater thickness while carrying a given weight on your back.

Fill power is measured when the down is puffed up, not compressed. When it's compressed, it gives very little insulation. For this reason, the part of your sleeping bag underneath you is extremely ineffective at keeping you warm. This is why people use sleeping pads, and some people use down blankets instead of sleeping bags.

Because insulation depends so strongly on the amount of compression, it's not possible to say in general that x centimeters of (uncompressed) down will provide y amount of insulation. For example, a down jacket will not be compressed as much as the bottom of a down sleeping bag in normal use.

It also depends on the wind. In a strong wind, there is a certain amount of difference in comfort between a thick layer of down and no layer of down. In totally still air, this difference is much smaller. Essentially all the down does is to provide you with a still layer of air, preventing convection. If the air is completely still anyway, you don't need the down for this.

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The primary advantage of higher fill power down is that it is warmer per ounce (or gram, if that's your thing) than down of a lower fill power.

If you had two nearly identical sleeping bags: same shell material, same weight of down, but one had 850 fill power down and the other had 700, the 850 fill power would be about 3˚F warmer.

More insight into this and backing for my results can be seen at Down Fill Power Impact on Quilt Temp Rating on backpackinglight.com.

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