What Makes Down Nature's Best Insulator
03/01/2019
Down is an amazing insulator. You simply cannot get a warmer
insulator that packs well, keeps the weight low, and has tremendous
longevity. Whether used in outerwear, sleeping bags, or bedding,
down is the best at keeping you warm.
When talking about down one of the most important factors to
consider is fill power, and fill power is the most commonly
misunderstood purchase decision factor. Usually the manufacturer
will advertise a "fill power" of 650, 700, 850, etc. Fill
power is the measurement of loft that down provides. So, the
numbers that manufacturers use are a reference to the quality of
the down insulation used.
Fill power is measured by volume; the amount of cubic inches one
ounce of down occupies when compressed will determine the rating of
the garment. One ounce of down is placed in a graduated cylinder,
then a weighted disk is placed on top of the down. Once the disk
settles, the compressed volume, measured in cubic inches, is the
fill power. For example, one ounce of 850 fill power down will
occupy 850 cubic inches when compressed.
Higher fill down is higher quality because it is loftier. A fill
of 850 down will be warmer for its weight than 650 fill down. If
two jackets have the same amount of down, but one has a higher fill
power, such as 850 versus 650, the jacket with 850 fill down will
be loftier, and thus warmer. That is why high-end light down
jackets usually have higher fill power down, it allows the jacket
to be very warm but very light. A common misconception is that a
jacket with a high fill power number is, therefore, warmer than a
jacket with a lower fill power, which comes down to the fill weight
as well as the fill power. When comparing down jackets, the jacket
with the most loft is going to be the warmest regardless of fill
power.
