Following my recent post on 10 Ways to Keep Warm While Camping, a friend asked me about the best clothing to wear when it’s cold.  The answer to that question is technical, and you should see my article on Layering for more detailed information.  We ended up talking about down jackets.  I told her that the best down jackets are thick, have a hood, fall low on the hips, and have a high fill power.  But what is fill power?

What is Down?

Down is a layer of extremely fine feathers that grow on aquatic birds like geese or ducks. It forms in clusters and adds volume under the outer layer of plumage on these birds.  Its purpose is to keep the birds warm.  It is used by humans fill blankets, jackets, sleeping bags and pillows.

How Does Down Keep you Warm?

It doesn’t.  Down is used to puff up garments and give them loft.  It separates the outer and inner layer of a jacket or sleeping bag so that the inside will fill with air.  Air trapped in down keeps you warm because air is a very poor conductor of heat energy.  Down also prevents air from circulating through the garment.  If the air could circulate, you would lose heat by convection.  For more information on convection and conduction, please see my post, The Science of Cold.

What is Fill Power?

Fill power is a unit of measure for the density of down.  It measures volume vs. weight.  The volume of down is measured in cubic inches.  Weight is measured in ounces.  One ounce of 900 fill power down will take up 900 cubic inches when uncompressed.  One ounce of 450 fill power down will only take up half as much space.

Why Does Fill Power Matter?

Down is used to trap air within sleeping bags, jackets, and other such devices.  Let’s imagine a small blanket designed for use in 60 degree weather.  Such a blanket would have to be 1 inch thick to give the proper thermal insulation.  If our blanket was 30 inches by 30 inches, it would have a volume of 900 cubic inches.  If you were to fill it with 900 fill power down, the down would weigh only 1 ounce.  It would take 2 ounces of 450 fill power down to fill the same blanket.

But what’s an ounce?  Well, our blanket is very small, probably only large enough for a baby.  It is also only thick enough for use in warm weather.  Imagine now you were making a sleeping bag for use at 20 degrees.  Such a sleeping bag would be twice as thick and would need much more down.  The weight saved by using higher fill power down would be considerable.

But it isn’t just the weight! When you squeeze the air out of a down, it compresses very small. How small it will compress depends on its mass.  Mass is the amount of matter in a material.  Mass is also measured by weight!  900 fill power down has half as much mass as 450 fill power down and will pack down to half the size.  When you’re trying to pack as much warmth into the smallest space possible, compressibility really matters.

Any Questions?

This is a tricky subject.  I’d be more than happy to answer any other questions you might have.  Please leave a reply below!