I’ll admit it, before working in the outdoor industry I had no idea what the little fill power numbers were embroidered on the sleeve of a jacket . It was one of those examples of “you don’t know, what you don’t know” I thought I knew so it never occurred to me to ask the question—the higher the number the puffier and warmer the jacket right? Well, not always.
There are a couple different factors in determining how warm a down jacket is, the quality of down used aka the fill power, the amount of down used aka how puffy a jacket is, and the fabrics it is sandwiched in-between as well as the construction of the garment.
Let’s talk about the fill power number first, it actually refers to the quality of down—meaning how fluffy the actual piece is. For example, 800 quality down is a fluffier individual down piece resulting in the ability to use less fill to make a garment warmer. In each of these test tubes, the weight of down is the same, but as you can see the 900 quality down takes up more space. The way all insulation works is by trapping air—your own body does the work of warming up the spaces in-between the down and the down fibers trap the air keeping your body warm. The higher the fill power, the less down you need to create those pockets of air to be trapped.
The next factor in determining the warmth of a garment is the amount of down used. The test tube with 900 fill is going to be the warmest, but if you tripled the amount of 550 down it would fill up the most space and thus, be the warmest. What you gain with 900 fill power vs. 550 fill power is warmth-to-weight, meaning you sacrifice the weight of the garment for more warmth with a lesser fill power. The best warmth-to-weight is 900 fill power, but determining what garment is warmer depends on both the fill power and actual amount of insulation in a garment.
The last factor of warmth in a garment are the fabrics and construction of the garment. Less stitch lines = less cold spots so an overall warmer garment. The more wind proof a fabric, the higher likely hood that the air will stay trapped in the garment keeping you warm as well. Most of the time the concessions of making a garment depend on weight, waterproof fabrics are usually heavier so although they are warmer, they will add more weight to the garment.
There is a lot to explore in the world of insulation! How ethical is the down being sourced (all of the down Merrell uses is certified RDS—meaning it’s from a food source)? How does synthetic insulation fit into the warmth conversation? What about duck insulation? Why aren’t garments usually temperature rated? But hopefully this helped you understand the differences in what to look for in down insulation specifically.