First off, I absolutely love this jacket. I spent hours doing extensive research on down jackets, I bought 3 different styles (this, a patagonia, and a marmot), and this is what I found:
One of the greatest misconceptions about down jackets is that "fill power" (800-fill, for example) is the most important spec to consider-- but if you're only considering fill power you're missing half the story! A down jacket's warmth is actually derived from a variety of factors: fill power (or loft), fill weight, and wind resistance.
Fill power relates to down "loft" or "fluffiness", and has to do with feather size- larger feathers means better loft and more air an ounce of the down can trap; increasing warmth and contributing to a lighter and more compressible jacket, but not all 800 fill jackets are made equal...
Even before you try them on, if you lay two 800-fill jackets side-by-side and you can immediately tell which jacket is going to be warmer: it's the one that's thicker! It's common sense. The more insulation shoved into the jacket, the warmer it's going to be. The problem is, browsing online you can't put products side-by-side and squeeze them. You have to look at pictures, and read numerical stats. Unfortunately fill weight isn't always publicized (I had to actually look at the physical tags for some of these to get their fill weight numbers). So let's take a look:
Fill Weight (Primary Determinant of Warmth):
- MontBell UL Inner = 2 oz
- Patagonia Down Sweater = 3 oz
- Marmot Zeus = 3.5 oz
- MontBell Light Down (this jaceket) = 4.3 oz
The numbers don't lie: this jacket has the best fill weight out of all that I considered. No wonder it's by far the warmest!
As for the third factor, wind resistance, the 30 denier ballistic ripstop nylon nylon shell is the strongest, lightest, most wind resistant shell fabric made.
So there you have it! I hope my legwork helps you make a fully-informed purchase decision... to get this jacket!!