February 23, 2001


How do goose-down bag ratings work?

Please explain goose down bag ratings. I have a Foxfire from The North Face that is 800-fill and rated from five degrees to 20 degrees. My friend has a Marmot Never Summer that is 600-fill and rated to 0 degrees. However, when you put the bags next to each other, the Foxfire has greater loft. Both bags have been used close to their lowest temperature ratings and were warm. Shouldn't the bag with the greater loft have a lower temperature rating? Will the Foxfire actually go lower than its rating? I'm confused.

— Laura Mac Texas

Understandable. You're exactly right —- the loft of a bag is good indicator of its warmth, and a bag with nine inches of loft will be warmer than one with five inches. What I suspect you're seeing is simply the greater durability of 800-fill down, which comes from more mature geese and is typically better able to withstand repeated stuffings than 600-fill. Without knowing the history of the two bags, my guess is that the Foxfire is newer or has been better cared-for.

In any event, rating bags is a wildly unscientific process. Even if there were a foolproof way to test two bags to see which was really "warmer," you'd still be stuck with all the human variables. That's why The North Face rates their bag across a range (Nowadays, Marmot does as well). My guess is that in reality both bags, when new, were pretty comparable. And if you're a warm sleeper, you could easily push the Foxfire to sub-zero temps by dressing warmly, wearing a warm hat and perhaps using a vapor barrier liner.

As an aside, what "fill ratings" measure is the ability of one ounce of down to fill a predetermined amount of space. Down rated to 600 can fill 600 cubic inches; down rated to 800 (rare stuff) can fill 800. Most high-end downs fall in the 700-750 range, as I think most bag makers have concluded that 800-fill down is a bit of a fluke. For bags rated to 0 or warmer, the type of down makes has little effect on the weight -— it's the shell and the zipper that contributes most to the weight. But higher-fill down, as I mentioned earlier, tends to be a bit more durable, so sometimes is worth the extra money.