Jim,

I have often thought that my old 600-fill TNF down sleeping bag was warmer than the 800+ fill WM replacement. Heavier, yes -- but about the same loft, and warmer. I wonder if there are more, smaller air spaces in the 600-fill down to collect and keep warm air.

Everyone says that LOFT = WARMTH. That 8 inches of loft is the same warmth no matter what. Well, here's a thought experiment: consider the amazing, yet rare, million-fill-power down. That means one ounce fills a million cubic inches. Great stuff -- a quarter of an ounce is enough for a winter bag lofting to 8 inches, which should be just as warm as my current winter bag. Well, that's crazy.

Somewhere between 550-fill, 800-fill, and 1,000,000-fill down, we run out of actual insulation. I don't know what that point might be -- it could be that we've already reached it, which is why the ultra-light super-high-fill down products don't feel as warm. But that's just a guess.

Oh, and my down jacket is an old Sierra Designs 600-fill with the stretchy baffles and lots of nice warm down. Easily warm into the teens over a long sleeve shirt. The only downside is the hood isn't insulated. It weighs a pound and a half, which I have no problem carrying in the winter.