For what it's worth, many people consider Western Mountaineering's ratings to be conservative and much more accurate than other manufacturers. From personal experience, I can tell you that I have pushed the limits of my Summerlite and have stayed warm. This isn't any scientific input but out of all the manufacturers out there, WM is one of the ones I would trust the most. smile
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Gear Talk There's no such thing as having too many sporks!

Backpack Flyfishing Tight lines,light packs