I've been amazed since I got my WM Ultralight Super at how the thing puffs up and puffs up and puffs up when it comes out of its stuff sack!

One one trip I got it rather damp and was really concerned about packing it up (i.e. compressing it) when damp. I need not have worried. Once I got it out in the sun, it quickly puffed up again to its original volume.

Just be sure to store it in its large cloth bag, and don't leave it in its stuff sack for several days after you return from a trip. In other words, preserve your investment and it will last for years.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey