U-shaped baffles curve across the top of the bag, so the walls catch the down as you move side to side. This makes the feathers less likely to migrate to one end of the baffle, creating cold spots. Short-staple synthetic insulation on the bottom is more resistant to compression than down, so it stays loftier—and warmer.
The DWR-treated nylon shell kept insulation dry in damp conditions. After our tester had his entire kit soaked while crossing a rainy pass on the PCT, he was surprised to find the inside of his bag still dry. Note: One tester found the temp rating generous and recommended sleeping in a puffy in near-freezing temps. Want more warmth? Pick up the 20°F Ion for another $50 and 3.5 ounces.
Lightweight materials (a 12-denier shell; 850-fill, hydrophobic goose down) and design (a ¾-length zipper) put this bag at the lighter end of its class, even with the synthetic fill.
A 10-inch zipper on the opposite shoulder from the main opening allows you to flip open the top of the bag for more breathing room in mild temps. However, the ¾-length main zip prevents you from venting your feet.