Sea to Summit Alpine -40°F

Warmest sleeping bag

Brand: Sea to Summit

Model: Alpine -40°F

Our take On the coldest nights, this is the bag we want. Every detail is about conserving heat, including the nearly 3 pounds of 850-fill down and the 4-inch thick draft tube and collar. The Alpine has a nylon shell with DWR and an interior PU coating, which in tandem proved impervious to the snow and frost accumulating inside our tent on Denali. A mix of vertical and horizontal baffles blend weight savings with down distribution (the vertical baffles keep more down above your core).

The details Our crew gave the Alpine high marks for livability. The roomy expedition cut (a whopping 63 inches around at the shoulders) means you can comfortably wear a puffy while inside the bag, and the 52-inch-circumference footbox leaves room for gear you’d like to keep warm. “It was even big enough that I could change clothes inside,” one tester says. The Alpine isn’t light or cheap, and it only compresses to the size of a basketball, but if you’re sleeping this cold you’re probably not cutting corners.

Trail cred “My favorite detail is the second zipper opposite the main one, which allowed me to flip down the top half of the bag while I was holed up in the tent,” one tester says. “It let me sit upright and write in my journal without leaving the bag.”

4 lbs. 2 oz.; -40°F

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