With breathable Polartec Alpha insulation in the core and Power Wool (also by Polartec) in the sleeves and down the sides, this layer proved indispensable during our gear editor’s trek to Everest basecamp last April. “Over a tee, it was the perfect amount of core warmth in 30 to 50°F temps as we climbed up the Khumbu Valley,” she says. “Yet the stretchy Power Wool let my body heat escape when I worked up a sweat.” But she wished they skipped the Alpha in the back area: “When I was wearing a pack, that’s the first place that overheated.”
The silhouette is trim, but not sausage-tight; it pairs well with other layers. A high, stand-up, fleece-lined collar traps warmth.