Is there such a thing as a midlayer that’s as warm as a puffy but breathable enough to keep you comfortable even in heated indoor environments or during high-exertion activities? Until I started wearing the Kora Xenolith sweater ($250), I’d have told you no. But now I barely take the thing off.
With a body-fabric mix of 30 percent yak wool and 70 percent merino fibers, and lined with Polartec Alpha insulation, the Xenolith is made from three different materials, each known for warmth and breathability.
Outdoorsy folks will already be familiar with merino wool, but you may not know exactly what makes it so comfortable: A wool fiber is scaly and hollow. Water vapor expelled by the wearer’s body can fit between those scales and is absorbed by the hollow interior. There, a chemical process breaks the bond between water’s hydrogen and oxygen molecules, forming heat. In cold weather, the heat produced by this process, along with the wearer’s body heat, is trapped in pockets created by the natural chaos of wool’s kinks and bends. In warm weather, wool draws moisture away from your body and facilitates evaporation by spreading that moisture out across a larger surface area. The cool air produced by that evaporation is then trapped in the fabric. Wool keeps you dry and stays warm when it’s cold out, and, accordingly, keeps you cool when it’s hot out. Merino wool is made from a variety of sheep known for its fine, soft fibers. The fibers work just like normal wool but are less itchy next to your skin.