Is there such a thing as a听midlayer that鈥檚 as warm as a puffy听but breathable enough to keep you听comfortable听even in heated indoor environments听or during听high-exertion activities? Until I started wearing the ($250), I鈥檇 have told you no. But now I barely take the thing off.
With a body-fabric mix of 30 percent yak wool and听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听what makes it so comfortable: A听wool fiber is scaly听and hollow. Water vapor expelled听by the wearer鈥檚 body can fit between those scales听and is absorbed by the hollow interior. There, a chemical process breaks the bond between water鈥檚 hydrogen and oxygen molecules, forming heat. In cold weather, the heat produced by this process, along with the wearer鈥檚 body heat, is trapped in pockets created by the natural chaos of wool鈥檚 kinks and bends. In warm weather, wool draws moisture away from your body听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听stays warm when it鈥檚 cold out,听and, accordingly, keeps you cool when it鈥檚 hot out. Merino wool is made from a variety of sheep known for its fine, soft fibers. The听fibers work just like normal wool听but are less itchy next to your skin.
Yak wool is even finer and softer than merino (if rarer听and more expensive as a result), and Kora has conducted its own听 that suggest the material to be 40 percent warmer, 66 percent more breathable, and 17 percent better at moving water away from your skin than merino. The company purchases its yak wool directly from producers in Tibet.
The third component of the Xenolith, Polartec Alpha, was developed for Special Forces soldiers deployed to the mountains of Afghanistan in the early 2000s. Designed as a more breathable alternative to high-loft polyester fleece, it鈥檚 made from a mesh chassis that holds together a loose collection of polyester fibers. In static conditions, those lofted fibers trap a lot of air, providing a lot of insulation. Get moving, and the barely there fibers present听virtually no obstacle to the air pressure generated by your increasing temperature. Alpha also wicks moisture听and spreads it out for quicker drying times.
Alone, a midlayer made from any single one of these fabrics will add a surprising amount of warmth听and remain comfortable across a wide range of temperatures and activities. Together, they create a sweater capable of providing outstanding insulation that stays听comfortable when things heat up.
A xenolith, which the sweater is named after, is 鈥攁听rock sandwich, if you will. Kora鈥檚听Xenolith is made from a thin outer layer of tightly woven 30 percent yak wool听and 70 percent merino wool. Beneath this is听a layer of Alpha that covers your entire torso, your shoulders, and the outer side of your upper arms听but doesn鈥檛 cover your armpits, your inner upper arms, or your forearms. On in the inside, there鈥檚听another layer of that wool around your arms and the front of your torso听but not your听back.
The end result听is practical and versatile on its听own, but something that will help听you get more out of your other layers, too. Because the tightly woven wool fabric serves as a rudimentary shell, blocking some wind, the doubled-up front of the sweater听keeps you cozy while cycling or skiing听without preventing your back from losing听heat. This arrangement also听keeps your back dry while wearing a pack. Thin听and slim fitting, it鈥檚 easy to wear under additional pieces听and perfect听under a hard shell, reducing bulk and making the most of the limited air permeability offered by waterproof jackets by providing no other barrier to breathability.
While bird hunting last month in South Dakota,听conditions were positively dreadful. Single-digit temperatures combined with high winds and light precipitation to create bone-chillingly damp weather that instantly stung any exposed skin with blown ice particles. Conditions听were so bad that I lost feeling in one of my fingers, which has yet to听recover.听But听layered over a wool base layer听and beneath听a lightweight puffy and hard shell, the Xenolith enabled me to hike around all day in otherwise听complete comfort.
One night in the remote backcountry of southwest Montana last fall, just outside Yellowstone National Park, temperatures plummeted unexpectedly into what also felt like the single digits. I鈥檇 only packed a 20-degree sleeping bag听but was able to sleep soundly in it wearing听nothing but my base layers and this sweater.
Over the holidays, while my wife and I were hiking around our cabin in northern Montana, just outside Glacier National Park, temperatures ranged from the low teens to the mid-forties. For the entire trip, all I wore were听base layers, this sweater, and a hard shell.
Right now听I鈥檓 sitting in front of the fireplace at home in Bozeman, Montana, wearing the Xenolith over a thin merino T-shirt. The thermostat tells me it鈥檚 69 degrees in my living room, and I鈥檓 as comfortable now as I will be when I take the dogs for a walk after I wrap up this article. It鈥檚 20 degrees outside today, and all I鈥檒l need to do is put on shoes.