Is there such a thing as a mid-layer that is as warm as a pouf, but breathable enough to be comfortable in heated interiors or during high-exertion activities? By the time I started wearing the Kora Xenolith sweater ($ 250) I would have told you no. But now I hardly ever take it off.
With a body-fabric mixture of 30 percent yak wool and 70 percent merino fibers and Polartec Alpha insulation, the Xenolith consists of three different materials, each known for its warmth and breathability.
People outdoors already know merino wool, but they may not know exactly what makes it so comfortable: a wool fiber is flaky and hollow. Water vapor expelled from the wearer’s body can pass between these scales and is absorbed by the hollow interior. There a chemical process breaks the bond between the hydrogen and oxygen molecules in the water and creates heat. In cold weather, the heat generated by this process is trapped, along with the wearer’s body heat, in pockets created by the natural chaos of the kinks and bends in the wool. In warm weather, wool draws moisture from your body and facilitates evaporation by distributing that moisture over a larger surface. The cool air created by this evaporation is then trapped in the fabric. Wool will keep you dry and warm when it’s cold and accordingly cool when it’s hot. Merino wool is made from a wide variety of sheep known for their fine, soft fibers. The fibers act like normal wool, but are less itchy on the skin.
Yak wool is even finer and softer than merino wool (albeit rarer and more expensive), and Kora has run its own laboratory tests that suggest the material is 40 percent warmer, 66 percent more breathable, and 17 percent better than running water away from your skin Merino. The company, based in Maharashtra, India, sources its yak wool directly from producers in Tibet.
The third component of the xenolite, Polartec Alpha, was developed for Special Forces soldiers who were stationed in the mountains of Afghanistan in the early 2000s. Designed as a more breathable alternative to high-loft polyester fleece, it’s made up of a mesh chassis that holds a loose collection of polyester fibers together. In static conditions, these raised fibers trap a lot of air and provide a lot of insulation. Get moving and the scarce fibers are virtually no barrier to the air pressure created by your rising temperature. Alpha also wicks away moisture and distributes it for faster drying times.
Just an intermediate layer made of one of these fabrics provides a surprising amount of warmth and remains comfortable over a wide range of temperatures and activities. Together they make a sweater that provides great insulation and stays comfortable when things heat up.
A xenolite, after which the sweater is named, is a stone that forms in another stone – a stone sandwich, if you will. Koras Xenolith consists of a thin outer layer made of 30% yak wool and 70% merino wool. Underneath is an alpha layer that covers your entire upper body, shoulders, and the outside of your upper arms, but not your armpits, inner upper arms, or forearms. Inside, there is another layer of this wool around your arms and the front of your torso, but not around your back.
The end result is practical and versatile on its own, but something that will help You also get more out of your other shifts. Since the tightly woven wool fabric serves as a rudimentary shell and keeps some wind out, the double front of the sweater keeps you cozy while cycling or skiing without your back losing heat. This arrangement also keeps your back dry while carrying a backpack. Thin and slim cut, it is easy to wear under additional pieces and perfect under a hard shell, reducing bulk and taking advantage of the limited air permeability of waterproof jackets by not offering any other barrier to breathability.
During the last month of bird hunting in South Dakota, The conditions were positively terrible. Single-digit temperatures, combined with strong winds and light precipitation, create bone-happy, damp weather that stings any exposed skin immediately with blown ice particles. Conditions were so bad that I lost the feeling in one of my fingers that has not yet recovered. Layered over a woolen pad and under a light, puffy, and hard shell, the xenolite allowed me to wander around comfortably all day.
One night in the remote backcountry of southwest Montana last fall, just outside Yellowstone National Park, temperatures unexpectedly dropped to single digits. I had only packed a 20-degree sleeping bag, but could sleep well in it and only wear my underwear and this sweater.
While my wife and I hiked our cabin in northern Montana just outside Glacier National Park for the vacation, temperatures ranged from the low teens to the mid-1940s. During the entire trip, I only wore underwear, this sweater and a hard shell.
Right now I’m 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 is 69 degrees in my living room and I am as comfortable now as I will be walking the dogs after completing this article. It’s 20 degrees outside today and I just have to put on my shoes.
Main photo: Nathan Norby