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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 ($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.

Yak wool is even finer and softer than merino (if rarer��and more expensive as a result), and Kora has conducted its own�� 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’s 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��virtually no obstacle to the air pressure generated by your increasing temperature. Alpha also wicks moisture��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��and remain comfortable across a wide range of temperatures and activities. Together, they create a sweater capable of providing outstanding insulation that stays��comfortable when things heat up.

A xenolith, which the sweater is named after, is —a��rock sandwich, if you will. Kora’s��Xenolith is made from a thin outer layer of tightly woven 30 percent yak wool��and 70 percent merino wool. Beneath this is��a layer of Alpha that covers your entire torso, your shoulders, and the outer side of your upper arms��but doesn’t cover your armpits, your inner upper arms, or your forearms. On in the inside, there’s��another layer of that wool around your arms and the front of your torso��but not your��back.

The end result��is practical and versatile on its��own, but something that will help��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��keeps you cozy while cycling or skiing��without preventing your back from losing��heat. This arrangement also��keeps your back dry while wearing a pack. Thin��and slim fitting, it’s easy to wear under additional pieces��and perfect��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,��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��were so bad that I lost feeling in one of my fingers, which has yet to��recover.��But��layered over a wool base layer��and beneath��a lightweight puffy and hard shell, the Xenolith enabled me to hike around all day in otherwise��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’d only packed a 20-degree sleeping bag��but was able to sleep soundly in it wearing��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��base layers, 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’s 69 degrees in my living room, and I’m as comfortable now as I will be when I take the dogs for a walk after I wrap up this article. It’s 20 degrees outside today, and all I’ll need to do is put on shoes.