The Intraknit Active Long-Sleeve Baselayer Top is our favorite for winter running, hiking, cross-country, and other cardio-heavy cold-weather sports. While merino wool offers moisture management and light insulation, Tencel provides quick cooling and aids with wicking and drying. Both major materials are soft as can be, making this next-to-body layer comfortable as can be. Smartwool's Intraknit tech seamlessly integrates ventilation in key areas, so we stay cool as our hearts race.