Regulates body temperature perfectly without getting sweaty or overly hot. You can wear it on its own or as a cold weather base layer. This is Smartwools warmest base layer that can be used as a single layer in cool weather or as a base layer in cold weather. This Merino 250 Men's 1/4 Zip Base Layer is great for all cold weather activities. The base layer is slim fit but just got a little more refined with an improved fit. Chafe-free seam construction and panels that offer more comfort.