Harkila Base Warm Zip NeckHarkila Base Warm Zip Neck The long sleeve Harkila Base Warm undershirt is the warmest in Harkila's collection. It is made from a very soft, warm, breathable, and naturally temperature regulating wool blend, making it an ideal mid layer in cold weather. The undershirt has a high collar that you can close snugly around the neck, or zip open to the chest when you need to cool down. To see more from Harkila, visit the Harkila Collection