This jacket does not look like much, thin and scant and just does not look like it provides a lot of insulation. Could not be farther from the truth, this jacket is incredibly warm, providing surprising warmth and surprising comfort in the cold. I generally only wear this stand alone if I am belaying on a nice winter/fall day, or if I want some warmth and don't need a shell, other than that I use this exclusively as a mid layer and it works incredibly well. Warm, lite compact and plenty of flex. I love it. I wear a medium in most products at 5'10 160 and I wear medium in all of montbell products.