I thought how could something sleeveless help me in the icy cold hills of South Korea, I would use this just as a layer before another layer.  I am happy to report that once on and a nice winter top pulled over it I felt the warmth.  I live in the tropics so investing in bulky things I have to store for the down time I wanted to avoid.  The bonus is that I get to wear this now at home with a cotton shirt thrown over it.