This is the best base layer I have owned. The warmth and breathe-ability are unmatched. I wear it skiing and shoeing in varying weather. I don't perspire much; for what it's worth, this piece stayed comfortable and dry. Layers: 35-40 deg., I wore my Phase SV and Sentinel shell. 15-30 I added the Atom Hoodie. Both were perfect for the conditions. Arc'teryx runs slightly small. I am a street size 4, 5'4" and 130lbs. I wear a size Medium/6 in Arc'Teryx. I highly recommend the Phase line of products.