The difference in poly... materials is only in the plastic it's made from. Any fabric that is not fleece won't insulate well. But, it will not absorb moisture , dry quickly, and will wick moisture through to prevent you from getting chilled. It is good material to wear under fleece. Cotton is bad because it absorbs moisture rather than wicking it through and does allow you to get chilled. I never wear cotton in cold weather. Wool, poly, or fleece only.