I generally agree with Wingbone, although I don't think the plastic fabrics totally lack insulative power. I wore mid-weight polypropylene long underwear for years as an underlayer. It seems to me they made me warmer than if I didn't have them on at all.

I now wear merino wool as an underlayer. It also wicks away moisture and is warm.

Wool as an outer or middle layer has a lot of advantages. A mid-weight wool shirt and mid-weight wool pants are excellent. They are silent. They are warm even when wet. They have a wide temperature comfort range. They breath and let perspiration out. They dry quickly in a warm tent (don't dry them in a clothes dryer).