Not cotton for sure. I do wear cotton briefs ....

I was given a top-bottom set of Under Armour brand cold weather gear. I had been using a Cabelas brand of base layer ... a more loosely fitting poly-wool blend. It worked great. The Under Armour fits tightly, especially on my rotund body .... but it is warm. This surprised my old fashioned self. I liked the stuff so much that I bought another set for layering.