*chuckle* When I was little, people bitched that cotton was unsuitable for thermals, and they should all be made of wool still. Of course, I couldn't wear wool but could wear the cheap cotton ones. Polyester is near-useless as thermal underwear. Bamboo or hemp, now, I suspect those would work quite well but I've never seen any. I've only seen silk in catalogs, but would pounce on it if I ever saw any in person.