Hi LIVE2HUNTNFISH,

The military uses polypropylene for its thermals. They come in two distinct weights, (so you can adjust for expected temperatures). Many civilian manufacturers make it as well.....in many different styles and weights. It is a very good insulative undergarment.

If you can afford a little more take a good look at Capilene garments. It works as well or better than polypropylene...and is more comfortable against the skin. You will be able to find either by using "search engines" on the web....or at better "Mountain/climbing outfitters". Also some of the better sporting/hunting stores carry either.

If you really want to increase the comfort level try wearing a silk garment against your skin, followed by either polypropylene or capilene, then an appropriate weight insulating garment, covered with wind/water proof (breathable) outer shell. For my insulating layer I normally have a fleece vest or jacket and a down vest or jacket. (Either or both can be worn or carried in your daypack to adjust for temperature and exertion levels.)

Layering is much more efficient....rather than wearing just one heavy garment!

There are some other "brand name" thermals that work to varying degrees. But as was previously stated....avoid cotton like the plague.

Good luck!