Men's Classic Waffle-Knit Thermal Long Sleeve Shirt - Warm Winter Base Layer for Outdoor Activities & Casual Wear

Avoid the medium weight option which is really a Knocker brand.I got the medium size Cottonbell which fit fine and is plenty warm, even though it is really medium weight, not heavy weight as claimed. It is also light enough and soft cotton comfortable. The last few cotton thermal tops I've had were barely acceptable but this one is a winner, so I hope it lasts.It says to wash in cold but I wash all my colors in warm, and it did shrink. It still fits fine but hugs a bit which may make it even warmer. I'm thin, so if you are not or you plan on washing on warm, you will probably want to buy a size larger than your normal size.