Under Armour Men's ColdGear Armour Compression Mock Long Sleeve Shirt, White (100)/Steel, XXX-Large
This is the first layer you put on every time the mercury dips. It's the weight of a T-shirt, fits comfortably next-to-skin, wicks away sweat, and keeps you amazingly warm. If you work out when it's cold, you better do it in ColdGear Armour.