Panther,
For keeping warm in cold weather I typically try to incorporate three things. One: Base layer that retains warm even when wet (from sweat) and dries quickly. Outside layer that is wind proof. An insulation layer either natural like down or artificial like prima loft. With those three components. I usually just have to adjust the amount of wind block and thickness of insulation to regulate temperature. I suspect Underarmour does a decent job at keeping a person warm when wet and drying quickly. If so I think it would work fine as a base. Most common base layers used in the mountains seem to be; polypropylene, silk or wool (Smartwool is great brand on non itchy wool).