well in my opinion, the problem is the materials Under Armour uses. Their base layers are made with synthetic materials. These materials are supposed to "wick moisture" but once you start sweating they get wet and the wicking cannot keep up with the amount of water they hold. Once this material gets wet it does not retain heat. Also, when it is dry, it still isn't as warm as wool. I had some under armour base layers. I gave them away. I prefer merino wool. I use KUIU merino wool, and it is the warmest highest quality stuff I have had. Merino wool still retains heat when it is wet, and it also wicks moisture. It also controls scent very well. I can wear it for a week straight in the backcountry and it controls my scent way better than any synthetic stuff does. In my experience the synthetic gear smells very bad by the end of the first day... and that doesn't cut it for me.