Power Stretch Pro is an excellent fabric with a smooth nylon outer that resists abrasion and keeps out light winds, and a soft, brushed polyester inner that pulls moisture away from the body. It works well as both a mid layer and, in really cold weather, a base layer. The fabric is very stretchy and the Aconcagua is body-hugging without restricting movement. It’s warmer than Polartec 100 so this is a warm garment for the weight, which is quite low. The design is simple with just a high collar, thumb loops and a roomy chest pocket. The Aconcagua is comfortable to wear and efficient at wicking. However it is expensive and doesn’t have handwarmer pockets or a hood. I’d only choose it as a cold weather base layer. For a fleece mid layer I’d rather have a garment with more features such as the Mountain Equipment Shroud or more warmth such as the Berghaus Scorch or EDZ Yeti, all fleeces that cost considerably less than the Aconcagua.