For the Mesa, we use patented 37.5® premium insulation, which makes you more comfortable by helping control your core temperature. The insulation retains body heat, while 37.5 active particles embedded in the insulation accelerate vapor movement preventing overheating and moisture build-up. Alpine’s nylon shell has a soft, buttery hand and a vertical diamond-quilt pattern to stabilize the insulation. The overall result is a flattering, comfortable, ultra-lightweight technical vest that is warm, without the bulk.