The Filson insulated Field Flannel Shirt combines a classic cotton flannel with high-performance insulation for a shirt that excels in cold weather. The midweight flannel fabric, heavily brushed on both sides for softness and warmth, is tightly woven for durability. The 60g PrimaLoft Gold insulation features the highest warmth-to-weight ratio in synthetics and is quite packable. Originally developed as an alternative to goose down, this polyester insulation breathes well and retains 98 percent of its insulating abilities even when wet. Virtually unbreakable melamine buttons are used for the front closure and adjustable cuffs. Center-back box pleat for ease of motion.