MOUNTAIN WEIGHT BASE LAYER - Wrapping your little one in warmth has never been easier! The Youth La Montaña Zip-T Base Layer Top features lightweight and ultra-soft micro-denier polyester yarns for incredible warmth, breathability, and durability; This thermal shirt for kids is designed for performance and wearable comfort in cold weather ACTIVE FIT & UPF 40+ PROTECTION - The youth base layer top features a zip-through collar with a rubber tip reversed zipper; This long sleeve shirt for boys and girls is designed with mechanical stretch; Active fit garments move with your child, allowing a full range of motion while staying close to the skin; In addition, this Hot Chillys thermal underwear shirt has UPF 40 protection WARMTH FACTOR 10/10 - Ranked from Mild to Hot, the Warmth Factor Thermometer tells you how warm is Hot Chillys thermal clothing; Warmth Factors 9 and 10 are considered great as an extra warm base layer or mid layers for extremely cold conditions; The Hot Chillys La Montaña Zip-T thermal underwear for kids has a warmth factor 10 out of 10 - mountain weight for exceptional warmth in extreme conditions MOISTURE TRANSFER FIBERS - The Hot Chillys base layers are designed for warmth and performance, including but not limited to breathability and moisture management; An essential feature in all Hot Chillys youth underwear is exclusive moisture transfer fibers (MTF) that move moisture away from the body by employing hydrophobic yarns to wick away perspiration, thereby keeping your child dry and comfortable SMELL-BLOCKING PROPERTIES - This Hot Chillys kids' base layer has smell-blocking properties; Fabrics that feature this label have been specially treated with Hot Chillys secret "chilly seasonings"; These products are designed for optimum freshness and will remain in the fabric for the first 20-30 wash cycles