The ActionHeat Vest Liner is a great option for those who want to stay warm without all those bulky layers. Designed to be worn over another garment and under a jacket, the ActionHeat heated Vest Liner keeps your torso warm. ActionHeat battery heated apparel utilizes innovative technology designed to warm the core body temperature. These revolutionary garments feature built-in heating panels and are made with ActionFlex material that provides lightweight warmth, comfort, and versatility.