An air permeable envelope has a gripper for securing the envelope to a seat. A mixture contained in the air permeable envelope can react exothermically upon exposure to air. A sealed bag that is relatively air impermeable, initially holds the air permeable envelope and mixture together with the gripper. Upon opening and unsealing the bag, the mixture is exposed to air in order to start an exothermic reaction. The envelope and the gripper are removed from the bag and the gripper is used to attach the envelope upon the seat to warm it.