The humidity from the dog's breath would make the box a damp and then cold place if it is not allowed to escape through vents or the door. Most boxes have a large opening covered with some form of metal grating. When the temperatures are colder than what the dog normally experiences a portion of the metal grating or other opening may be partially covered to conserve heat, yet still allow humidity to escape. Mushers put a variety of materials in the boxes for bedding. The most common bedding is fresh straw.
Straw provides padding and insulation.
It must be changed regularly so it does not become wet, molded or soiled. The dog boxes quickly become a sort of mobile home for the dogs providing a safe haven and a familiar environment no matter where the team travels. The dog truck is equipped to haul everything from sleds to dog food and is rigged with a number of special devices to make travel easier. Lights on the side of the boxes and the rear make it easier to feed at night. Eye bolts around the bottom of the truck give the musher a place to attach short leashes, called 'drop chains', to the truck so that the dogs can get out, stretch and relieve themselves while remaining securely attached to the truck and out of harms way.