In a refrigerated truck, insulating a compartment and then sealing it create the self-contained atmosphere. The insulation is made from high-density polymer foam, which is quite similar to polystyrene, although it’s constructed from a higher density and more durable polymer than you’d find in typical polystyrene insulation. The insulation, combined with a seal around the door, creates an atmosphere that is fairly heat-tight. Then all you have to do is remove any heat that remains inside, and any heat that comes in when the door is opened. This is accomplished using a refrigeration system with a reasonably simple mode of operation. It has three different mechanisms all working together: a condenser, a compressor, and an evaporator.