A filter box mounts to an air intake opening of an air handler. The filter box includes an inner shell, insulator, and an outer shell. The inner shell is adapted to confine one or more filters therein. The inner shell defines an airflow channel from a front end opening of the inner shell through the filter(s) to a rear end opening of the inner shell. The outer shell defines a chamber from a front end opening of the outer shell to a rear end opening of the outer shell. The outer shell contains the inner shell in the chamber and the insulator is sandwiched between the outer shell and the inner shell. The inner shell and the outer shell are separated and spaced apart all over by the insulator and the insulator insulates and seals the inner shell from the outer shell.