"Jeff" wrote in message
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Think about it. The heat in the furnace room must go somewhere. If the
room is too well insulated it will get hotter and hotter until the heat
loss equals the heat from the furnace walls. This will cause the furnace
to be hotter, maybe shortening the life of the motor. You might put a
small opening in the inlet duct to draw house air into the furnace, making
sure there is space under the door for the air to flow in.

First check your local codes about the benefits of installing a return vent
in a furnace room. It may not be legal because of the potential of creating
negative air pressure in the room resulting in the flames not staying where
their supposed to be in the furnace, and the possibility of drawing carbon
monoxide and/or even natural gas or propane into the household air
distribution ducts if there should be a leak.