Why is one bedroom colder than the rest of the house?
In my experience as a Home performance contractor, situations like you describe are almost always a problem of air sealing. There is likely something about the way the room was constructed that allows more air infiltration. Possible causes would be an attic stairs located in the room, one of the exterior walls having an overhang, the room is connected to a cantilevered porch or maybe it's over a garage. It's highly unlikely that 8 feet of properly insulated wall would cause the temperature differences you're describing.