Back in the seventies I had a my first house built with the ducts running inside the living space. Ranch house so each room had a "box" duct chase down one of walls of each room. My neighbor did the same. We had central electric resistance heat and three months into winter me and him were comparing notes. His electric bill was four times what mine was. And his house was cold. We went over and climbed up into his attic and they had forgotten to enclose the tops of the duct chases and so he was blowing hot air right up into his attic all winter. THAT is a major duct loss. :ahhh: