I had an HVAC company give me some advice yesterday for adding some ductwork in my soon to be finished basement. He said it would be best to us "flex". Now I was thinking this meant the very flexible stuff, similar but more sturdy than what you connect your dryer to the exhaust outlet. But when I was in the hardware store there was some of the traditional metal duct that was labeled "flex". So which one did he mean? I know it would be easier to use the truly flexible pipe because there are so many obstacles coming from the ceiling, but is it less efficient? I am only running maybe 2 lines in to the main room.