Our current ductwork (all in the attic) is probably as old as our house, built in 1984. The plenum is metal and the rest is insulated flex that leaks. We attempted to tape/seal everything seveal years ago but all the flex needs to be replaced. At some point in the next couple years we will probably replace the A/C and furnace, however, we'd like to get the ductwork done and add insulation first. So my 1st question is this: if we simply replace the flex with an equivalent to what's currently there, will it work with the new A/C and furnace that goes in later? In other words, is ductwork "standard" as far as fittings, etc? 2nd inquiry: Should we be considering changing any of the duct sizing for any reason? 3rd: Should the plenum be insulated? If so, how do we go about insulating the current plenum? We don't plan replacing that if we don't have to. As far as I know, there's nothing wrong with it. I hope that makes sense. This is our 1st HVAC project so doing lots of homework!