I am about to install my first Attic air handler and duct work. I know the duct has to be insulated. Is there a "standard" size to the insulation in R value? I will be using metal ducting. What other precautions should I take with this installation? Any suggestion are welcome. I have done far too many installations of furnace systems but have always stayed away from the attic jobs, leaving them to the younger guys. But this job is for family.