The most fundamental building science idea to understand is that a house is a system of many interacting components that all affect how well it performs. The insulation and air barrier, which make up what we call the building envelope, have a huge impact on the performance, of course. The insulation must be complete and installed correctly, and the air barrier must be continuous. Just as important as having both there, however, is that the insulation be in contact with the air barrier.
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Oh, but that's why we have shiny insulation on ducts, right? To keep that air cold as it travels perhaps 50 feet or more through that blazing hot air, the building code requires insulation with an R-value of 8. Well, the truth is, putting an air conditioner in an attic is one of the worst places you can put it.