Even if you have a perfectly balanced system with no leaks, your air ducts can still lose energy. Imagine how sweltering hot the air inside your attic gets in the summertime. Now imagine how effective your bare sheet metal ductwork is if it passes through your attic without insulation. All the cold, conditioned air you are trying to cool your house with is being warmed as it passes through ducts that are not insulated. Some studies have shown that the same amount of energy lost via leaks can also be lost due to a lack of insulation.