If you want to do something really stupid with the ducts for a heating and air conditioning system, put them in an unconditioned attic. In cold climates, designers, builders, and contractors know this and, for the most part, avoid it. They also have basements as an alternative location. In hot climates, designers, builders, and contractors do the really stupid thing all the time. I've written about alternatives in the past, but over the holidays I came across a paper showing what might just be the solution you've been looking for.