I had lunch yesterday with a friend who's an HVAC designer and HERS rater in North Carolina, and he mentioned the idea of separating HVAC design from HVAC contractors. For that to happen, though, HVAC contractors first have to start doing the design part because most residential HVAC systems are thrown in with little to no actual design work.
Not only does the duct displace insulation, it's also right up against the roof deck. In the summer, that roof deck can get up to 150° F or higher. Inside the duct is air that should be about 55° F. That's a 95° F temperature difference, and all that lies between that hot roof deck and the cool air is, in this case, a little bit of fiberglass insulation with an R-value of 6. Do you think that air's going to come out of the vent at 55° F? In the winter, you've got warm air in the duct next to a cold roof deck, and the temperature differences can be similarly large.
Point for third party HVAC design. Yes, there are a few HVAC contractors out there who do good work, but overall, the HVAC industry needs an overhaul. If you've read my articles and white paper about Version 3 of the ENERGY STAR new homes program, you know that I've talked a lot about how difficult the HVAC part will be. In fact, it may not work without third party HVAC design.