Would it be worth it to add a bubble roof on top of an ordinary flat roof? On a triple decker or something similar? The main benefits would be greater insulation and waterproofing. There's energy in needing to pump in air to make up for leaks, but I don't know how that compares to the energy saved in heating and cooling. Because people don't have to be able to inhabit it you can seal it tighter than usual, which should cut down on fan usage. It can be self-monitoring for leaks in a way that a normal roof can't because the amount of air you have to put in to maintain a given pressure tells you how much you're leaking.