Skyscrapers are already very efficient. They fit more usable floorspace into their ‘footprint’ than any other type of building does. They conserve energy that gets used for heating and cooling by exposing less of that floorspace to the outside weather. For all that usable floorspace, they need only a single roof.
Think about it. A relatively short ten story building contains as much floorspace as ten single story buildings that occupy the same area individually, but collectively occupy ten times as much area! Nine of those stories loose heat during cold weather, and collect heat during hot weather, only around the exterior walls. Only the top floor loses and collects heat through the roof, and only if there is not an upper utility ‘attic’ floor that insulates it. Ten single story buildings of the same area are all exposed on top, as well as all the way around. Of course…