Nothing against single-family homes, but apartment buildings and condos are much more efficient for a number of reasons. For one, residents share walls, floors, and ceilings with their neighbors, surrounding them with excellent insulation. Secondly, the square footage of each unit tends to be smaller than detached homes, so there’s less air to manage. Accordingly, it takes less energy to climate-control apartment units and keep people comfortable: The typical resident of a downtown high-rise emits one-third as much greenhouse gases as a resident of a detached house in the suburbs.