New homes are built specifically for energy efficiency, with better windows, energy efficient appliances and AC units, and a better layout for AC ducts and airflow. Many older homes were built before the use of AC was widespread, are not design for best airflow or insulation. In subsequent years, technology has improved with every decade to increase energy efficiency. You can make your home even more energy efficient by sealing any gaps and cracks that lead to the outside and installing Energy Star appliances & lighting.