Skyscrapers are thus material and energy intensive buildings. Skyscrapers have considerable mass, requiring a stronger foundation than a shorter, lighter building. In construction, building materials must be lifted to the top of a skyscraper during construction, requiring more energy than would be necessary at lower heights. Furthermore, a skyscraper consumes much electricity because potable and non-potable water have to be pumped to the highest occupied floors, skyscrapers are usually designed to be mechanically ventilated, elevators are generally used instead of stairs, and electric lights are needed in rooms far from the windows and windowless spaces such as elevators, bathrooms and stairwells. Skyscrapers can be artificially lit and the energy requirements can be covered by renewable energy or other electricity generation with low greenhouse gas emissions. Heating and cooling of skyscrapers can be efficient, because of centralized HVAC systems, heat radiation blocking windows and small surface area of the building. There is Leadership in Energy and Environmental Design (LEED) certification for skyscrapers. For example, the Empire State Building received a gold Leadership in Energy and Environmental Design rating in September 2011 and the Empire State Building is the tallest LEED certified building in the United States, proving that skyscrapers can be environmentally friendly. The 30 St Mary Axe in London, the United Kingdom is another example of an environmentally friendly skyscraper. In the lower levels of a skyscraper a larger percentage of the building floor area must be devoted to the building structure and services than is required for lower buildings: More structure – because it must be stronger to support more floors above The elevator conundrum creates the need for more lift shafts—everyone comes in at the bottom and they all have to pass through the lower part of the building to get to the upper levels. Building services – power and water enter the building from below and have to pass through the lower levels to get to the upper levels. In low-rise structures, the support rooms (chillers, transformers, boilers, pumps and air handling units) can be put in basements or roof space—areas which have low rental value. There is, however, a limit to how far this plant can be located from the area it serves. The farther away it is the larger the risers for ducts and pipes from this plant to the floors they serve and the more floor area these risers take. In practice this means