There are several reasons for installing an air conditioning unit in an attic space. Most people prefer cooler homes for sleeping - and bedrooms are usually located on the upper levels. Most of the heat generated in your home comes from the roof and attic, adding heat gain to the upper levels. Cooling from the attic benefits your home by conditioning the air at the source of greatest heat gain - the attic. Since cold air flows downward, attic air conditioning units cool the top levels of a building first, ensuring the upper levels will never be too warm. To prevent overcooling, the thermostat is installed on the top floor. In a conventional set up, forced air systems with the air handler or furnace in the basement must force the air to the upper levels (which is more difficult for the system) and the upper levels of most air-conditioned homes experience unsatisfactory cooling.