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A condition that exists in a tall building when outside temperature is significantly lower than the temperature of the spaces in the building is called stack effect. Stack effect is the phenomenon in which a tall building acts as a chimney in cold weather, with the natural convection of air entering at the lower floors of the building, flowing through the building, and exiting from the upper floors.
The cause of stack effect is the difference in density between the cold, denser air outside the building and the warm, less dense air inside the building. The pressure differential created by stack effect is directly proportional to building height, as well as to the differential between the warm temperature inside the building and the cold temperature outside the building.
When the temperature outside the building is warmer than the temperature inside the building, the stack-effect phenomenon is reversed. This means that in very warm climates, air will enter the building at the upper floors, flow through the building, and exit at the lower floors. This downward flow of air is known as reverse stack effect. The cause of reverse stack effect is the same in that it is caused by differences in density between the air in a building and the air outside a building, but in this case the heavier, denser air is inside the building.
While reverse stack effect would seem to be a problem in tall buildings in warm climates, this usually is not the case. The reason is that the difference between temperatures inside and outside a building and the resultant difference in density in warm climates is significantly less than the difference in temperatures inside and outside the building in very cold climates.
There is a neutral pressure level (NPL) in any building. This is the point at which the interior and exterior pressures are equal at any given temperature differential. The location of the NPL in any building is governed by the actual building, the permeability of its exterior walls, the internal partitions, and the construction and permeability of stairs and the shafts, including elevator shafts and shafts for ducts and pipes. Also influencing NPL are air-conditioning systems, with exhaust systems tending to raise NPL in a building (thereby increasing the portion of the total pressure differential experienced at the base of the building) and any excess of outside air over exhaust air in supply-air-conditioning systems tending to lower the neutral pressure level in the building (thereby decreasing the portion of total pressure differential experienced at the base of the building).
Air movement through high-rise apartment buildings is believed to have considerable impact on building performance in the areas of occupant comfort, energy consumption, indoor air quality and deterioration of exterior envelope components. The traditional approach to reducing air leakage has been to seal the building envelope. An alternative that has been suggested for apartment building is to incorporate a compartmentalization sealing strategy that seeks to isolate the living units from other compartments so that they equalize pressure with the exterior. This will reduce the driving force that causes air leakage to or from the outside
