Stack Effect and Building Design

In the simplest term, stack effect acts like wind, moving large volume of air through a building envelope. This phenomenon contributes a big part to the air flow, cooling and heating system of all kinds of buildings. There are two basic principles that we need to know to understand stack effect.

First, heat follows cold. Air is always seeking for equilibrium state to obtain a uniform temperature throughout the region it flows. Second, hot air rises and cold air sinks. This explains why upper level is generally warmer than the lower level. Convection of air happens due to this law of physics causing lower pressure at the bottom of building. The pressure difference and flow of air draws colder outdoor air into cracks or air gaps at the bottom.

stack effect image-300x295 Stack_Effect

Due to stack effect, buildings always act like chimneys. For instance, in the winter when one heats the inside of house, the hot air rises and due to convection, it tries to escape through holes or gaps in the ceiling between the top floor and the attic. The pressure difference when warm air leaves causes cold air from the outside to be sucked into the building from the cracks and tiny air gaps in the foundation or windows. The merry-go-round of air flow — sucking air up the bottom, heating it up, and blowing it out the top makes the furnace work harder to keep the building warm. The reverse effect happen in the summer which leads to inefficiency of air conditioning.

Green building advisor reported that “the differences in temperature and pressure aren’t as great during the summer as they are during the winter. When it’s cold outside, the pressure created by the stack effect is 4 pascals per story of height; when it’s hot, about 1.5 pascals per story of height.” Studies show that the amount of air leakage in a typical American home is the equivalent of leaving a window wide open, 24 hours a day, 365 days a year.

To increase the efficiency of the heating and cooling systems in the building, revolving doors are invented especially for high rise building where stack effect is more apparent. In fact “when skyscrapers were first developed at the turn of the century, people also had to invent revolving doors because you couldn’t open the front door due to the stack effect pressure,” says Straube. “The cold air was rushing in with so much pressure that it was difficult to push the exit doors open.” Air sealing and insulation by specialists are another way to lower energy consumption and saving energy bills.




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