Stack effect is the movement of air into and out of buildings through chimneys, or flue gas stacks.  The stack effect is also referred to as the “chimney effect”, and it helps drive natural ventilation and infiltration through buoyancy.  Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The result is either a positive or negative buoyancy force. The greater the temperature differences from inside the building and the outside temperature, combined with the height of the stack structure, the greater the buoyancy force that is created.  This buoyancy is called the stack effect.

It is important to have stack effect as it helps to balance the air flow through your home.

The stack effect will always bring air infiltration, since buildings are never completely sealed.  During the heating season, warmer indoor air rises up through the building and escapes at the top either through open windows, ventilation openings, or leakage. The rising warm air reduces the pressure in the base of the building, forcing cold air to infiltrate through either open doors, windows, or other openings and leakage. During the cooling season, the stack effect is reversed, but is typically weaker due to lower temperature differences.

High rise buildings, that have a well-sealed building envelope, must account for significant air pressure differences due to the height of the building.  There must be a well planned mechanical ventilation system.  Stairwells, shafts, elevators, and the like, tend to contribute to the stack effect, whereas interior partitions, floors, and fire separations can mitigate it.  In case of fire, the stack effect needs to be well controlled to prevent the spread of smoke and fire.