This reduces the dry-bulb temperature of the air within the building relative to the outdoor ambient air. Mechanical refrigeration equipment provides sensible and latent cooling during summer months. Warm air will attempt to exfiltrate the building envelope through floors above the neutral axis. The net negative pressure on lower floors can induce outdoor air to infiltrate the building through doors, windows, or ductwork without backdraft dampers. This presents a situation where floors underneath the neutral axis of the building have a net negative pressure, whereas floors above the neutral axis have a net positive pressure.
Consequently, it rises from lower levels to upper levels through penetrations between floors. Warm air within the building has a low density (or high specific volume) and exhibits a greater buoyancy force. Normal stack effect occurs in buildings which are maintained at a higher temperature than the outdoor environment. Two regimes of stack effect can exist in buildings: normal and reverse. A cavity between the outer aluminium cladding and the inner insulation formed a chimney and drew the fire upwards. The Grenfell Tower fire, as a result of which 71 people died, was in part exacerbated by the stack effect. Smoke extraction is a key consideration in new constructions and must be evaluated in design stages. While natural ventilation methods may be effective, such as air outlets being installed closer to the ground, mechanical ventilation is often preferred for taller structures or in buildings with limited space. Especially in case of fire, the stack effect needs to be controlled to prevent the spread of smoke and fire, and to maintain tenable conditions for occupants and firefighters. Stairwells, shafts, elevators, and the like, tend to contribute to the stack effect, while interior partitions, floors, and fire separations can mitigate it. In a modern high-rise building with a well-sealed envelope, the stack effect can create significant pressure differences that must be given design consideration and may need to be addressed with mechanical ventilation. During the cooling season, the stack effect is reversed, but is typically weaker due to lower temperature differences. The rising warm air reduces the pressure in the base of the building, drawing cold air in through either open doors, windows, or other openings and leakage. During the heating season, the warmer indoor air rises up through the building and escapes at the top either through open windows, ventilation openings, or unintentional holes in ceilings, like ceiling fans and recessed lights.
Since buildings are not totally sealed (at the very minimum, there is always a ground level entrance), the stack effect will cause air infiltration. 3 Stack effect in flue gas stacks and chimneys.
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