What is the stack effect?

A frequently discussed phenomenon in air quality, natural ventilation, and smoke mitigation studies is the so-called stack effect. Sometimes called the chimney effect, the stack effect is a naturally-induced vertical flow of air through a structure. In low-rise structures, the effect is often small enough as to be negligible, but in high-rise towers, the stack effect can be a powerful driver of airflow.

A fireplace with chimney intentionally exploits the stack effect for ventilation purposes and is perhaps the most illustrative example. As warm air escapes from a fireplace and rises through the chimney, ambient air from the room is drawn into the fireplace to replace it. The temperature difference between the fireplace and the room—more properly, the pressure difference induced by that temperature difference—sets up a flow of air from cool room to hot fire.

The net effect is that warm air escapes from the top of the chimney, while cold air is drawn in at the bottom of the chimney (the fireplace), creating a self-sustaining upward flow of air. Engineers optimize chimneys to carry away pollutants by adjusting the chimney’s height and cross section: Larger cross sections and taller chimneys both improve the flow.

If we now think of buildings as chimneys, we can understand how the stack effect manifests itself in other ways:

  • A tall building with a central atrium is like a chimney without a top opening. If a fire breaks out on the ground floor, smoke and hot air rise but have nowhere to go. Smoke can collect at the building’s upper floors, hampering evacuation efforts.

  • In winter, heated air rises toward a building’s ceiling, setting up a situation very much like that of a fireplace, but with much smaller temperature extremes. Ground level openings like doors and windows allow the cold, outside air to rush in and displace the warm, rising air, while upper-level openings permit warm air to escape. The net effect decreases heating efficiency more so than do thermal differences alone.

  • A building specifically designed to take advantage of solar heating can exploit the stack effect to induce a natural flow of air during warm months, particularly when windows, doors, and other openings are optimized for this purpose.

All buildings experience the stack effect to some degree. In a single-story, single-family residence, the effect may be virtually unnoticeable. But as we have seen, in a tall commercial building, hotel, or high-rise residence, the stack effect is significant and should be considered in the design of any such structure.AirTemperature