Reverse stack is defined as which condition?

• A. Hotter air inside, cooler air outside
• B. Equalized temperature between inside and outside
• C. Hotter air outside, cooler air inside
• D. No air movement

Answer: (C)

Air movement in a building is driven by buoyancy: warmer, less-dense air tends to rise, while cooler, denser air tends to sink. In the typical stack effect, inside air is hotter than outside, so it rises and exits at the top, and cooler outside air is drawn in from below. Reverse stack happens when the outside air is hotter than the inside air. The denser interior air then moves downward, and hot outside air can push into higher openings and move downward inside via vertical shafts or stairwells. This downward air movement inside is what defines reverse stack, which is described by having hotter air outside and cooler air inside. If the temperatures were equal, buoyancy effects would be neutral and there would be little to no vertical air movement; if the interior were hotter, you’d see the normal stack effect rather than reverse stack.