Which statement describes the reverse stack effect?

• A. In the cooling season, air may be drawn in at the top of the building where pressure is lowest.
• B. In the heating season, warm indoor air moves downward due to gravity.
• C. It refers to wind creating a high pressure on the leeward side.
• D. It occurs when mechanical ventilation cycles on and off.

Answer: (A)

Buoyancy-driven air movement caused by indoor–outdoor temperature differences is being tested. In the reverse stack effect, during the cooling season the indoor air is typically warmer than the outdoor air, and the resulting pressure pattern with height makes the top of the building a site where air is drawn in. The top has relatively lower pressure, so outside air can infiltrate through ceiling-level leaks. This infiltration at the upper levels is the hallmark of the reverse stack effect.

This differs from the usual winter stack effect, where warm indoor air rises and exits at the top, pulling cool air in from the lower levels. It also isn’t about wind-created pressure or about just the building’s mechanical ventilation cycling.