Buoyancy‑driven vertical movement of air within a building or vertical shaft caused by density differences between interior and exterior air due to temperature (and thus density) differences; typically produces inflow at low levels and outflow at high levels when interior air is warmer than exterior.

Principle

The magnitude and sign of stack‑driven pressure differences increase with building height and the indoor–outdoor temperature difference; taller structures and larger ΔT produce stronger stack pressures and sustained vertical flow paths in discontinuities and openings.

Demonstration

Illustrative scenario → In winter a heated multi‑storey atrium is internally warmer than outside. Lower levels draw cold air into basements and ground‑floor cracks while warm interior air exits through upper openings, increasing infiltration heat loss and creating draft paths that affect comfort and ventilation.

Misapplication

Attributing all observed vertical airflow solely to stack effect while ignoring wind pressures and mechanical ventilation. The reasoning error is treating stack effect as the exclusive driver rather than one of multiple simultaneous drivers.

Consequence

Stack effect affects infiltration/exfiltration rates, heating and cooling loads, moisture transport and potential smoke movement; it can necessitate changes in sealing, vestibules, stair pressurization, or mechanical balancing to control unintended flows.

Reversal

When interior air is cooler than exterior (e.g., in some cooling seasons) the direction of stack flow reverses; in low‑rise buildings or when temperature differentials are very small the effect is negligible and other drivers dominate.

Boundary

Clearly within: multistorey buildings, vertical shafts and continuous cavities with openings that allow vertical flow. Boundary case: short vertical ducts or buildings with effective air‑barriers where stack effect is present but minor. Clearly outside: closed, single‑storey enclosures without vertical continuity and situations dominated by mechanical ventilation or strong wind loads.

Semantic Tension

Natural Ventilation (passive drivers) ↔ Mechanical Ventilation (controlled systems) — stack effect can support or oppose designed ventilation strategies and must be reconciled with mechanical control for predictable indoor environment.

Synthesis

Stack effect is an unavoidable physical driver of vertical air movement in buildings with vertical continuity; effective airflow control requires quantifying its contribution alongside wind and mechanical drivers rather than assuming it will behave independently.