The stack effect, often called the chimney effect, is a fundamental principle in building science governing air movement within and through a structure. It describes natural air movement driven by air buoyancy, resulting from temperature and density differences between indoor and outdoor air. This phenomenon creates measurable pressure imbalances across the building envelope, constantly pushing and pulling air through unsealed openings. Understanding this effect is important for controlling a building’s energy efficiency, internal climate, and overall indoor air quality.
The Physics Driving Air Movement
The stack effect is rooted in the physics of buoyancy. When a building is heated, the warmer indoor air becomes less dense than the colder, heavier air outside. This density difference creates a pressure differential that acts vertically across the structure’s height.
The warmer, lighter air inside rises, similar to smoke moving up a chimney flue. As this air ascends, it lowers the air pressure at the bottom of the building while simultaneously increasing the pressure at the top. This pressure gradient is the driving force of the stack effect. The force is amplified by two primary factors: the temperature difference between inside and outside, and the vertical height over which the pressure differential acts. Taller buildings experience a significantly greater stack effect because the longer air column results in much higher pressure differences between the base and the roof.
Inflow and Outflow Dynamics in Structures
The pressure differences created by rising air manifest as distinct zones of air movement across a building’s exterior boundaries. In a heated building, the upper sections experience positive pressure, meaning the internal pressure is greater than the external pressure. This forces conditioned indoor air to leak out, a process known as exfiltration, through gaps in the attic, roof, and upper windows.
Conversely, the lower sections of the building, such as the basement or ground floor, experience negative pressure relative to the outside. This lower pressure draws in cold, unconditioned outdoor air, a process termed infiltration, through cracks in the foundation, window frames, and doors. The air then moves upward through internal vertical pathways, such as stairwells and utility chases, completing the cycle.
Between the positive and negative pressure zones is the Neutral Pressure Plane (NPP). At the NPP, the air pressure inside the building equals the pressure outside, and theoretically, no air movement occurs across the building envelope at that level. The location of the NPP is dynamic, determined by the distribution of air leaks, and its height dictates the severity of the pressure imbalance above and below it.
Managing Energy and Air Quality Impacts
The uncontrolled movement of air driven by the stack effect has significant consequences for building performance and occupant comfort. The constant exfiltration of warm, conditioned air and infiltration of cold air force HVAC systems to work harder, leading directly to increased energy consumption and higher utility costs for the owner.
The stack effect also causes comfort issues, primarily drafts on lower floors where cold air is drawn in. Furthermore, infiltration into the lower levels can compromise indoor air quality by pulling in contaminants. Air drawn from basements or crawl spaces often carries moisture, mold spores, soil gases like radon, and odors, distributing them throughout the living spaces as the air rises.
To manage this phenomenon, the most effective strategy is controlling air leakage points to reduce the severity of the pressure difference. Air sealing involves identifying and sealing gaps, cracks, and penetrations in the building envelope, particularly in the attic and basement areas. For very airtight or modern structures, mechanical ventilation systems, such as Heat Recovery Ventilators (HRVs) or Energy Recovery Ventilators (ERVs), can be used to balance internal pressures and provide controlled fresh air.