The stack effect, also known as the chimney effect, is a naturally occurring phenomenon that drives air movement into and out of buildings. This movement is caused by differences in air density, primarily resulting from temperature variations between the indoor and outdoor environments. Like the draft in a fireplace chimney, this effect creates vertical airflow through unsealed openings in a structure. The greater the temperature difference and the taller the building, the stronger this force becomes.
The Physics Behind Air Movement
The mechanics of the stack effect rely on thermal buoyancy. Warmer air is less dense than cooler air, causing it to be more buoyant and rise within a structure. During the cold season, heated indoor air rises and escapes through leaks high up in the building, such as the attic or upper windows.
As this warm air exits the top, it creates a negative pressure differential near the bottom of the building. This pressure difference pulls colder, denser outdoor air inside through cracks and openings near the foundation or lower floors.
The Neutral Pressure Plane (NPP) is the location within the building envelope where the internal air pressure equals the external air pressure. Below this plane, the internal pressure is negative, leading to air infiltration from the outside. Conversely, the internal pressure is positive above the NPP, causing air exfiltration to the outside.
The position of the NPP is not fixed; it is determined by the balance of air leakage areas across the building’s height. For example, if a home has more air leaks in the attic than in the basement, the NPP will be pushed lower down in the structure.
Real-World Consequences for Structures
The continuous, uncontrolled air movement caused by this effect has significant consequences for a building’s performance and occupant comfort. One result is the loss of conditioned air, which forces heating and cooling systems to work harder. This strain on HVAC equipment leads to poor energy efficiency and increased utility costs.
The stack effect creates comfort issues, especially during extreme temperatures. In winter, cold air infiltration near the base of the building results in cold floors and uncomfortable drafts on the lower levels. Simultaneously, the exfiltration of warm air at the top can cause upper floors to feel overheated, leading to inconsistent temperatures throughout the structure.
The movement of air also carries moisture, leading to indoor air quality (IAQ) and structural problems. When warm, humid indoor air exfiltrates through wall or attic cavities, it can meet cooler surfaces, causing condensation. This trapped moisture promotes the growth of mold and mildew, which compromises IAQ by circulating spores, allergens, and dust.
Strategies for Minimizing the Effect
Controlling the stack effect involves reducing uncontrolled air movement through the building envelope. The most effective strategy is comprehensive air sealing, which targets unintentional openings where air can enter and exit. These leaks are often concentrated in the attic and the foundation.
Sealing involves applying materials like caulk, foam, and weatherstripping to common leakage points such as electrical outlets, plumbing penetrations, window frames, and the joint between the foundation and the framing. Limiting the air that escapes at the top and infiltrates at the bottom reduces the pressure differential driving the stack effect.
Mechanical ventilation systems can also aid in pressure balancing. Systems like Heat Recovery Ventilators (HRVs) or Energy Recovery Ventilators (ERVs) bring in fresh air and exhaust stale air in a controlled manner. These systems help maintain healthy indoor air quality without creating uncontrolled pressure imbalances.