The stack effect is the movement of air into and out of a structure, driven by temperature differences between the inside and outside. When warm air inside a building is lighter than the cold air outside, it rises through vertical shafts and exits at upper levels, pulling colder air in through openings near the ground. The phenomenon goes by several names, including “chimney effect,” and it influences everything from energy costs and fire safety to disease transmission in tall buildings. Though the basic physics is simple, the consequences are surprisingly far-reaching and sometimes dangerous.
What Drives the Airflow
Air behaves like a fluid: warmer air is less dense and tends to rise, while cooler, denser air sinks. Inside a heated building during winter, the indoor air is warmer and lighter than the outdoor air. That temperature gap creates a pressure difference across the building envelope. Near the bottom of the building, outdoor pressure exceeds indoor pressure, so cold air leaks in through cracks, doors, and other openings. Near the top, the reverse is true: indoor pressure exceeds outdoor pressure, pushing warm air outward. The boundary where inside and outside pressures are equal is called the neutral pressure level, and it typically falls somewhere around the middle floors of a tall building.
Vertical shafts act as highways for this airflow. Elevator shafts, stairwells, mechanical chases, and even plumbing stacks all connect the bottom of a building to the top. Once air enters at ground level, these shafts give it a continuous path upward. The taller the building and the greater the indoor-outdoor temperature difference, the stronger the driving force. A 50-story tower on a bitterly cold day can generate pressure differences strong enough to make lobby doors difficult to open and send noticeable drafts whistling through corridors.
The Reverse Stack Effect in Hot Weather
The stack effect is not a winter-only problem. In hot climates, or during summer months, the direction flips. When outdoor air is warmer than the air-conditioned interior, the denser cool air inside the building sinks, and outdoor air enters at the upper floors and exits at the lower ones. This “reverse stack effect” is weaker in most climates because the temperature difference between a cooled building interior and hot outdoor air tends to be smaller than the winter gap in cold regions, but it still affects air distribution, energy use, and smoke movement.
A fire safety study described the pattern plainly: in cold weather, air enters the warmer building from the lower stories, flows upward through vertical shafts, and exits at the upper stories. In hot weather, the opposite occurs, with air penetrating the cooler air-conditioned building at upper levels, flowing downward, and exiting at lower levels.1ScienceDirect (Elsevier / Fire Safety Journal). The effect of fire location and the reverse stack on fire smoke transport in high-rise buildings This reversal matters for engineers designing smoke control systems, because the smoke transport path flips depending on the season.
Drafts, Noise, and Doors That Won’t Close
For the people living and working in tall buildings, the stack effect shows up as everyday annoyances that can escalate into serious complaints. Strong airflow driven through corridor gaps and apartment entrance doors produces audible whistling and drafts. Research on high-rise residential buildings in Korea found that complaints about airflow noise and uncomfortable drafts were concentrated on floors where the pressure difference across apartment entrance doors exceeded about 25 pascals.2Elsevier. Identifying stack-driven indoor environmental problems and associated pressure difference in high-rise residential buildings: Airflow noise and draft That may not sound like much, but 25 pascals is enough to create an audible rush of air through the gaps around a standard door and make the door noticeably harder to open or close.
On the lobby level of a tall building, the problem is often worst. Cold outdoor air gets sucked in forcefully through the main entrance, creating a blast of wind that makes the lobby uncomfortable for anyone standing nearby. Elevator doors on lower floors can struggle to close against the pressure, and the rushing air picks up dust and pollutants from outdoor sources and channels them through the building’s interior.
How Buildings Fight Back
The most visible countermeasure is the revolving door. By keeping the building entrance sealed at all times, even as people pass through, a revolving door prevents the direct exchange of indoor and outdoor air that a conventional swinging door allows. In theory, this should cut off the stack effect’s main air supply at ground level. In practice, the benefit is limited. A study on revolving-door performance found that while they reduce drafts at the lobby floor, their effect on the building as a whole is modest because air finds alternative paths through loading docks, parking garage ramps, and other unsealed openings throughout the structure.3Journal of Asian Architecture and Building Engineering. Characteristics of Revolving Door Use as a Countermeasure to the Stack Effect in Buildings A separate analysis reached a similar conclusion: revolving doors help locally at the entrance, but addressing the building-wide problem requires considering the pressure distribution across all walls and openings.4Indoor and Built Environment. Quantitative Reduction Method of Draft in High-Rise Buildings, Using Revolving Doors
A more aggressive strategy targets the vertical shafts themselves. One approach studied in a Korean high-rise involved cooling the air inside elevator and ventilation shafts. When shaft air was cooled from 22 °C down to 12 °C, simulations showed the stack effect pressure dropped by roughly 27 percent. Real-world measurements in the same building confirmed that wind velocity through elevator doors on the lobby floor fell by about 25 percent, with smaller reductions of around 10 percent on upper floors.5Elsevier / Building and Environment. A study on the development and application of the E/V shaft cooling system to reduce stack effect in high-rise buildings The logic is straightforward: if you cool the shaft air so it is closer to the outdoor temperature, you shrink the density difference that drives the flow. The tradeoff is the energy cost of running a cooling system inside the shafts, which adds to the building’s operating expenses.
Other mitigation tactics include compartmentalizing stairwells with vestibules, sealing elevator lobbies with airtight doors on each floor, and tightening the building envelope overall. No single measure eliminates the problem in a very tall building, so most designs layer several approaches together.
Fire and Smoke Transport
The stack effect becomes genuinely dangerous during a fire. In cold weather, airflow travels upward through a building’s vertical shafts, and smoke from a fire on a lower floor can ride that current to upper stories far from the fire’s origin. Occupants on high floors may encounter smoke before they realize the fire is many stories below them. Stairwells, which are supposed to serve as escape routes, can instead become smoke-filled chimneys.
In subway tunnels and underground passages, the dynamics are similar but the geometry creates its own complications. Sloped tunnels generate a stack effect because air at the higher end of the slope is at a different elevation than air at the lower end, producing a pressure difference even without mechanical ventilation. Experiments on tunnel fires found that the slope of the tunnel had a stronger influence on how far smoke traveled backward against the natural airflow than the intensity of the fire itself.6Building Services Engineering Research and Technology. Investigation of the fire hazard of underground space fire scenarios in urban metro tunnels under natural ventilation: Analysis of the impact of tunnel slope on smoke back-layering length Additional research on subway station passageways developed predictive models for how the stack effect moves smoke based on the fire’s heat output and the elevation change within the passage.7Case Studies in Thermal Engineering. Prediction of smoke backflow length and smoke outflow rate in subway station passageways under weak stack effect
Fire engineers design smoke control systems to counteract the stack effect, using pressurized stairwells and mechanical ventilation to keep escape routes clear. But these systems have to account for the fact that the natural airflow direction reverses between seasons, which means a system calibrated for winter conditions may behave differently in summer when the reverse stack effect takes hold.
Disease Transmission Through Vertical Shafts
One of the more unsettling consequences of the stack effect came into focus during infectious disease outbreaks in high-rise buildings. If contaminated air from one floor enters a vertical shaft, the stack-driven airflow can carry pathogens to other floors. A study of a high-rise hospital found strong evidence that airborne contaminants released by infected patients on lower floors could travel to upper floors through the building’s vertical chutes, driven by the stack effect.8PubMed Central. Predictions and measurements of the stack effect on indoor airborne virus transmission in a high-rise hospital building
During the COVID-19 pandemic, this pathway received renewed attention. Investigators in Hong Kong identified at least 15 outbreaks of COVID-19 in high-rise residential buildings linked to aerosols traveling through building drainage stacks. Wastewater containing viral particles generated aerosols inside the drainage pipes, and the chimney effect within those pipes pushed the aerosols upward and into apartments through pipe leaks or dry drain traps.9PubMed Central. Aerosol transmission of SARS-CoV-2 due to the chimney effect in two high-rise housing drainage stacks This was not airflow through corridors or elevator shafts but through the plumbing system itself, a pathway most residents would never think about. The cases provided direct evidence that the chimney effect in drainage pipes could facilitate long-range aerosol transmission of a respiratory virus.
These findings reinforced the importance of maintaining water seals in floor drain traps, which act as barriers preventing sewer gases and aerosols from entering living spaces. A dry trap, common in bathrooms that go unused for extended periods, opens a direct path from the drainage stack into the apartment.
Underground Spaces and Subway Climatology
The stack effect is not limited to buildings that reach into the sky. Underground spaces, including subway systems and caves, experience the same physics in a different orientation. In subway systems, the tunnels connect to the surface through stairwells, ventilation shafts, and elevator shafts. Because temperatures underground tend to be higher than outdoor temperatures from late summer through winter, the warmer subway air rises through these openings, pulling cooler outdoor air into the tunnels. Researchers have found that without mechanical ventilation or the piston effect of moving trains, the stack effect becomes the dominant driver of airflow in subway stations, pushing air toward stairwells and ventilation shafts. The pressure differences generated by the stack effect in these conditions are much larger than those produced by other natural forces like wind.10PubMed Central. Environmental and Health Effects of Ventilation in Subway Stations: A Literature Review
In natural caves, the effect goes by “chimney effect” and is the most common driver of airflow through karst systems. The outside air enters the cave, and as it travels deeper, it approaches thermal equilibrium with the surrounding rock along a characteristic distance. The resulting temperature and density contrast between the cave air and the outside air determines how strongly air is pulled through the system.11PubMed Central. How do caves breathe: The airflow patterns in karst underground On cold winter days, warm cave air rises out of upper entrances while cold air pours in through lower ones. In summer, the direction can reverse. Cavers and cave scientists often feel this as a noticeable breeze blowing through passages, and it plays a critical role in transporting moisture, carbon dioxide, and radon gas through cave networks.
The Stack Effect in Nature
Long before humans built skyscrapers, other organisms had already been exploiting the same physics. Termite mounds are perhaps the most famous natural example. The mounds of certain African and Asian species feature elaborate networks of internal tunnels and surface conduits that facilitate gas exchange for the colony buried below. The most widely accepted mechanism for ventilation in these mounds is solar-powered convection: sunlight heats the thin outer walls of the mound during the day, warming the air in surface conduits and causing it to rise. This pulls cooler, COâ‚‚-rich air from the nest upward and outward, drawing fresh air in from below. At night, when the mound surface cools below the nest temperature, the airflow reverses but ventilation continues.12PubMed Central. Termite mound architecture and climate control: a review of X-ray tomography and flow field simulation approaches
Direct measurements inside Macrotermes mounds in Africa have shown that the solar-driven convection is not uniform. The eastern side of the mound warms first in the morning as the sun hits it, followed by the northern and then western sides, while the southern face stays coolest. This sun-tracking pattern means that air in some surface conduits flows downward during the day, specifically on the shaded side, adding complexity to what might seem like a simple chimney.13Journal of Experimental Biology. Solar-powered ventilation of African termite mounds The mound’s architecture effectively acts as a passive solar ventilation system, one that adjusts airflow direction throughout the day without any moving parts.
Even mushrooms use a version of convective airflow. The cap of a mushroom releases water vapor, which cools the surrounding air through evaporation. That cooler, denser air sinks and is replaced by warmer air rising from below, creating a tiny convective cell. Research has shown that this self-generated airflow can carry spores upward at speeds of centimeters per second and lift them 10 centimeters or more off the ground, enough to catch ambient breezes and disperse.14PubMed Central. Mushrooms use convectively created airflows to disperse their spores For a mushroom fruiting in still air on the forest floor, this convective trick is essential for getting spores away from the parent organism.
Solar Chimneys and Passive Ventilation Design
Engineers have borrowed from the same physics to design passive ventilation systems that reduce the need for mechanical air conditioning. A solar chimney is essentially a channel attached to a building that absorbs sunlight, heats the air inside, and uses the resulting buoyancy to draw fresh air through the building. The concept has been around for centuries in traditional architecture across hot climates, but modern computational modeling has quantified how different design variables affect performance.
A recent simulation study found that raising the temperature of the absorber wall in a solar chimney could increase the flow of incoming air by up to 1.9 times, while increasing the window area at the chimney outlet produced the strongest effect of all, boosting airflow by up to 9.2 times.15Energy Science & Engineering. Passive Solar Chimney Ventilation Efficiency in a Single Enclosed Space In practical terms, a well-designed solar chimney can ventilate a room without electricity by simply being oriented to catch sunlight and sized with a large enough outlet. The principle is identical to the stack effect in a building shaft, except here the buoyancy is deliberately harnessed rather than fought against.
Coastal Vegetation and Wave-Driven Flows
Stepping further afield, the word “stalk” sometimes appears in discussions of how plant stems interact with water flow in coastal and estuarine environments. Although this is a different physical system from the thermal stack effect in buildings, the way vegetation modifies flow patterns shares a conceptual thread: structures protruding into a fluid alter the movement of that fluid in predictable ways.
Aquatic plants reduce water velocity within the vegetation canopy while increasing turbulence. Flume experiments with submerged vegetation showed that plants create a zone of slower flow within the canopy and enhanced turbulence below the canopy top. When waves were added to the current, the combined effect dramatically increased flow speeds above the vegetation and boosted turbulence throughout the water column.16Estuarine, Coastal and Shelf Science. Wave and vegetation effects on flow and suspended sediment characteristics: A flume study The drag that plant stalks impose on passing waves is one reason coastal marshes, mangroves, and seagrass beds provide natural shoreline protection. A re-analysis of wave attenuation studies across kelp, mangrove, marsh, and seagrass habitats found that vegetation characteristics and the way drag changes with flow conditions explain much of the variation in how effectively different habitats dampen waves.17Ecosphere. Quantifying wave attenuation to inform coastal habitat conservation
Different plant species achieve this protection through different strategies. Some, like the stiff-stemmed Schoenoplectus maritimus, tolerate high drag forces by investing in rigid, lignin-rich stalks. Others, like the more flexible Schoenoplectus tabernaemontani, avoid stress by bending with the flow, reducing the forces they experience while still slowing the water around them.18PLOS ONE. Ecosystem Engineering by Plants on Wave-Exposed Intertidal Flats Is Governed by Relationships between Effect and Response Traits From a coastal management perspective, the practical takeaway is that the structural traits of plant stalks directly determine how much wave energy a habitat can absorb, which feeds into decisions about which species to plant for shoreline restoration.
Airtightness and Energy Waste
For anyone paying heating or cooling bills, the stack effect is fundamentally an energy problem. Every cubic meter of conditioned air that leaks out of a building through the stack effect has to be replaced by unconditioned air that needs to be heated or cooled. Airtightness, defined as a building envelope’s resistance to air leakage through unintentional gaps, is the first line of defense.19Elsevier. Assessing the airtightness performance of container houses in relation to its effect on energy efficiency The three main forces driving infiltration through those gaps are the stack effect, external wind, and mechanical ventilation imbalances, and in tall buildings during extreme weather, the stack effect typically dominates.
Older high-rises with poor sealing can lose enormous amounts of energy to stack-driven infiltration. Modern building codes in many countries now mandate blower-door testing and minimum airtightness standards specifically because of this. Passive house standards, which originated in Germany, push airtightness to extreme levels partly to minimize the energy penalty from uncontrolled airflow. For homeowners in single-family houses, the stack effect is gentler but still present: warm air leaking out of the attic in winter draws cold air in through the basement, which is why insulating and air-sealing the attic is one of the most cost-effective energy upgrades available.