Stack effect
Stack effect is air movement caused by density and pressure differences between warmer and cooler air columns. In buildings, it can pull air in through lower leaks and push air out through upper leaks in cold weather, or reverse direction in some cooling conditions, affecting comfort, energy use, moisture, smoke, odors, and ventilation.
What stack effect is
Stack effect is the movement of air caused by buoyancy. Warm air is less dense than cool air, so a warm indoor air column can rise through a building, shaft, stairwell, flue, or chimney. As air exits high in the building, replacement air is drawn in lower down. The same physics can be useful for natural ventilation, troublesome for uncontrolled air leakage, and dangerous when it moves smoke or combustion gases.
Winter pattern
In cold weather, heated indoor air is usually warmer and lighter than outdoor air. It tends to rise and leak out through upper openings such as attic bypasses, roof penetrations, shafts, stairwells, and high windows. That outward flow creates lower pressure near the base of the building, pulling colder outdoor air through basements, crawlspaces, rim joists, doors, and lower wall leaks.
Summer and reverse stack effect
In hot weather with air conditioning, indoor air may be cooler and denser than outdoor air. Some buildings can experience reverse stack effect, with air moving downward and leakage patterns changing direction. The effect is often weaker than winter stack effect in many climates, but it can still bring humid outdoor air into cooled assemblies or pull warm air through upper leaks.
Height and vertical paths
Stack pressure grows with building height and temperature difference. Tall buildings, atriums, elevator shafts, stairwells, mechanical shafts, chimneys, and open chases can create strong vertical air pathways. Even low-rise homes can have meaningful stack effect if they have large attic leaks, open wall cavities, chimney chases, or a connected basement-to-attic path.
Neutral pressure plane
The neutral pressure plane is the height where indoor and outdoor pressures are roughly equal. Below it, air may tend to leak inward; above it, air may tend to leak outward during winter conditions. The plane shifts depending on leak distribution, exhaust fans, supply fans, open windows, chimneys, wind, and mechanical ventilation.
Energy and moisture
Stack-driven air leakage can increase heating and cooling loads because conditioned air leaves and unconditioned air enters. It can also carry water vapor into walls, roofs, attics, and shafts. If the air cools below its dew point inside an assembly, condensation can damage insulation, framing, finishes, or indoor air quality.
Ventilation uses
Designers can use stack effect intentionally for natural ventilation by placing low inlets and high outlets, sometimes with atria, vents, clerestories, solar chimneys, or stairwell exhaust paths. Intentional stack ventilation needs controllable openings, weather protection, fire and smoke strategy, security, acoustics, and a backup plan for still or extreme weather.
Controlling unwanted stack effect
Air sealing is the first defense against unwanted stack effect. Sealing attic bypasses, chases, shafts, rim joists, and top and bottom plates can reduce the vertical leak paths that drive the problem. Balanced ventilation, compartmentalization in multifamily buildings, vestibules, pressure control, and correctly operating exhaust systems also help manage stack-driven flows.
Why it matters
Stack effect matters because it explains why air leaks are not random. The same crack may behave differently by season, height, wind, and fan operation. Understanding stack effect helps diagnose drafts, odors, smoke movement, moisture problems, hard-to-heat rooms, elevator-door pressure issues, and why air sealing near the top and bottom of a building can have outsized value.