Technical Portal for Sustainable Architecture
What is the stack effect and why does it affect the health of a building?
The stack effect is one of the most powerful and complex thermodynamic drivers governing building physics. Often overlooked or dismissed as a mere spontaneous ventilation phenomenon, this convective flow significantly impacts the durability of structures, heat loss, and indoor air quality. For contemporary designers, understanding and managing the dynamics of the stack effect is a fundamental requirement to ensure compliance with NZEB standards and preserve the integrity of materials over time.
How the chimney effect works: physics at the service of comfort
The chimney effect (or thermosiphon effect) is a natural physical phenomenon based on the difference in density between hot and cold air. Inside the air gap that we separate between the thermal insulation and the VM Group external cladding, this cycle activates automatically, following five precise phases:
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The sun heats the external cladding: The panels (in stoneware, stone, or metal) absorb the solar radiant heat.
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The air in the cavity heats up: The heat accumulated by the cladding is transmitted to the layer of air enclosed in the ventilated chamber.
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The hot air rises naturally: Becoming lighter than the outside air, the heated air rises, creating buoyancy generated by the temperature and pressure differential.
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New air enters from below: At the base of the building, special perforated grilles continuously introduce fresh air from outside to compensate for the air that has risen.
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The continuous cycle (Dynamic Ventilation): This movement generates a perpetual upward flow that continuously removes excess heat and water vapor.
Insulation during the winter
The presence of the insulating layer on the outside of the wall achieves the best result in terms of phase shift and dispersion of the heat produced internally.
Weather protection
The presence of the cladding prevents rain from reaching the insulating layer, thus avoiding the accumulation of humidity on the walls of the building.
Isolation during the summer
The presence of a screen distant from the insulating layer reduces the amount of heat radiation incident on the insulation itself.
Sound insulation
The correct choice of facing increases the effectiveness of the wall in terms of transmission of airborne noise inside the home.
The physics of the stack effect: pressure gradients and temperature differentials
The stack effect is governed by precise physical laws related to the difference in density between the internal and external air columns, induced by the thermal gradient (Delta T). The warm, less dense air moves toward the upper floors, generating an increase in pressure (overpressure) at the top of the envelope and a geometrically mirrored depression at the bottom. The inversion point of these forces, called the Pressure Neutral Plane (PNP), determines where the air tends to enter or exit the building, a crucial factor for calculating infiltration and exfiltration flows.
Impact on structural durability: interstitial condensation and vapor migration phenomena
From a building disease perspective, an uncontrolled stack effect is one of the main causes of interstitial condensation. During the winter months, overpressure in the upper part of the building pushes warm, moist air through micro-cracks in the building envelope. When it encounters the colder layers of the insulation package or structure, the moisture condenses, triggering rotting, deterioration of load-bearing elements (especially in wooden structures), and loss of effectiveness of insulation materials.
Thermodynamic control of the envelope: airtightness and thermal stratification
In multi-story buildings, the stack effect accentuates thermal stratification, compromising overall comfort and overloading air conditioning systems. To mitigate this phenomenon, modern design must focus on maximum airtightness of the building envelope (verified by the Blower Door Test according to UNI EN ISO 9972) and vertical partitioning of spaces (stairwells, elevator shafts). The goal is to stabilize the Neutral Pressure Level and optimize the efficiency of Controlled Mechanical Ventilation (CMV) systems with heat recovery.
Passive ventilation strategies: the stack effect as a resource for summer comfort
Despite the winter challenges, the stack effect is a valuable resource for passive cooling in the summer (night purging). By integrating solar chimneys, ventilated facades, or automated skylights at the top of the building, it is possible to activate the natural extraction of heat accumulated during the day. This strategy drastically reduces the primary energy requirement for cooling, aligning the project with the most stringent energy sustainability protocols.
Conclusions: Designing for longevity
Considering the stack effect from the preliminary design stages or during renovations is not optional. A building capable of managing natural airflow is healthier, more efficient, and designed to last longer, while also ensuring a higher quality of life for its occupants.
