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Thermal draft effect in high-rise buildings

Chimney effect

The thermal draft effect is a major challenge for skyscrapers, but it can also be a significant factor for buildings with two or more floors. The structure acts like a giant chimney, effectively channeling the hot air upward until it eventually leaves the structure completely. Generally these phenomena are favored by the open stairwells distributing all the levels of a building.

The chimney effect occurs when the outside temperature is significantly lower than the inside temperature. Cold air is denser than warm air, so when cold air enters the structure from below, it replaces the warm air above. This creates an airflow that draws in more cold air and intensifies the drafts. The higher the structure, the stronger this airflow. This is why revolving doors were developed shortly after the first skyscrapers. The suction force at ground level was so strong in winter that people had trouble opening the doors! We still find this phenomenon today for the elevators which sometimes connect the lower parts of the building with the roof areas for the ventilation of the hopper. However, in the presence of wind (see effect of draught combined with wind), the draught becomes such that the elevator doors can be blocked or important whistling can appear in the airlocks of the lower levels.

The obvious problem is that the treated indoor air is lost, and therefore wastes energy. But another factor is that this problem can get worse over time. If the airflow is particularly strong, it puts pressure on fine parasitic air inlets, causing the seals to crack and creating new vulnerabilities. With sustained pressure, these gaps can widen and expand, intensifying the airflow and accelerating energy loss.

The stack effect works because the warm air has to go somewhere when it reaches the highest level of the building. In many cases, it escapes into the attic through cracked ceilings, leaky ductwork, recessed light fixtures, or simply through excessive air permeability of the attic floor. Once the hot air reaches the last level, the air escapes to the outside through any small vulnerability it can find.

Neutral pressure plane

The neutral pressure plane is an imaginary horizontal plane, where the internal pressure is equal to the external atmospheric pressure. At this altitude, the pressure difference between the inside and the outside is zero, the air does not enter or leave the building. The neutral pressure point depends on the building’s HVAC systems which pressurize or depressurize depending on their distribution area in the building.

CFD simulation - La Défense - Paris

Draw effects combined with the wind

CFD simulation - La Défense - Paris

Beyond the thermal effects of the processes, the depressions at the outlets are the driving force behind the natural ventilation of a building. To this, it should be understood that for high-rise buildings, the depression in the roof is strong enough to convert all the openings in the lower part into an air inlet, even on the facades downstream from the wind.

Examples of CFD simulation applications

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Example of CFD simulation projects:


CFD design office

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Losses – CNIT