What Is the Stack Effect?

The stack effect is the trend for warm air to move upwards in a building, driving circulation and creating pressure differentials. It is driven by differences in air density and must be accounted for in the design and maintenance of structures. With very large buildings like skyscrapers, it can be a significant issue. The stack effect can add to heating and cooling expenses, for example, by causing heat loss at the top of a building, especially in the cold winter months.

Warm air is less dense, and tends to move upwards in a building while the cooler air sinks. This process can be accelerated with ventilation, including opening doors and windows as well as running fans. In very tall structures, it can be particularly acute and to some extent unavoidable. At the upper stories, the air can escape. This creates positive pressure, as air pressing in from the outside cannot get into the building because of the pressure of the escaping air.

Meanwhile, at the bottom of the building, a negative pressure environment is created. The air pulling up towards the roof creates an opening for cooler air to move in from the outside. This can cause the lower stories to be much cooler. In a poorly designed building, people may close windows and turn up heat to address this, which can accelerate the stack effect by heating air and forcing it upstairs to escape, drawing in more cold air.

Some circulation of air is desired. The movement of air inside a building can keep it fresher and make it a more pleasant work environment, especially when fresh oxygenated air cycles in from the outside. Designers can control for it by adding insulation to limit the loss of air, installing shielding on windows to minimize heat transfer, and advising occupants of the building on climate control. For example, isolating a lobby to limit blasts of cold air when people open the doors can help.

For homes and small structures, the stack effect shouldn’t play a significant role unless the structure was not well built or maintained. Some leakage is inevitable, but the biggest problems, like seeping around window frames and skylights, can be resolved with measures like better flashing. It can also help to design entries and doors thoughtfully to discourage the flow of air into a house during the winter months. Designing with the stack effect in mind allows architects to create homes that stay ventilated, but do not lose excess heat.