What is the reverse stack effect primarily caused by?

The reverse stack effect is primarily related to density differences between indoor and outdoor air. This phenomenon occurs when there is cooler indoor air that is denser than the warmer outdoor air. When the indoor air cools down (for instance, when heating systems are turned off during the winter), it creates a situation where the cooler, denser air tends to sink, while the warmer, less dense exterior air rises.

This temperature gradient can lead to pressure differences within the building, which in turn drives airflow from the outside into the building at lower levels and pushes the warmer air out at higher levels, creating a reverse stack effect.

Understanding this concept is crucial for building performance professionals as it influences the design and operation of ventilation systems and can significantly impact energy efficiency and indoor air quality.

In contrast to other choices, the equal density of indoor and outdoor air would not create any stack effect, nor would pressure differences alone explain the mechanics at play without considering the role of temperature and density.