In which type of walls are stud wall cavities prohibited from being used as air plenums?

Stud wall cavities are prohibited from being used as air plenums in outside walls primarily due to concerns regarding energy efficiency, moisture management, and the integrity of the building envelope. Outside walls are designed to provide a thermal barrier between the interior environments and external elements, such as weather conditions and humidity. Utilizing these cavities for air movement could compromise this function by allowing unconditioned air to infiltrate, thereby increasing energy costs due to inefficient heating and cooling and potentially leading to condensation issues within the wall assembly.

Moreover, when air is routed through these cavities, there is a risk of introducing contaminants from the outside environment into the indoor spaces, which could negatively impact indoor air quality. The mechanical code aims to ensure that outside walls maintain their primary purpose of providing insulation and protection from the elements, and not to be used as pathways for air distribution. By prohibiting air plenums in outside walls, the code promotes overall building performance and occupant comfort.