Under which condition is it acceptable to use stud cavities for return air ducts?

The use of stud cavities for return air ducts is acceptable if the ductwork does not go from one floor level to another. This is primarily due to building codes and safety considerations. Stud cavities are typically located within wall assemblies, which can create challenges when transferring air across different levels. Without a clear pathway and appropriate construction methods to ensure adequate airflow and prevent air contaminants from traveling between floors, the ducting could facilitate the spread of odors and pollutants.

Additionally, if a return air duct were to go across floors without properly designed ducts, the risk of creating pressure imbalances and inefficient airflow increases. This can lead to various issues, including poor indoor air quality and increased energy consumption as HVAC systems work harder to maintain temperature and airflow.

In contrast, considerations such as maximum temperature, potential for condensation, and the specific use of spaces like kitchens and bathrooms typically influence duct design and material choices, rather than the fundamental acceptability of using stud cavities for return air ducts in a structural sense. Hence, while there might be additional guidelines pertaining to these factors, the most critical condition for using stud cavities relates directly to the vertical airflow implications.