Plenum

[Woodrow]

I have a contractor in my jurisdiction that is wanting to use the space between the 1st and 2nd story of a residential home as a return air plenum due to space restraints. All the duct work for supply air is in place. The contractor used web truss in place of solid lumber. Has anyone else ran into this? Is it allowed?

 

[Sifu]

There are conflicting answers to that question, which you need to speak with your AHJ about. Unsure of exactly what the methods you are proposing are, but using the 2018 IRC, there are provisions with limitations for them at 1601.1.1. However, again with the IRC (and IECC) using any building cavity as a duct or a plenum is prohibited by 1103.3.5. So depending on the adopted codes and the opinions or interpretations from the AHJ the answer could go either way.

 

[klarenbeek]

You will also run into issues with the electrical code regarding wires, lights, boxes, etc. in the ceiling cavity, plus anything else in the floor system that may be a problem, including openings into places where taking return from is prohibited. No gas appliance vents, No no openings into mechanical rooms, etc.

Based on the prohibition of 1103.3.5 we only allow stud cavities and solid floor joist cavities as described in 1601.1.1 to be used as return plenums in a residence because they are specifically called out as being allowed.