My client said the city inspector told him that the return vent for the HVAC system can’t be made out of wood. I think he was referring to the exhaust register. But I was looking in the code book with no luck. Anybody know the answer?
2018 IRC, not sure what your state is using.
That is still under construction so I do not know what the final product will be. They may be adding a filter cabinet.
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My guess it’s referring to an all wood plenum. Which I don’t see here…
Right…they have to finish that out somehow. (which is why I mentioned a filter cabinet). Or, just drop it down flush with the ceiling edge.
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IMO, they are referring to the stud and joist bays being used as return chases, as they have for decades. The change is that they now must be lined.
As for the plenum register frame, what do you expect them to use, steel? It will be finished out when the plenum in installed. It’s a non-issue.
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Sort of in the same mindset. Maybe the finished set is a metal cabinet as opposed to just a cover… 
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What I typically see is a return air filter box made of sheet metal. It’s a purpose made box for a certain dimension filter. There are components built into the filter box to accommodate a hinged grill.
Looks like they cored a hole in OSB and smeared duct mastic on it to seal the flex duct. That also means it isn’t insulated as required.
