Just a general discussion. Do any inspectors call out wood plenums/ducts? Is it common or acceptable in your area? What concerns might you have?
Below are a couple of photo’s of wood returns. The second photo, we obviously have a problem.
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Yes, the buyer needs to know what they are buying. For all the obvious reasons, dedicated ductwork is preferred. Your milage may vary 
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Using the joists as in your 2nd pic is very common for return air in my area. I have it in my home as well. I don’t include it in reports.
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In my area as well, except the joists are always sealed on bottom with sheet metal.
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At the minimum, I would say that would be the preferred method as we can see the results in photo 2 
This is all about when the house was built. It is obvious this is an older house. If there are no other issues (leaking air sucking in Radon, Mold, Outdoor Air), I would not be playing the “Code Game” about this. There are hundreds of thousands of houses like this out there…
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Agreed!
Secondly, do you think these wood plenums are prone to moisture issues such as fungal/microbial type growth? If so, is it notable (not as a defect but more as an FYI?)
As you know, “conditioned air” is moving through the plenum, not OA. It is the worst case indoor air that needs conditioning, but it’s humidity is well below the OA moisture that prevents condensation and fungal growth. The SA is more prone to problems when they get dirty, than the return.
Take a look at the psycrometrics properties of that air.
Leaks are the primarily issue, but wood duct is easily repairable.
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Thanks for that information. As in the above photo’s, the return is passing through an unconditioned space, does that increase the risk of condensation and fungal growth on the inside of the plenum or likely a non-issue? Or, no way to tell because of the numerous variables?
There is wood in all returns because of the studs or joists. So, no I did not call the wood “returns” out, if they were in good shape.
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Exactly. Panned off spaces between floor joists are very common here in Michigan.
Although I frequently find the Thermopan product has been damaged and always recommend repair/replacement.
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That looks like particle board in the first photo. Particle board would be the worst option besides OSB or cardboard.
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