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rafter through ductwork


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The first picture is a return air duct that was rigged around a roof rafter in a new construction home.


The second picture is a supply air plenum that was installed over a ceiling joist (joist runs through the plenum to the other side).


My question is have you guys seen this before and how do you report it, if at all, provided that it is properly sealed around the framing members. Thanks.

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I have never seen anyone attempt anything like this. IRC says that duct material has to meet specific performance requirements. Since this is IN the duct, I would suspect that a AHJ who was thinking at all would question this installation. Sec. M1601 of the IRC gives the flame spread requirements. Obviously wood would not meet those requirements. In addition to all that, common sense would dictate that the wood in the ducts would be subject to different heat and humidity conditions that just couldn't be a good thing. I guess I would call this out and recommend consulting a heating professional.

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Darren is correct. 2000 IMC

602.3 Stud cavity and joist space plenums. Stud wall cavities and the spaces between solid floor joists to be utilized as air plenums shall comply with the following conditions:

1. Such cavities or spaces shall not be utilized as a plenum for supply air.

602.2.1 Materials exposed within plenums. Except as required by sections 602.2.11 through 602.2.1.4, materials exposed within plenums shall be noncombustible or shall have a flame spread index of not more than 25 and a smoke developed index of not more than 50 when tested in accordance with ASTM E 84.

Jeff Euriech

Peoria Arizona

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Check with your AHJ. Regardless of the code, lots of municipalities allow stud and joist bays to be used for return air. I've seen plenty of homes around here like that. In fact, I'll go so far as to say that I think the majority of homes with forced air that I look at are configured that way. It's been allowed around here since the 1940's.



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I think I may need to clarify. In both cases they are not using joist or stud bays for the return air, they "carved" the duct board so that there ducts would fit around the rafter in pic 1 and around the ceiling joist in pic 2.

Pic 1 I could almost see not making a stink about since it is minimal and is thoroughly gooped with mastic (not pulling in any air). But in,the second pic, they carved a 2"x6" notch on two sides of the supply air plenum and set it on top of the HVAC unit in the closet below leaving the ceiling joist to run directly through to the other side of the plenum.

Steve, that "dryer vent" is their half-hearted attempt to provide the lower combustion air. The only problem....it was sitting on the decking. Thanks for the responses everyone.

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