Thread: flex ducts in concealed space?

I'm getting conflicting info on this one. Are flexible ducts allowed to be installed in a concealed space - like in a floor joist bay that runs between two finished levels in a house?

I had never really thought about it since I don't see a lot of concealed spaces as I inspect houses but I'm running ductwork for an addition I'm putting on and can't get a straight answer.

Duct supplier says no they have to be sheetmetal (big surprise, he sells sheet metal and fabricates ducts). A couple calls to local HVAC contractors left me more confused then when I started. Just lots of 'maybe'

An inspector friend in a neighboring county to where I live said no problem as long as they're not penetrating a fire separation wall.... of course.

Anyhow, the attached pictures are of the metal ducting at the end of the existing house which I plan to tap into and extend to the new rooms (pictured at framing stage). Flex ducts would be much easier.... I'm calling my AHJ in the morning. I'm just curious what you guys think.

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Yes you can run lex duct. But in saying that you are wise to check with the city inspector. I inspect new homes constantly and the flex duct runs in the ceiling and the vents are in the ceiling.

Originally Posted by Matt Fellman

I'm getting conflicting info on this one. Are flexible ducts allowed to be installed in a concealed space - like in a floor joist bay that runs between two finished levels in a house?

Yes.

Just make sure they are supported properly, not being crushed, etc.

Thanks guys... it turns out I'm going with rigid. The cost is not much different for the short distance and installer says the flow will be better since the inside is smooth. I'm not sure how much difference to expect but if for no other reason I like the durability.

i like how the rigid breaks down and adds fibers to the air.