Hi Gang, new 2024 build in Florida. Normally I call out Flex Duct in a garage due to fire separation. I know the whole 26-gauge metal or 1" ductboard is the standard bit, but code R302.5.2 also says “other approved material.” Would a “Class-1” Flex Duct be acceptable here? Haven’t been able to get a straight answer on Google. I’m sure it would be okay if fire dampers were installed, but I can’t verify those. Thanks for your help!
I wouldn’t have a problem with it.
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Good question which makes it impossible for us to inspect to code without having full access to the code inspector/approved plans.
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Physically check the duct. Sometimes it is rigid duct with insulation (looking like flex duct), which I would not have concerns about.
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In my opinion, it is not suitable for use in your application. I looked at the manufacturer’s website and none of their offerings have a rigid metal interior.
I did a Google search for “flex duct to garage” and came up with the below file. It is from a Fire Marshal in NC, so although that is not your area, I still think it is a viable resource.
My recommendation would be to build a drywall chase around it or swap it out with a more appropriate duct.
https://www.ncosfm.gov/residential/030252-flexible-air-duct-garage-single-family-dwellings-0/open
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I’d probably recommend clarifying a proper install with the building department or, if I could find the HVAC company info, I might just make a call. I’ve done this many times over the years and the techs are usually very helpful. “Hi, help me understand this installation…” and they’ll usually have a good explanation and you learn something for next time. Liability-wise this is pretty low… it was just permitted.
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This is a fire code issue, not a mechanical/HVAC issue. 
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