I currently have uninsulated HVAC ductwork running the width of my garage. There's no gap between the ducts and the plaster ceiling above it. There's also very little clearance between the ducts and vehicles.

Based on some recent plumbing work I've done, I doubt there is any insulation between the garage plaster ceiling and the living space above the garage.

Most guides I've reviewed start with the assumption there is a gap where insulation can be wrapped entirely around the duct. Since that really isn't an option here, how can I successfully insulate them ?

  • Possibly inject foam into the joist space, but that has to be not overdone, as overdoing it can cause problems. On the underside just pick the highest R-per-inch foamboard you can find (usually polyisocyanurate, but see what's available)
    – Ecnerwal
    Sep 28, 2019 at 15:52
  • Ahh so basically wrap it like a “U” with foam board instead of wrapping ? Sep 28, 2019 at 16:18

1 Answer 1


There are different profiles of duct available. In height restricted areas you can switch to a rectangular flat profile which might give you the ability to better insulate.

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