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HVAC guys are running a vertical duct (supply and return) from my basement to my 2nd floor attic. It will run adjacent to an exterior wall and will eventually be enclosed in a drywalled-in vertical soffit. My question is in regards to the insulation in the exterior wall cavity: Would the metal of the ducts touching the kraft-faced insulation be enough to provide the fire protection that kraft-faced insulation needs?

Or should I go with unfaced insulation and just use 6 mil poly as my vapor barrier? Or maybe a combination of unfaced insulation and a foil faced foam board?

  • Can you simply cover it with fire-taped drywall? That would make me feel better than the other alternatives. – bcworkz Mar 10 '13 at 20:44
  • You should go ahead and type that up as a formal answer since that's what I ended up doing. I didn't want to drywall behind it because I was already squeezing a 16" duct into a 16" spot, but I can make the extra 1/2" (from the drywall thickness) work so I might as well go with something I know for sure the inspector won't think twice about. – milkboneUnderwear Mar 10 '13 at 22:00
  • Thanks milkbone, done. I'm usually reluctant to enter quick comments as answers, but there really isn't much else to say this time :) – bcworkz Mar 11 '13 at 21:34
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You should simply cover it with fire-taped drywall. That would make me feel better than the other alternatives. It's really as cheap and easy as anything and will do the job without question.

  • On a related note, I ended up using 3/4" plywood as fireblocking above the 2" (pink) foam board above my interior basement/foundation walls. Drywall is also typically used here but I'm not ready to drywall that area yet, so I went with 3/4" plywood because the city (Madison, WI) website says that 3/4" plywood meets code the same way drywall does. I'll probably use drywall everywhere else down there, but in a pinch the plywood should be fine. – milkboneUnderwear Mar 12 '13 at 17:41

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