Can R-30 insulation catch fire if placed over flex ducts? I do not have air conditioning - only heat. Thanks, Victor

  • 1
    What insulating material are we talking about here? Jun 3, 2018 at 21:22
  • 2
    And what sort of duct? Most ductss do not carry anything nearly hot enough to start a fire.
    – isherwood
    Jun 3, 2018 at 21:45
  • Do be careful how it is placed, flex ducts can be compressed flatter restricting air flow, with enough weight on top
    – Jack
    Jun 4, 2018 at 0:12

1 Answer 1


The air in a properly sized heating supply system should only reach somewhere between 100 and 150 deg if all is well. That temp is too low to catch anything on fire. Not only that, most flex ducting comes with insulation already integrated in it.

  • That’s good “design” to keep the ductwork on the “living” side of the insulation envelope. In fact, if you can extend the insulation up tight to the side of the duct and then lay a blanket of insulation over the top of the duct, that’s better. (You’ll eliminate the little triangle area on each side of the duct. Otherwise, your duct will just heat that space too from heat loss through the duct.)
    – Lee Sam
    Jun 3, 2018 at 19:28

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