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Open cell on roof deck in michigan, I’m so confused…

shrews01 | Posted in General Questions on

I’m looking for some guidance on insulating my attic. Location is Michigan near Detroit. I am getting quotes on spray foam in my attic. After doing generic research online, I am seeing that open cell on the deck should have a vapor barrier, but every installer around me says it doesn’t need it. I am having trouble with what to believe (I could be reading the wrong articles). My duct work all runs through the attic, so I thought spray foam on the roof deck would be best. Now I’m not sure what to do?

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  1. Jon_R | | #1

    To convince them, look up your code, specifically something like:

    In Climate Zones 5, 6, 7 and 8, any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation.

    Also investigate encapsulating the ducts with closed cell spray foam, leaving the attic unconditioned.

  2. Expert Member
    BILL WICHERS | | #2

    It's safer to just use closed cell spray foam under the roof sheathing. If you want to build up a thick layer, you can use several inches of closed cell to get the barrier you need, then fill the rest with open cell. This saves a little money compared to using exclusively closed cell spray foam.

    Jon's idea about sealing up the ducts in closed cell spray foam could work, but that would make maintenance a huge problem. I wouldn't go that route myself.

    BTW, don't let the contractors get away with "we always do it this way". It's entirely possible to always do something incorrectly.


    1. Expert Member
      MALCOLM TAYLOR | | #3


      It's funny how some parts of the construction industry think it's a positive when they say they have been doing things the same way for decades. I'm trying to think of how that approach would sound if said by dentists? doctors? automotive designers? appliance manufacturers?

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