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Mineral Wool in Ceiling Cavity With Rigid Foam to Interior

Alex Halpern | Posted in General Questions on

Hi,

Is it okay to use mineral wool insulation in ceiling cavity between framing with a few inches of rigid foam bellow on the interior side of ceiling framing? The house will also have an intello vapor retarder. Just trying to get a higher R value in the ceiling. Is this an ok ceiling design?

Zone 5. With a small attic space. Vented at the ridge and will try to put in venting at eaves as well.

Anything wrong with this plan?

Thank you,
Alex

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Replies

  1. Expert Member
    Zephyr7 | | #1

    If you'll have a vent channel of at least 1" (which is code minimum, 1.5" is better) between the top of the insulation and the bottom of the sheathing, then your plan should be fine. Remember that you need soffit vents at the opposite end of those vent channels from the ridge vent.

    If you did NOT have a vent space, then your proposed assembly could be a problem.

    Bill

  2. Alex Halpern | | #2

    Ok thanks , and what it we do a flat ceiling with trusses and gable roof. You just insulate the flat ceiling and don't insulate the roof sheathing in the attic space correct ?

    Thanks !

    Alex

    1. Expert Member
      Dana Dorsett | | #3

      It's fine to add a layer of foam on the interior side of the flat ceiling under a vented attic, but it's usually much cheaper & easier to hit the same performance point with deeper fluff in the attic, skipping the foam. In new construction specifying an "energy heel" truss with sufficient depth for the desired performance level is usually pretty cheap upgrade compared to a few inches of foam.

      https://www.greenbuildingadvisor.com/article/raised-heel-trusses-make-better-enclosures

      If the plan is/was to put ducts & air handlers in the attic, a "plenum truss" to acccomodate the mechanical systems fully inside the pressure & thermal boundary of the house is also cheaper than insulating at the roof deck:

      https://www.greenbuildingadvisor.com/article/use-plenum-trusses-to-keep-ducts-out-of-your-attic

      The concepts are not mutually exclusive- most plenum trusses would also be energy heel trusses. Just be sure to specify the target insulation depth & density to the truss designer.

  3. Alex Halpern | | #4

    Awesome,

    Raised heel trusses ! Great info as a potential second option.

    Thank you,
    Alex

  4. Expert Member
    Zephyr7 | | #5

    If you'll have attic space above (I was thinking you'd have a cathedral ceiling arrangement), I would use trusses like Dana suggested, then blown cellulose over the attic floor. This is usually the cheapest type of insulation you can use in this application, and it's also one of the best performing.

    Bill

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