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Community and Q&A

Insulating ceiling of garage

mgensler | Posted in Green Building Techniques on

I’m looking to air seal and insulate the ceiling of my garage. Our kitchen is above it and we have quite a few pipe penetrations as well as three beams in the garage. The joists are 2×10 and 12″ OC. The ceiling already has type x drywall on it and the rim joists have 3.5″ of closed cell on them.

I’m thinking of strategically cutting the drywall and installing 3.5″ of closed cell foam. Does it make sense to then install mineral wool if it can be compressed by the drywall? Areas around the beams, plumbing, etc could have some mineral wool but it would be difficult to compress due to the soffits. I’m trying to be judicious with the closed cell as the ceiling is 700 square feet. Any other recommendations? We’re in climate zone 4a.

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  1. Expert Member
    Zephyr7 | | #1

    You don't need to compress mineral wool batts, just put in the correct thickness batts for the spaces you have to insulate. You are right to be stingy with the spray foam, you don't need to use it everywhere -- try to use it only for areas where it's best suited.

    Dense pack cellulose is another option you could try. Dense pack cellulose can probably be installed with less drywall demolition too, which is a plus.

    Either way I'd try to air seal the penetrations as much as you can prior to insulating.


    1. mgensler | | #3

      Thanks for the reply. I've finished the air sealing with 3.5" of closed cell. Planning on installing mineral wool in the remaining of the joist space. Will the mineral wool be effective if the drywall isn't tight against the joists? We have a bunch of soffits below the joists so the drywall won't be tight against the joists in many places.

  2. Charlie Sullivan | | #2

    I agree with Bill. It's a good scenario for blowing in cellulose. And it's important to do good air sealing.

  3. Walter Ahlgrim | | #4

    I dislike spray foam maybe more than anyone but for air sealing an existing building like yours it seems to me to be a good option.

    I say strip all the drywall off fill the joists with foam and replace the drywall.

    Note your garage ceiling is an important fire break and a life safety feature I think it would be a mistake cut it full of holes and piece it back together.


    1. Andy S | | #5

      Blowing in cellulose wouldn't put too many or too big holes in the ceiling, Maybe two per cavity. Typically they're cut out with a hole saw and then the same pieces are used to patch the holes/taped/mudded. I don't think it's a fire hazard once patched.

  4. mgensler | | #6

    OK, thanks everyone for the feedback.

    What I'd really like to know is can I pack the joists with mineral wool without having the drywall tight to the joists? I've already put in 3.5" of spray foam and have ruled out dense-pack cellulose as I don't trust myself to hire a contractor that can do it correctly.

    1. Expert Member
      Zephyr7 | | #7

      You should be ok putting in mineral wool regardless of how the drywall is installed. Mineral wool batts are pretty easy to get in place well, and can be easily fitted around obstructions.


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