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Insulating addition over unheated garage

3NyQ6AX4gs | Posted in Energy Efficiency and Durability on

I’m a homeowner tackling an addition on my own (so far). I’m located in zone 6 (Twin Cities, MN). I searched for this topic and most of the answers had to do with warmer zones (<4) and I’m unsure of the approach for my (cold) area. The project is an addition over an existing garage. The garage is unheated and built with 22″ deep web trusses, 2X6 walls. Part of the house overhangs the garage and is already insulated with fiberglass batt only. Yes the floor over that space is cold. The new addition will adjoin the existing space and I want to have a consistent insulation strategy for the entire garage ceiling.The advice so far, 1) closed-cell foam – prohibitively expensive 2) fiberglass batt – simplest installation, lowest cost, seems to be the lowest performer as well 3) blown-in cellulose/fiberglass – I have no experience with blown-in 4) have considered 1.5″ rigid against floor sheating, then fill batt (almost like the flash and batt) The walls of the addition – I’m planning traditional R-19 fiberglass batt. Looking for advice. Thanks

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  1. GBA Editor
    Martin Holladay | | #1

    My recommendation is to install as much rigid polyisocyanurate foam under your joists as you can afford -- 2 inches or more would be better than 1.5 inches -- and then blow the cavities full of dense-packed cellulose. Hold the polyiso in place with 1x4 strapping.

    Your plan for wall insulation is weak (and probably won't meet code, which calls for at least R-20). Fiberglass batts are your worst choice. Almost anything would be better -- dense-packed cellulose (especially if you thicken your wall with horizontal interior 2x4 strapping) or rigid foam plus cellulose.

  2. 3NyQ6AX4gs | | #2

    Thank you for your response. I have a question and a comment.
    Q. When blowing cellulose, is there a 'proper' density measurement that achieves the desired R value? In other words how does one know when the cavity is packed vs filled?

    Comment about the wall insulation. The local building code is actually R19. For the wall extension, this 2x4 strapping will result in a 10" thick exterior wall. Any issues with vapor inside the wall that thick. I'm leaning towards the rigid plus cellulose, since it is consistent with the suggested floor (garage ceiling) method. Would that be 2" polyisocyanurate, filled with cellulose? I already have a (small) amount of R-19 fiberglass. Can I use that to fill with the rigid foam?
    Like the addition, what started as a comment has become more....
    Thanks for your straightforward advice.

  3. GBA Editor
    Martin Holladay | | #3

    You want to install cellulose at a density of at least 3.0 pounds per cubic foot -- 3.5 is better.

    If you've never done it, and you don't have the equipment, this is a job best performed by an experienced cellulose contractor. Ask the contractor about dense-packing; if they sound as if they don't know much about it, find another contractor.

    Two inches of polyiso plus dense-packed cellulose would be a good combination for your walls. If you want to use your R-19 batts instead, you can, but I think it would be best to place the batts in a dumpster.

  4. 3NyQ6AX4gs | | #4

    Thanks. Your advice is appreciated.

  5. jbmoyer | | #5


    Martin has given you some good advise. When you are inquiring about the services of an insulation contractor ask them the following questions:

    What's your experience with cellulose insulation?
    -They should have years of experience with wet sprayed and dense pack installs.

    At what density do you blow in the cellulose?
    - As Martin explained it should be blown in at 3.0 to 3.5 pounds/cubic foot. At less than 3, settling can occur, more than 3.5 will cause problems installing drywall.

    How do you assess the density?
    -The contractor should be taking an accurate bag count (most manufacturers include a density chart on the packaging listing # of cubic feet per bag at various densities) and ideally should be taking core samples (removing random samples from the wall and weighing them on a digital scale).

    If the contractor isn't doing these things, find a different contractor.

  6. user941025 | | #6

    Man, I wish I'd had access to this forum before I did my addition three years ago (also Minneapolis). Yesterday there was a team here with an infrared camera doing a blower door test and I saw so many things I'd have done differently now.

  7. user-788447 | | #7


    While you insulate the ceiling of your garage I would highly recommend establishing as best you can a continuous air barrier. This is not only important for the performance of the insulation but with a living space above a garage (not recommended in my book) you want to ensure that car fumes don't enter your living area.

    Drywall acts as an air barrier but the trick is detailing the seams and protrusions correctly with tapes, caulks and/or spray foam.

    For reputable cellulose insulation contractors in the Twin Cities I would consider Houle Insulation Inc. and those listed by the Minnesota Building Performance Association:

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