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Community and Q&A

New construction: Integral garage insulation help

Wayne321 | Posted in Energy Efficiency and Durability on


I’m building a house in Zone 5 that will have a 2 car integral garage with living space above 1/3 of the garage ceiling and will share a rear wall and 1/3 of a side wall with main house.

My plan is to use the Zip sheathing system (walls and roof) for the entire house with 4″ rigid foam attached to the zip sheathing on the exterior of the walls and then cellulose in 2×6 bay studs.

The shared wall sections will be 2×4 and I was thinking to use 3″ closed cell spray foam here.

The entire ceiling of the garage I was going to use closed cell spray foam as well, making it thicker where there is roof decking (2/3 of ceiling) and would be applied directly to the subfloor above the shared space and to the bottom of the roof deck.

The garage is on fill and as such will have an excavated basement area beneath. The garage floor slab is scheduled to be 4″ with mesh and with metal sheeting and support beams underneath. The electric panel will be housed in the garage and I am pushing for no other systems to be run through the garage except a hose bib on the shared wall.

The garage is not planned to be heated or cooled currently. I plan to get a high performing overhead garage door, but they all leak pretty good anyway.

I am concerned about possible mildew or mold in the garage with this approach. Am I making a mistake? Is there anything I could do better?

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    I don't see any reason why your garage will be any more susceptible to mold than any other garage.

    If you every want to heat your garage, you may have a hard time installing continuous insulation, especially with your suspended slab. Ideally, this type of slab is within the thermal envelope of the building, and there are no thermal breaks in the building's envelope, if this type of garage is ever heated. Think through the concrete slab details (especially at the slab perimeter) if insulation matters.

    Another point: Why are the shared walls between the garage and the house framed with 2x4s? They are exterior walls, and should be framed with 2x6s and insulated to the same level as your other exterior walls.

  2. Wayne321 | | #2

    Thank you Martin for the quick response. Very much appreciated.

    The architect drew the shared walls as 2x4. I'll ask to have them increased to 2x6.

    The exterior walls of my house will have 4" rigid foam and then cellulose stud bays. To your last point, should the garage side of the shared wall also be covered in 4" rigid foam and changed to 2x6 with cellulose bays?

    Or do you mean simply switch to 2x6 on the shared walls and use 5 inches of closed cell spray foam (or 3 inches + cellulose) in the bays?

    Also thank you for the point on slab perimeter insulation. I'm going to consider asking for 2"-4" of foam around the edge of the garage slab. In the future then I may add foam to the underside of the slab assembly.

    The house will have 4" of EPS type II on the exterior of CMU basement walls, under the footing (type IX), at the edge of the basement slab, and under the basement slab. An eventual goal is to DIY rigid foam insulation to the interior basement walls as well.

  3. GBA Editor
    Martin Holladay | | #3

    It's up to you to determine what your R-value target should be. If you want your exterior walls to have 2x6 studs filled with cellulose plus 4 inches of continuous exterior rigid foam, that's great. I'm just saying that the wall separating your house from your garage is an exterior wall, too -- and there is no logical reason to skimp on the R-value of that exterior wall.

    If your basement walls have exterior rigid foam, then you don't necessarily need rigid foam at your slab perimeter. The idea is to make sure that your insulation is continuous. It seems as if the exterior rigid foam at your house will be continuous -- so you appear to be all set on that score.

  4. Wayne321 | | #4


    Thank you for all of your valuable wisdom. Much appreciated!

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