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Best way to insulate the wall between an attached garage and the house?

user-1135533 | Posted in Energy Efficiency and Durability on

Hi Everyone,

Just finished wrapping my house in 1 inch foil faced tuff-R polyiso during my siding update and now have the last exterior wall left – the shared wall between the garage.

I took down the old sheetrock (not to code thickness anyways) and took out the old batts. The only thing left is the interior sheetrock. The house was built in the early 70’s so no vapor barrier. What is best option to insulate this space? I’m struggling between these two.

– Roxul in the stud bays. Place polyiso sheets over the studs and then firecode sheetrock over that.

– Cut polyiso to fit into the stud bays, spray foam the gaps, then fill with roxul. Foam over the studs and then sheetrock.

Any thoughts are much appreciated! Thank you in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    The first question is: where do you live? It only makes sense to install 1-inch-thick polysio on the exterior of your walls if the foam is thick enough to keep your stud bays above the dew point in winter. If you have 2x4 walls, your R-6 foam will work in climate zone 5 (and anywhere warmer). If you have 2x6 walls, your R-6 foam will work in climate zone 4 (and anywhere warmer).

    For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Assuming that you are installing rigid foam that is thick enough, the best approach is to install the rigid foam as a continuous layer on the garage side of the studs. Fill the stud bays with the insulation of your choice; Roxul batts would be fine. Pay very close attention to air sealing, especially at penetrations through the bottom plate and top plates, as well as at the seams between the sheets of rigid foam.

  2. user-1135533 | | #2 in zone 5 coastal Connecticut with 2x4 construction.


  3. GBA Editor
    Martin Holladay | | #3

    So go with Option 1.

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