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Exterior Rigid Foam Insulation Between House and Attached Garage

BdLouis | Posted in General Questions on

Been a lingerer for a while, but first time posting. Looking for some advice on something I can’t seem to find a definitive answer for online.

I’m needing to have a fire-rated door installed between the house and the attached garage, but before a new door and jamb go in, I would like to know if installing rigid foam insulation against the sheathing on the shared wall garage side is worthwhile before I hang drywall – and – is there any concern about moisture on that side that a vapor retarder might be necessary?

I live in zone 6-A, and the wall does have batt insulation, but will require some sealing to close up gaps before I get to putting anything up. The rest of the garage is not insulated, and don’t have any plans to do so for now. I’m sure I’m leaving some relevant info out – so please ask what else is needed.


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  1. walta100 | | #1

    When I built mine I skipped the foam on the garage exterior walls and no foam on the interior common wall to the house, The garage 2x4 exterior walls are fill with cellulose as is the 2x6 common wall. My zone 4 garage stays about 40°

    Will you have any other vapor impermeable layers in your wall besides the foam?

    I did find it necessary to upgrade the fire door hinges to ball bearing hinges be sure to check that box when you order the door.


  2. BdLouis | | #2

    @Walter I appreciate the feedback and good additional tip on the hinges!

    I'm not aware of any other vapor barrier on that wall, however I do not have visual confirmation whether there is poly sheet or faced fiberglass batt on the home-interior side - my understanding of insulation would tell me there should be; assuming the shared wall of the attached garage is considered exterior.

    On the garage side of that wall, the sheathing is bare up against the stud and no vapor barrier exists.

    Do I need to confirm whether a vapor impermeable layer exists first in order to prevent trapping moisture?

  3. walta100 | | #3

    A walls with 2 vapor impermeable layers will often end up with moisture trapped in the wall sometime rotting and molding, so you want to be sure.


    1. BdLouis | | #4

      @Walter There does appear to be a film between the drywall and studs on the house side. So I guess no rigid foam on the garage side is in order.

      Unless there are more/other considerations, I expect just putting up the sheetrock and sealing up to stop air flow to be my plan forward.

      Thanks, Walter!

  4. maine_tyler | | #5

    You could use semi-rigid roxul

    1. BdLouis | | #6

      @Tyler I have not used rockwool before, but believe it to fill the stud bay rather than sandwich between the sheathing and drywall. I'd consider taking the sheathing down and replacing the existing batts, but not sure I'm gaining much for the extra work - this could just be my unfamiliarity with rockwool.

      My primary focus is to block up the wall with sheetrock before the new door goes in, but was exploring whether that wall might be better insulated before I close it up.

      Additional thoughts on the rockwool welcome. Thanks Tyler!

      1. Expert Member
        BILL WICHERS | | #7

        There are two types of rockwool (mineral wool): the usually batt stuff, and a lesser rigid board type. The rigid board type can usually be used in place of rigid foam, but rigid mineral wool is very vapor open by comparison.

        If you take that sheathing off, you can easily remove any interior side vapor barrier with a knife. It’s possible (if this is a newer build) that you might have a “smart” interior side vapor barrier like MemBrain, in which case it can be left in place when you add exterior side rigid foam.


        1. BdLouis | | #8

          @Zephyr7 The house was built in '64, and wouldn't expect it to have anything like MemBrain, but who knows. In my climate, I do believe the vapor barrier on the inside of the existing batt insulation is correct - which is why it does appear that rigid rockwool is an excellent suggestion while allowing me to leave the existing system in place.

          The more I learn about rockwool's properties, the more I like it. The only considerations I think I have left is 1.) whether I should run some furring strips around the sheathing for rockwool and anchor the sheetrock; and 2.) how the rockwool + 5/8" sheetrock impacts a new door with the added wall thickness.

          Thank you!

  5. Jon_R | | #9

    Make sure you end up with something that meets the specs below. Note that like rockwool, exterior unfaced EPS is almost always beneficial. And it can help in cases where AC use + high exterior perms would cause excessive moisture accumulation on interior poly.

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