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Insulating a wall that is exterior on 2nd floor, interior on 1st.

whitenack | Posted in General Questions on

I hope the topic of this question is understandable. Imagine a floor plan that is a simple 2-story box (ie, 40×40) that has a single-story wing to one side. The common wall between the main structure and the wing will be both an interior (1st floor) and exterior wall (2nd).

My thoughts for the rest of the house’s insulation would be 2×6 frame construction with blown in cellulose, and 2-4″ of exterior rigid foam, a rain gap, and then brick veneer. For the attic, I am leery of the dangers of spray foam, and attic floor insulation makes sense to me, so I’m thinking 16-18″ of blown in cellulose on the attic floor.

I have read on GBA that you should bring the wall insulation down to the level of the ceiling insulation of the attached wing. I’m just trying to imagine how that would work with exterior rigid foam. The exterior foam from the 2nd floor wall would come down the wall, inside the 1st floor roof line, until it meets the blown in cellulose from the 1st floor ceiling. Is there any complications attaching the roof of the wing to the exterior foam ? What about the brick veneer?

And an additional question: Let’s say that this wing then attaches to the garage. The garage’s roof line is self contained and the only connection from the garage to the house is the single shared wall. I’m assuming I just need to worry about sealing and insulating that shared wall and leave the rest of the garage uninsulated. Do I still need the same amount of insulation for this shared wall, or does the garage give me any protection? I figure an uninsulated garage won’t give me much, but wondering if I still need 8-10″ of insulation.

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  1. GBA Editor
    Martin Holladay | | #1

    If your garage is unconditioned (like most garages), then the partition separating your house and your garage should be treated like an exterior wall. It should be insulated like any other exterior wall. Air sealing is particularly important for this wall, because there are often a lot of chemicals and fuels stored in a garage.

    Concerning an insulated second-story wall that intersects with the roof of an attached single-story addition:

    1. Make sure that the insulation barrier is continuous and uninterrupted.

    2. Make sure that the air barrier is continuous and uninterrupted.

    3. Make sure that you have a good plan for supporting the brick veneer on the second-floor wall.

    4. Make sure that you know how to step up the through-wall flashing at the base of the brick veneer where the brick veneer meets the roofing on your single-story wing. Most masons make mistakes with this flashing.

    The rafters of your single-story wing don't depend (structurally) on attachment to the adjacent two-story wall for support, but a few lag screws to attach the nearest rafter through the wall foam to the studs on the two-story wall are a good idea.

  2. whitenack | | #2

    Thanks Martin. For the shared wall to the garage, I assume that I just use one layer of sheathing between the garage and the wing, correct? 2x6 studs with cellulose, sheathing, and then the rigid insulation between the studs of the garage framing? Or does that allow two much thermal bridging? In that case, I would need 2x6 cellulose, sheathing, rigid foam, sheathing, garage framing. Only a portion of the garage wall will be connected to the house, so I can't just put the rigid foam on the garage side of the garage framing, or else I'll have a bump out inside the garage in the amount of the foam.

    I think I follow you on advice 1-4, other than I'm not sure about the support for the brick. I'm not sure what is typical. I assume I have to be careful with thermal bridging down the brick and through whatever is supporting it.

  3. GBA Editor
    Martin Holladay | | #3

    If you have an attached garage, there is usually a common wall. That mean just one row of studs.

    If you want to install a continuous layer of rigid foam facing the garage, you have to protect the rigid foam on the garage side with 5/8 inch (fire code) drywall. In some jurisdictions, you may have more stringent code requirements for this wall, so check with your local code official.

  4. whitenack | | #4


    If my insulation strategy for the rest of the house is 2x6 plus rigid foam on the exterior, how do I insulate the shared garage wall with the garage with only one set of studs? Do I just leave out the rigid insulation on this wall? Do I have a bump out in the garage to account for the extra foam? Do I put the rigid foam on the house side of the studs?

    *ETA: Or, do I switch to double stud construction for this particular wall?

  5. GBA Editor
    Martin Holladay | | #5

    Your question is unclear to me. The wall system you are talking about -- "2x6 plus rigid foam on the exterior" -- has just one row of studs. Why would you want two rows of studs? Two rows of studs are used for a different approach -- namely, a double-stud wall.

    For more information on the approach I thought we were talking about -- "2x6 plus rigid foam on the exterior" -- see this article: How to Install Rigid Foam Sheathing.

  6. whitenack | | #6

    Sorry I'm not explaining this correctly, and thanks for your patience with the uneducated. If I am understanding you correctly, you are saying to have a single, 2x6 wall between the garage and the house. My question would be: If I am filling the 2x6s with cellulose, where do I put the rigid foam? If, like the rest of the house, the rigid foam goes on the outside of the 2x6 studs, that means the foam will be sticking out into the garage (since the connection to the house would not run the entire length of the garage wall). Is that how it is done? Just make sure the foam is covered by the approved drywall and have a bump out?

  7. GBA Editor
    Martin Holladay | | #7

    Your wall can be as thick as you want. You can have a bump-out if you want, or you can make your wall co-planar so that no one ever sees a bump out.

    It's all up to you and your framer. The usual approach would be to attach OSB or plywood on the exterior side of your 2x6s, and to attach the rigid foam to the exterior side of the sheathing with cap nails. If the rigid foam faces a garage, it has to be covered with 5/8 inch drywall.

    If the garage continues past this common wall, you can frame the rest of the garage walls (the ones that aren't part of the common wall) so that the drywall on the interior is co-planar. Not too hard to do -- you just have to plan ahead. If everything on the garage side is co-planar, there is no bump-out -- right?

  8. whitenack | | #8

    For example. See the attached floor plan that has a small hallway to the garage. This is like something I have in mind. Do I just put the rigid foam where the red line would be, which means the wall would bump out right there?

  9. GBA Editor
    Martin Holladay | | #9

    Let's say that North is up, as it is on most maps.

    Just north of your red line is a black line -- that's the exterior wall of your garage that is not a common wall. Move that wall 3 or 4 inches west, or as many inches as you need to move it to make the garage-side drywall co-planar.

    If you don't want to do that -- because this suggestion may complicate (or not) setting your garage roof trusses, you can always install horizontal strapping on your garage studs to make your drywall co-planar -- if, for some reason, it is really, really important to you that your garage drywall is co-planar.

  10. whitenack | | #10

    Thank you so much, Martin. It's obvious I don't know enough about construction to get these simple concepts. I didn't realize you could break the 2x6 wall like that to make everything flush. I assumed the 2x6 wall had to be straight, which would either mean 2x10s on the rest of the "north" wall or adding 4" blocks to the 2x6s to make them flush.

  11. whitenack | | #11

    I wrote that last reply while you were editing your last paragraph. I started to ask about the complicated roof trusses, but figured I had bothered you enough! It isn't important for my garage drywall to be co-planar. I just want to know what to expect. In some garages, losing 4" in certain places can be a big deal.

  12. GBA Editor
    Martin Holladay | | #12

    Wood-framed walls can have many types of interruptions, including bump-outs and changes in thickness. That said, moving part of your garage wall west may not be worth it. If it were my house, I wouldn't mind the fact that a section of the drywall on one side of my garage was not co-planar with an adjacent section -- and most framers would prefer to keep the framing simpler.

    If you are designing the house, and you aren't familiar with how wood framing works, I suggest that you hire an architect or designer.

  13. whitenack | | #13

    Thanks Martin. Yes, it is obvious I need some professional assistance. I have a builder that I have worked with before that does great work. I just like to have a little knowledge about it so I have proper expectations. You know, trust but verify.

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