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Closed Cell Foam On The Interior & GPS on the Exterior

sulli370 | Posted in General Questions on

I’ve asked a similar question but nobody chimed in so I figured I’d try again.

I’m wrapping my electrical work next week in an 1880s farm house in Chicago (Zone 5) and will be starting insulation right after. I currently have 3 inches of exterior GPS (neopor) over a Blueskin WRB which is attached to the original 1×8 plank sheathing. I’ve built out double studs within the house and have anywhere from 8 – 10 inches of room for interior insulation depending on where the house was straight and where it wasn’t.

After removing the plaster and lathe the house seemed to sway in the wind a bit more than I am comfortable with and I’d really like to give it at least an inch of closed cell spray foam against the interior side of the sheathing to improve racking strength but I’m afraid that it won’t be able to dry properly.

Can anyone confirm if its a good idea or bad idea to attempt to use spray foam  with that exterior GPS? If its a bad idea for 2 or 4 inches would it be possible to try and target the installation just in the gaps between the boards to allow for them to dry?

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  1. Expert Member
    BILL WICHERS | | #1

    I don't see a problem here, since spray foam is fully adhered. You would want to use closed cell spray foam, but that's the one that adds a bit to the structure so that's what you'd want anyway.

    I would not trust spray foam as a structural element though. If your house "sways in the wind", you have structural issues. I would try putting up heavier exterior sheathing (OSB or plywood), making sure it ties the top and bottom plates together along with a number of the studs. It's important that it tie the top and bottom plates together for proper shear strength on the wall. If you can't put structural sheathing on the exterior, it will work on the interior too but will complicate some of the other construction if you do it that way. You could also try interior side let-in 1x4 bracing if you can't put up OSB or plywood.

    Note that you can also engineer shear walls if you don't want to try to brace with a continuous layer of sheathing. Check with a structural engineer if you want to go that route.


    1. sulli370 | | #3

      Thanks Bill. I've added three shear walls and that did tighten the building up. We also had a recent wind storm with 50MPH winds and the building didn't move more than what is typical for a 140 year old 35ft tall structure . I just figured longer term I wanted the spray foam to effectively take the role of what that the plaster did to add bracing to the sheathing and balloon frame - at the very least it can't hurt as long as that sheathing can still try. I'm going to give it a go and ask the installer to try and keep the closed cell to 1-2 inches thick and fill the rest with cellulose.

  2. Expert Member
    Akos | | #2

    I think the better and much cheaper option is to use let in bracing (either wood or metal) for the walls. If you are doing 5/8 drywall, you can also look at bracing the corners with 3/8" plywood and cover it with 1/4" drywall to match the rest.

    Drywall itself does a pretty good job of bracing a house. Key is tight screw pattern and making sure to get screws into the bottom and top plate. Once you get the interior drywall on, even without any additional bracing, it will stop the movement.

    1. sulli370 | | #4

      The issue is that the exterior walls all had plaster on the inside of them making sure they stayed in place from the inside, now they will effectively have nothing since the drywall will be attached to the interior 2x4 walls of the double studs. The let in bracing doesn't make up for weakness of the old exterior plank sheathing when it doesn't have some sort of wall material on the interior of the side of the 2x4s.

      1. Expert Member
        Akos | | #5

        I would pop out the handful of studs that are in the way and install the bracing to the outer wall. If you use metal bracing, you might be able to fish it between the studs if the gap is big enough. Still way cheaper than spray foam over the whole thing.

        The old plank sheathing is nailed to the studs, by adding the diagonal bracing you complete the triangle. It will be very solid.

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