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Zone 7 but limited to 1.5″ of rigid foam. Worth it?

Canadan | Posted in Energy Efficiency and Durability on

Been reading a ton of good info on here but still have some questions. I’m planning to strip my Zone 7 2×4 house to the 5/8″ sheathing. I’ve already installed new windows made for 2×6 wall, so built out the frames 1.5″ with 2×4’s under the nailing fin.

I’d like to build the rest of the walls out 1.5″, WRB, then hardie plank siding.

I was considering 2 options:
1- 1″ XPS foam continuous, WRB, 1/2″ furring strips, hardie plank

2- 1 1/2″ XPS foam with furring strips set into furring strip channels, WRB, hardie plank

Problem is, based on the chart I should have 2″ of rigid foam for 11 r value.

Is a little foam better than nothing? or will less than 2″ cause issues down the road?

The house is not airtight with proper vapour barrier on the interior. Walls insulated with fiberglass batts.

Any advice on best route to take is appreciated.


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  1. GBA Editor
    Martin Holladay | | #1

    I stand by the minimum R-value recommendations for exterior rigid foam provided in this article:
    Calculating the Minimum Thickness of Rigid Foam Sheathing.

    I don't have a good recommendation for you. Unfortunately, you began building your house before researching the best practices for wall system design. (Note to other GBA readers: do your research before you begin framing your walls.)

    Perhaps you can alter your window details in a way that allows the use of thicker foam.

  2. charlie_sullivan | | #2

    You can also use mineral wool board instead of foam, to avoid the problem that too-thin foam causes. It's more flexible than foam, so installing siding over it is a little trickier,--you should be able to find some recent discussion of that here.

  3. Canadan | | #3

    Thanks Charlie I'll look into it!

  4. Canadan | | #4

    Thanks Martin, I appreciate the response. Judging by the depth of the window trim, I could notch the back of 2" foam so the extra 1/2" of foam lays over the nailing flange. I would have no room for an air gap though

  5. GBA Editor
    Martin Holladay | | #5

    Q. "So 1 inch of foam with a half inch air gap could lead to rot issues down the road?"

    A. Yes. Once again, I urge you to read my article on the topic: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    In your climate zone, 2x4 walls with exterior rigid foam need a minimum R-value for the rigid foam layer of R-10. One inch of XPS has an R-value of only R-5.

  6. Canadan | | #6

    Okay I read the article again and I see what you're saying. Looks like my options are to make the 2" XPS work or go with the mineral wool board that Charlie mentioned.

    $$ is a factor

  7. wjrobinson | | #7

    Martin my thinking is that too little mineral board is not going to stop rot conditions for the same reason as to little Foam. Rot in a wall needs the moisture and the right temperatures. Using some insulation could put the wall sheathing at rot temps often enough for problems, what say you?

    Bruce Brownell is and has always been the number one designer and Builder of external continuous insulation structures. The structure stays warm and dry. Details matter.

    Building Science has fantastic detailing sketches at their site right now.

    Cold sheathing or warm dry sheathing is what you want. Too little exterior insulation is worse than none.

  8. Canadan | | #8

    Okay. What is mineral board? My house is sheathed on 1x8 solid wood planks.

    I understand 2" XPS will be the only way to go if I put rigid foam up. Other than that, the mineral wool option.

    If I go ahead and put the 2" XPS on my house, it is okay to put the WRB under the foam, between the foam and the sheathing? Then hardie plank directly over the foam.

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