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Pros and cons of using 1″ foam for exterior wall thermal break

Baconaiken | Posted in Energy Efficiency and Durability on

I am building a garage in Northern Wi and want to use Tuff R against OSB sheathing. The insulation will be an additional 3″ of foam board in the cavity wall. (2×4) Some living spaces are involved, but moisture from occupation should be minimal as the building is not plumbed. Walls will be sealed inside and an interior vapor barrier used. Caulking applied to all exterior stud joints. I want to apply a vapor barrier between the OSB and the foam for release of moisture. I am thinking of using 15 felt which is typical for stucco installations. Any experience with this type of construction out there?

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  1. GBA Editor
    Martin Holladay | | #1

    1. Don't install an interior polyethylene vapor barrier. Rigid foam is already a vapor retarder.

    2. Don't install a vapor barrier between the OSB and the foam.

    3. A vapor barrier will not lead to the "release of moisture."

    4. #15 felt is a perfectly acceptable water-resistant barrier (WRB).

    5. It doesn't make much sense to stack narrow rectangles of rigid foam between your studs. If you want to use 3 inches or 4 inches of rigid foam, install all of the foam on the exterior side of the wall sheathing -- not between the studs.

  2. Baconaiken | | #2

    Thanks for the clarification. I found a site at which provides some better context to what I am trying to do. MY main concern was the OSB potential for moisture problems. It seems that the better answer is to eliminate the OSB and replace it with 1" exterior foam. DOW advised that I use this method as well and fill the wall cavity with fiberglass or cellulose and provide a type 2 or 3 vapor barrier which would allow the wall to breathe from both sides. There are plenty of ways to brace the wall adequately without the OSB. Underbuilding is always a concern, but overbuilding seems to be more of a problem in our times.

  3. GBA Editor
    Martin Holladay | | #3

    You are in northern Wisconsin, in climate zone 6 or 7. You can't use just 1 inch of XPS sheathing (R-5) in your climate zone.

    Re-read the article I linked to: Calculating the Minimum Thickness of Rigid Foam Sheathing. Depending on your climate zone, you need at least R-11.25 or R-15 foam for 2x6 walls. That means 2.5 inches or 3 inches of XPS, not 1 inch.

  4. dFArch | | #4

    I am looking at a similar situation.

    What about rigid foam board between the framing studs and the exterior sheathing and additional spray foam applied from the interior side ?

    Bridging reduced and better air / vapour seal from inside - sheathing can dry to outside.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "What about rigid foam board between the framing studs and the exterior sheathing?"

    A. That approach is usually frowned upon. For one thing, the OSB or plywood sheathing can't provide adequate bracing (racking resistance) with the intervening foam. If you really want to install rigid foam directly to the studs, you might as well skip the OSB or plywood, and provide your wall bracing by alternate means (for example, L-profile steel strapping).

  6. dFArch | | #6

    Thanks Martin for the comments pointing out structural concerns - have not built with a wall like that before but seemed to make sense from a thermal break and air/vapour barrier approach (at least in theory).

    I was looking at GBA plan detail showing "brick veneer with 2-part ties / 1.5" rigid insulation" and no sheathing at all posted on the website - that must really depend on the alternate bracing methods you mention.

    Closed Cell SFI can add rigidity to the inner frame but any structural advantages of direct adhesion to the exterior sheathing would be totally interrupted by a RI foam sheathing layer. Hmmm - one wonders about nailing exterior sheathing through horizontal strips of RI foam "strapping" with the SFI then applied from the inside to "grab" the back of the OSB / Plywood with the SFI locking around the stud framing members.

    I have heard about another similar method using extra 2x2 strapping between studs & sheathing- and just CCSFI (closed cell spray foam insulation) applied from the interior side eliminating the rigid foam board sandwiched layer. Only thermal bridging is at the overlap locations between vertical framing and horizontal strapping - more connection strength but less insulating value along the length of the strapping...

    Apologies for getting a bit tangential from the original topic (!) - but somehow adding foam beyond the face of framing is something I've wondered about for some time and appreciate the feedback !

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