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Could you use rigid foam with closed-cell spray foam if it’s between the studs and sheathing?

Kendog52361 | Posted in General Questions on

I have read several answers and blog posts stating that it’s a bad idea to use rigid foam on the outside of the sheathing in combination with closed cell spray foam, whether it’s the only inside insulation, with flash and batt, or flash and fill in the studs. That using closed cell spray foam inside the stud cavities still has the thermal bridging issue. My question is, could you use, either a “diy” or commercial product to place the rigid foam in between the studs and exterior sheathing. An idea of what I’m describing, as well as one of the commercial versions is Zip System insulated R-Sheathing. It includes either 1″ or 1.5″ of rigid foam on the back of the Sheathing that, I think, provides a thermal break between the studs and sheathing/wood.

http://www.huberwood.com/zipsystem/products/zip-system-rsheathing

Whether you use a commercial store bought product, or some kind of diy system, could this solve both issues of thermal bridging and avoiding the “osb sandwich” insulation issue.

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Replies

  1. Kendog52361 | | #1

    Above, that should be, "That only using closed cell spray foam inside the stud cavities still has the thermal bridging issue.

  2. Expert Member
    Dana Dorsett | | #2

    When putting rigid foam between the studs & sheathing you have to ensure that you still meet code on the structural issues. The rigid sheathing is necessary to deal with racking forces on the building from wind- if there is 2-3" of foam between the studs and sheathing there probably isn't ANY faster pattern that would guarantee that structural capacity.

    ZIP-R is a pre-engineered & tested system, and the fastener specs have to be followed to ensure it has the structural capacity. With a DIY you're on your own. You could get there with let-in bracing (indeed, foam sheathed buildings without any rigid sheathing have been build with let-in bracing.)

  3. Kendog52361 | | #3

    I was mainly including the "diy option" for anybody who doesn't like the Zip System/premanufactured systems that provide that. I fully realize that it would require additional bracing/structural work.

    I was focused more on the idea/possibility of using the foam on the inside in combination with closed cell spray foam to provide great warmth, no thermal bridging, and avoiding the whole "osb sandwich".

  4. Expert Member
    Dana Dorsett | | #4

    There's no such thing as "...no thermal bridging...", unless literally all the insulation is exterior to the studs.

    If you have the stud bays available and are putting suffient exterior-R for dew point control, it's a lot cheaper to stuff the stud bays with fluff than to hit a similar performance point with thicker exterior foam, even if the thermally-bridged performance is lower than the center cavity fluff's R value.

  5. Kendog52361 | | #5

    So I didn't choose my words perfectly. I meant limiting thermal bridging as much as possible, while still allowing closed cell foam inside the wall cavities. As I said in the previous responses, I am asking about using rigid foam to separate the sheathing from the studs and combining with closed cell spray foam. That is my question, not trying to play word games or trying to nitpick poor word choices.

    Edited at 17:18 Central US Timezone:

    I apologize if I seemed short or rude. I was getting frustrated that you seemed to be nitpicking my word choice instead of answering the question about closed cell spray foam in the wall cavity, combined with rigid foam between the studs and exterior sheathing like the Zip System R-Sheathing product.

  6. GBA Editor
    Martin Holladay | | #6

    Ken,
    If you decide to use an alternative bracing strategy -- diagonal metal strapping, let-in 1x4 bracing, or some other engineered approach -- then you can install a continuous layer of rigid foam on the exterior side of your studs. At that point, there is no building science reason to worry about closed-cell spray foam between the studs.

    If you go this route, you can either install the siding without any OSB or plywood, or you can install a layer of OSB or plywood on the exterior side of the rigid foam.

    All of that said, this is an expensive wall that has few, if any, advantages over more common approaches.

    For more information on these issues, see these two articles:

    Four Options for Shear Bracing Foam-Sheathed Walls

    How to Design a Wall

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