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Foam sandwich concerns

KSJeffery | Posted in Green Building Techniques on

I suspect I know the answer to this already, but it bears exploring as the “risk” will impact my wall design for the future house.

First, the pertinent data: I live in zone 6 Minnesota, and the structure in question is a garage/shop. In floor radiant heat, no humidifier, design temp of 60F. 2×6 at 16″ o.c. construction with 7/16″ OSB sheathing. One and one-half inch XPS was placed outide the sheathing (before I educated myself and learned that 2″ XPS would have been a better choice for 2×6 in zone 6), followed by WRB wrap. 3/4″ 1×3 stapping to create rain screen, then fiber cement lap siding

Now for the question: how stupid would I be if I proceeded to apply 2″ of spray foam to interior stud bays?

I know that creating a “foam sandwich” would not provide an avenue for drying the OSB if it got wet, and thus is rightfully discouraged. But, with the rain screen and WRB, it seems that water will have difficulty getting to the OSB; and if it did, it must have accessed it through a crack that perhaps then would provide an equal opportunity for drying? In fact, the XPS is full of “cracks” anyhow as the seams between the XPS sheets didn’t get taped before some overzealous hired hands applied the WRB.

I am a fan of closed cell spray foam – for air sealing, adding structural strangth, and good R-value per inch of thickness. However, while the above described wall is nearly complete, the stub bays sit vacant so filling with unfaced fibergalss batts is an available option. I want to spray foam, but I do not want to create a wall destined to fail if that’s the opinion of people far smarter that I!


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  1. jklingel | | #1

    I'd suggest Roxul batts if you fill the 2x6s. I wish I could advise you on the spray foam (I'd be leery of it, as you are), but someone more knowledgable will post.

  2. GBA Editor
    Martin Holladay | | #2

    Standard answer: closed-cell spray foam between the studs is OK, and so is exterior rigid foam. But not both. If you want to take the risk, you can -- it's your garage -- but it's not recommended.

    You can install open-cell spray foam if you really want spray foam. Or you can choose another type of vapor-permeable insulation (mineral wool batts, dense-packed cellulose, or fiberglass batts) between the studs.

  3. KSJeffery | | #3

    John - why do you advise Roxul? Is it for the fire safety given that this is a garage? While I like the product, I am doing my level best to keep costs to a minimum as it is just that... a garage.

    Martin - Is the "foam sandwich" a demonstrated risk at this point? Or is it more a theoretical risk based on well-founded opinions and understanding of building sciences? In addition, would you feel that the risk is more due to water infiltration getting past the rain screen and behind the WRB? Or do you feel this is more likely to be risky in consideration vapor pressure? I ask, quite simply, because I'm confident in my rain screen construction, but do not feel I can effectively combat vapor pressure resulting in OSB moisture.

  4. user-757117 | | #4

    Concerning Roxul batts...
    They have many qualities that are superior to fibreglass batts.
    Yes, they are more expensive than fibreglass batts, but I'd wager you'd still spend less going this route than if you decided to hire someone to install a spray foam product - especially if you are able to install the Roxul batts yourself (it isn't hard).

  5. LucyF | | #5

    I chose mineral wool batts for my brother's shop because of water and fire risk. The outside surface of the shop was also not detailed with a rain screen so I figured as a batt insulation mineral wool was the best way to go. The guys installing the mineral wool didn't do a great job, but the building is functions very well anyway.

    After reading stuff on Green Building Advisor for the last few years, I don't think the problems with "foam sandwich" are a theoretical risk. Martin or one of the building science guys who've done "autopsies" on failed building structures can answer that question better.

    How confident are you that you have a good air barrier over the OSB? Was the WRB taped and lapped appropriately? You said that the XPS was not taped. From what I can gather much more water gets through those leaky areas rather than through vapor diffusion.

  6. homedesign | | #6

    I do not advocate sprayfoam

    I believe that Lucas Durand has an excellent Air Barrier System in conjunction with the Roxul.
    and his air barrier is (rightfully) located on the warm side

    Kent, I believe that if you choose Cellulose or Roxul Batts... then you should also detail an Air Barrrier System.. on the warm side of the wall.

  7. LucyF | | #7

    I'm in South Carolina, a very different climate obviously. I am doing the same thing as you -using the shop as a learning tool before l build a house. I really think it helps you go over details and improves your learning curve.

  8. KSJeffery | | #8

    Lucy -- the exterior walls are simple in shape, and the WRB was able to be placed with great ease. No wrinkles, creases and minimum 6" overlap when overlapping req'd. The WRB was taped. Further, with all the 1x3 strapping, it is held snug against the foam. I have great confidence in my rain screen functionality.

    Yet, one can make the argument that any water that gets behind the cladding to the rainscreen could track beneath the strapping, find the FastenMaster structural screws and access the OSB and deeper wall structures in that manner. Which raises another question for me...

    ... need I worry about the strapping rotting? I did prime the strapping on corners, as trim boards will be face-on-face there, but elected to forego priming the field furring strips where fiber cement siding will rest. The cladding will all be back and end-primed.

    As mentioned, I figured I knew the answer, but wanted to pose it one more time to squelch the niggling voice that kept saying, "spray it!"

  9. GBA Editor
    Martin Holladay | | #9

    No, you don't need to worry that your furring strips will rot.

    A ventilated rainscreen gap tends to be quite dry. The furring strips are protected from rain and UV light, and are surrounded by gently moving air. For wood longevity, such an environment is close to ideal.

  10. KSJeffery | | #10

    Thank you for the reassurance, Martin.

    John -- when you refer to "Air Barrier System", do you mean a manner of construction to stop air movement through interior walls? Like airtight drywall?

  11. homedesign | | #11

    Kent, it sounds like you may not currently have a complete air barrier "system"....
    and that adding spray foam may be risky

    so, I was suggesting an interior side air barrier such as airtight drywall or perhaps interior side airtight sheathing
    similar to what Lucas Durand has done with plywood and 3M tape

  12. wjrobinson | | #12

    Spray open cell if you use spray foam as you want to.

    Go with Lucas and John if not spray foam.... since I see you headed that way.

    Your walls will be outstanding used for a shop, garage. No interior moisture loads it sounds and a rainscreen. Can't get better.

  13. Expert Member
    Dana Dorsett | | #13

    Kent: You can use closed cell foam between studs if you used a higher-perm/lower-R variant like MD-R-200 (an Icynene product), which has the further advantage of using water as the blowing agent instead of HFCs. Even at 5" the water blown Icycnene runs ~0.8 perms, still more vapor permeable than the 1.5" of foam on the exterior, and twice as vapor permeable as a kraft facer on a batt.

    But for less money you could use a open cell foam for similar air-tightness and use a variable-permeance vapor retarder like Certainteed MemBrain on the interior, which becomes much more vapor open during the warmer higher-humidity months giving the sheathing plenty of drying capacity when it needs it, but is under 1 perm during the dry-air winter months when the indoor relative humidity is below 35%.

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