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Rigid foam, OSB, spray foam: Can I have my cake and eat it, too?

kiwiMN | Posted in Energy Efficiency and Durability on

I’m in the process of designing a home in West Central Minnesota (Zone 6), which we will start building this spring (2017). I’m evaluating SIPS vs Stick frame for my walls but to do that I need to determine what my typical stick frame wall section will consist of.

After many hours of research (many here on GBA, thanks guys), I have settled on some “ideal” design parameters:

– Traditional OSB sheathing. We can have high winds in this area as well as tornados so this will give me some peace of mind.
– A thermal break for the studs and top and bottom plates. Rigid foam on the exterior seems the best way to acheive this, but it also introduces other problems. I’m thinking of only having 1.5″ to 2″ though.
– Closed cell spray foam in 2×6 wall cavities.

Now, the recommendation for installing rigid foam on the exterior in zone 6 is R11.25 (to keep the OSB warm enough to stop “condensation”), and the recommendation is to no vapour barrier on the interior when rigid foam is installed.
I don’t really see the need of installing 2.5″+ of rigid foam to create the thermal break…so if i did install say 1.5″ then the OSB will be too cold, right? But if I’m using spray foam on the inside, then won’t that create an airtight and vapour impermeable barrier? Or is that wishful thinking? If did create that barrier, then the only moisture coming into contact with the OSB is from the exterior, and given that the rigid foam is so thin, then can that moisture dry to the exterior?

One more question – I’m putting a lot of effort into stopping thermal bridging across the studs, but does anyone have a figure on how much energy is lost through TB? I’ve seen the infrared pictures but haven’t been able to track down how much efficiency this will gain.

Thanks in advance for any feedback and I’m looking forward to hearing what the experts have to say.

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  1. GBA Editor
    Martin Holladay | | #1

    Most of your questions are answered in this article: How to Design a Wall.

    Some quick comments:

    1. Stopping thermal bridging through the studs is a good idea. Don't skimp on the thickness of the rigid foam layer.

    2. Most brands of closed-cell spray foam are environmentally disastrous (because they use blowing agents with a high global warming potential) and expensive. Used as you propose, the spray foam (a) would prevent the OSB from drying to the interior -- that's bad -- and (b) would provide very little improvement in R-value compared to dense-packed cellulose.

  2. Reid Baldwin | | #2

    Andy, You say that you are putting a lot of effort into stopping thermal bridging, but much of your post is attempting to justify skimping on the exterior foam which is what reduces the thermal bridging. You mention that exterior foam introduces other problems. If you provide more details about what other problems you are concerned about, we might be able to find ways to address those problems besides skimping on the foam.

  3. kjhkjh | | #3

    I read in a post by Dana that 1.5" of closed cell is still vapor permeable. In that case, would this stack up not work as long as the interior layer of closed cell is thin?
    I'm considering a similar stack up for my project 4" of foam on the outside and 1.5" on the inside. That partly overcomes the risk of setting the dewpoiint at the sheathing if I fill the rafter bays with high density cellulose. Additionally it has the benefit of 'stiffening' the structure. I've noticed that in our current house which had closed cell sprayed into about 1/2 the exterior walls.

    I understand the global warming impact of closed cell but if used sparingly, would this negative be mitigated by the positive benefit of not asking roofers to apply 5" of foam (apparently going from 4 to 5" is a big deal as the longer screws are harder to source, more costly and more challenging to screw)

  4. GBA Editor
    Martin Holladay | | #4

    I've provided my advice (and the reasoning behind my advice) in the article I linked to (How to Design a Wall). Of course, if you prefer to proceed differently, it's your house.

  5. Sal_123 | | #5

    To quote Dana from another post:
    Closed cell polyurethane is wasted when applied between studs, and will hamper the capacity of the sheathing to dry toward the interior."

    I am not saying you can cook-book the statement for all situations, certain scenarios have their nuances, but he went on to explain:
    You're only calculating the center-cavity R, which is almost irrelevant. The number that matters is the whole-wall R with all thermal bridging and all layers factored in.

    My 2 cents from what I culled from Martin and Dana and the articles on GBA :
    - Make the wall as air tight as possible
    - Full cavity close cell SPF is NOT efficient
    - If you have the luxury of being able to use 2" or better yet, 2.5" rigid XPS "outsulation", depending on your final cladding system, use it. Be mindful of your fastening system. Let a local engineer look it over
    and be aware of Peter Baker's article: BA-1404: Initial and Long-Term Movement of Cladding Installed Over Exterior Rigid Insulation.

    - Be aware of your wall's drying design. With exterior XPS, wall should be free to dry to the interior, consider open cell SPF in bays vs cellulose vs old fashioned fluffy stuff installed right, with our without interior, unfaced EPS panels under the sheetrock.
    - Read Martin's link carefully and consult a local knowledgable contractor who understands the issues presented.
    My 2 cents
    Good luck

  6. Chaubenee | | #6

    Go with two inch or two and a half inch exterior foam, tape seams, tape seams on osb, use drain wrap. In the cavities use dense pack cellulose. Never skimp on the screws. Check the tables and add a few more where you think they might benefit the assembly.

  7. brendanalbano | | #7

    The standard stackup of: 2x6 studs with cellulose or fiberglass + as much rigid exterior insulation as required by your climate zone (or performance goals) is standard for a reason.

    It's probably not a good idea to try and invent a novel wall assembly without a really really good reason.

    If you want to save money, maybe look into using recycled foam for the exterior insulation.

    By sticking to a standard assembly, you have a wealth of existing details to draw on that have been stood the test of time so to speak. It's lower risk.

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